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1 Research with Rare Isotope Beams Nuclei Far From Stability

Overview

Beams of short-lived nuclei (frequently referred to as ‘rare isotope’, ‘radioactive’, or ‘exotic nuclear’ beams) have opened an important new window on the study of the strongly-interacting system which we call the atomic nucleus. The new facility proposed in this Conceptual Design Report (CDR) is intended to provide the tools for a far-reaching, in-depth exploration of this new frontier. In this chapter we describe the science and the instrumentation to realize these goals. Since the research involves the preparation and use of secondary beams, a close link exists between the experimental stations addressed here and the accelerator facility described in Section 3 of this report.

The nucleus is built of protons and neutrons, collectively called nucleons, not so much for ease of reference but to reflect the identity of these two nuclear constituents within an isospin doublet. The interaction between the nucleons is governed by the strong force. We know that the nucleons are built from quarks and gluons and that the strong force acts between these fundamental building blocks. To derive the properties of the nuclear (or nucleonic) many-body system from the underlying quark-gluon dynamics seems impossible or, at best, a (very) long-term goal. As in any many-body system, effective theories and interactions are developed to describe the physical behavior and the relevant degrees of freedom and, if correct, to extrapolate into unknown territory. Conversely, moving into new territory can reveal new sensitivities and deep insights into the effective theories which were previously hidden. This is also the expectation with radioactive beams, namely that pushing nuclei to their limits in neutron and proton numbers reveals new features and leads to a more comprehensive understanding of the nuclear many-body system. In this context, one should note that another, largely unexplored dimension in the chart of nuclides is spanned by hypernuclei which contain strangeness as an additional quantum number. Antiproton beams at the proposed facility will allow efficient production of hypernuclei even with more than one strange hadron, see the discussion in Chapter 2.

This expectation for substantial scientific achievements with radioactive beams, of course, is strongly based on the initial results from the first-generation facilities. These studies allowed the limits of nucleon binding to be reached – i.e. the proton and neutron drip lines. Neutron-rich nuclei in particular revealed new phenomena such as a neutron halo. But only the lightest neutron-drip line nuclei could be reached. It is worthwhile to note here that, for example, the best-known of these neutron halo nuclei, 11Li, had been known and previously studied for more than 20 years but that only its availability as an energetic beam, and thus the possibility to perform nuclear reactions, revealed the new topology. Much of what we know about nuclei comes from nuclear reactions. This is the key motivation for not only increasing production of nuclei far from stability, but to be able to prepare them as an energetic beam.

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Preparing these exotic nuclei at high intensity and/or as energetic beams also opens new opportunities for research in the area of nuclear astrophysics, for studies of fundamental symmetries and tests of the Standard Model. The specific studies envisioned for these three key areas of research with radioactive beams – nuclear structure and the nuclear many-body system, fundamental symmetries and nuclear astrophysics – are summarized in the following. There are also possibilities for applied studies using such beams; these are briefly discussed in Chapter 5.

Nuclear structure far from stability. The availability of intense and energetic beams of short-lived nuclei allows to explore the structure and dynamics of nuclei in regions far away from stability (Figure 1.1). Both the increasing proton-neutron asymmetry and the decreasing binding approaching the limits of stability lead to exotic nuclear properties. Stable nuclei are characterized by a constant, almost universal value of the nuclear density with a rather steep gradient at the surface. Loosely bound neutron-rich nuclei, for instance, are expected to exhibit rather extended surface zones of low nuclear density, which are occupied essentially by neutrons alone, the skins or halos. Nucleons tightly bound in the core are coupled to valence nucleons moving almost freely. The structure, the dynamics and the spectral response of loosely-bound quantum systems will be very different from those of stable nuclei. One expects to encounter novel types of shell structures, new collective modes, new pairing phases, or regions of nuclei with special deformations and symmetries. Effects of nucleon clustering should become more prominent, giving rise to unusual nuclear geometries. Theoretical concepts have to be developed further in order to achieve a profound understanding of the expected new data on exotic nuclei. Outstanding problems are the appropriate effective interactions in new regions of density and isospin, many-body methods that account for the weakness of the mean field, and structure and reaction models that can cope with the continuum in close vicinity to bound states.

Fundamental interactions and symmetries. Nuclear decay properties are particularly sensitive to specific properties of fundamental interactions in particle physics. Selected species of exotic nuclei provide the micro-laboratory for crucial tests of the Electro-Weak Standard Model. Precise tests of the conserved vector current hypothesis and searches for non V-A contributions to the weak interaction may be performed. Such studies require specific β-decay transitions in nuclei which are located at or near the symmetry line of equal proton and neutron number. Moreover, violations of fundamental symmetries such as parity or time reversal can be studied with extraordinary precision at the interface of atomic and nuclear physics experiments. Such low-energy tests of the Standard Model will benefit from a next-generation facility in a two fold manner, first from the large variety of unstable isotopes being available, and second from their much improved source intensity.

Nuclear Astrophysics. Cosmological processes are primarily driven by gravity and nuclear physics. Stars generate energy and synthesize elements in nuclear reactions which thus define the various stellar burning stages. The nucleosynthesis of heavier elements occurs in violent explosive events such as novae, supernovae or x-ray

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bursters. The synthesis pathways in such cataclysmic processes proceed mainly within regions of unstable nuclei, see Figure 1.1. Experimental studies involving exotic nuclei and exotic nuclear beams provide masses, decay properties and reaction rates, the most basic ingredients to be known in order to understand the evolution and the fate of stars.

Figure 1.1: Chart of nuclei. Stable nuclei, as found on earth, are marked by black symbols. The yellow area covers unstable nuclei already produced in laboratories. Many more unstable, but bound nuclei may exist, the region of these exotic nuclei is given in green. Red areas cover specific stellar nucleosynthesis pathways, the r- and rp-processes. Magic proton and neutron numbers are indicated. The insets itemize some of the key questions to be addressed at next-generation exotic-beam facilities.

During the past decade, a rich scientific harvest has been collected from exotic-nuclear-beam facilities which were installed worldwide. One of the reasons for this success stems from the possibility that unstable nuclei could be used in nuclear reactions. These facilities followed two different approaches: the In-Flight method where high-energy heavy-ion projectile fragmentation is used as the production method and the technique of Isotope Separation On Line (ISOL) where reaction products are stopped and post-accelerated. The former technique provides fast, clean and chemistry-independent separation, the latter is superior in terms of beam intensity for a number of elements and in terms of beam quality. There is a general consensus worldwide that both approaches should be pursued since they are highly complementary.

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A first-generation of in-flight nuclear-beam facilities is in operation at various laboratories in Europe, the USA, and Japan. Among those, GSI has played a particular role since it provides the only facility worldwide where beams of all elements are available with energies up to around 1 GeV/u. This has opened the unique possibility to utilize fragmentation reactions from even the heaviest ion beams and, moreover, to exploit in-flight fission of uranium for the production of isotopically separated, secondary exotic nuclear beams. The latter method was pioneered at GSI. In addition, the time structure of the beams from the synchrotron accelerator at GSI allows the injection of short bunches of projectile fragments into a storage-cooler ring, where they can be electron-cooled to extremely small emittances. At present, the storage-cooler ring facility at GSI is worldwide the only one which operates with exotic nuclei.

The experimental results from the existing in-flight facilities gave a new boost to nuclear structure and reaction physics with the discovery of unexpected phenomena. The most noted feature probably was the observation of neutron halos in light nuclei at the neutron drip line. Other unusual, partly related effects concern significant changes in the single-particle level sequences, sudden onsets of nuclear deformations, the particular role of residual pairing interactions among loosely bound valence nucleons, and the multipole response exhibiting considerable low-lying strength and perhaps ‘soft’ coherent modes.

Experiments at GSI have made substantial contributions to the field. The unstable doubly-magic isotopes 78Ni and 100Sn were observed for the first time. Proton- and neutron-halo nuclei were studied in high-energy reactions and revealed their intricate structure. The high energy of the secondary beams gave the first access to giant resonances in neutron-rich unstable nuclei. Information of astrophysical interest on proton and neutron capture reactions could be deduced from the inverse Coulomb breakup process. High-spin isomeric exotic beams were produced in fragmentation reactions and their γ decay was observed. Low-energy fission properties were deduced for unstable fissile nuclei between lead and uranium, not accessible anywhere else. At the storage-cooler ring, ultra-sensitive methods for mass and lifetime measurements have been developed, which are applicable to a single stored ion and allow the substantial extension of the number of known nuclear masses. Another experimental highlight was the first observation of an exotic decay mode, the bound-state beta- decay, by using a similar technique in the storage ring. Last, but not least, in-flight separation at low beam energies has led to the discovery of the heaviest, unambiguously identified new elements Z =107 to 112 at the velocity filter SHIP.

At present, efforts are under way worldwide to improve the potential of exotic nuclear-beam facilities in terms of beam intensity, separation efficiency, beam quality, and instrumentation. In view of the successful operation of GSI’s synchrotron driver accelerator coupled to a projectile-fragment in-flight separator and a storage-cooler ring, it is a great opportunity to install an international next-generation in-flight facility for Europe at GSI.

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The new facility at GSI will provide two major achievements which are essential for experiments with exotic nuclei. Firstly, secondary beam intensities will be superior by orders of magnitude compared to those presently available, and secondly, new experimental concepts and highly advanced instrumentation allows one to cope with the low intensities of the exotic beams. The building blocks of the proposed facility (see Figure 1.2 for schematic illustration) are based on the experience gained with the existing one. Each of these building blocks contains new features that expand significantly the scientific and technological potential of the new facility. The most important components are listed in the following.

• A synchrotron accelerator complex that provides primary ion beams of all elements from hydrogen to uranium with intensities that will amount to ≥1012 ions/s, i.e. at least two orders of magnitude larger than the ones presently available at GSI. In this high-intensity mode, the beam energy is variable up to 1.5 GeV/u.

• An improved Superconducting projectile-FRagment Separator (Super-FRS) that will accept a much larger phase space compared to the present FRS. Together with the improved primary beam intensity this will result in an overall increase of secondary beam intensities by up to a factor 10000. A pertinent feature of the Super-FRS will be the ability to collect projectile-fission fragments with the same efficiency as achieved at present for projectile-frag-mentation products. Fission of uranium is an indispensable source of very neutron-rich medium-mass nuclei. The Super-FRS serves an experimental area with equipment dedicated to high-energy ion reaction studies, allowing in-jection into storage rings, and includes an energy-bunching stage. The latter provides the possibility to stop beams in solid or gaseous material with strongly compressed range distribution. It also provides energy-focused low-energy secondary beams.

AGATA

2 m

Production Target

SIS

CR

NESR

eA-Collider

Main-Separator

Pre-Separator

Low-Energy Cave

Energy Buncher

High-Energy Cave

Super-FRS

Figure 1.2: Schematic view of the proposed exotic nuclear beam facility. The super-conducting two-stage fragment separator (Super-FRS) serves the double storage ring system (CR and NESR) including an intersecting electron ring (eA-Collider), and areas for high- and low-energy experiments.

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• A double-ring system for accumulation, storage and cooling of the large-emittance fragment beams. Its design aims at maximum collection efficiency and thus will remove a major difficulty at the present Experimental Storage Ring (ESR). Both fast stochastic pre-cooling in the Collector Ring (CR) and subsequent electron cooling in the New Experimental Storage Ring (NESR) will be applied. In conjunction with internal targets, scattering experiments on light probes such as on hydrogen or helium atoms become feasible. It is even conceivable to intercept one of the ion rings with a small electron storage ring (eA-Collider) and to detect electrons scattered elastically or inelastically off exotic beams.

Clearly, many of these features represent major technological challenges. This is particularly true for the exotic-beam electron collider and its instrumentation, for the equipment needed for successful in-ring experiments with stored beams, and for the super-conducting large-acceptance fragment separator.

The proposed exotic nuclear beam facility will provide outstanding scientific opportunities with important advances expected in the fields of nuclear structure and reactions, nuclear astrophysics and fundamental-interaction physics. The scientific potential of exotic nuclear beam physics and future options were assessed by european expert groups. A Working Group of NuPECC, an expert committee of the European Science Foundation, provided detailed recommendations [1]. The present proposal meets the criteria imposed therein on a next-generation european in-flight exotic-nuclear-beam facility. The physics program as proposed is highly competitive with that at the most advanced facilities of its kind, in particular with the large-scale facility RIA, currently under consideration in the USA, and the RIKEN exotic-nuclear-beam factory already under construction. In many respects, e.g. those of highest beam energies and of coupled storage rings, the facility at GSI provides unique, unsurpassed experimental possibilities.

This chapter of the Conceptual Design Report is organized in the following manner: the Introduction provides the main arguments for the present proposal of a next-generation radioactive-beam facility and its overall concept. In Chapter 1.1, the scientific opportunities offered by such a facility are discussed as well as the principal experimental methods to be used to reach the physics goals. The second Chapter, 1.2, provides design studies and projected performance parameters of the major installations and the experimental equipment. These include the fragment separator, the spectrometers linked to the storage-cooler rings, and the detector systems required for external-target experiments.

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1.1 Scientific Case

1.1.1 Introduction

Nuclei far from stability

Nuclei form the core of atoms, the building blocks of the chemical elements. Protons and neutrons are the constituents of the nucleus. In the charge-neutral atom, the nuclear charge carried by the protons is compensated by that of the electrons. The number of electrons determines the position of the atom in the Periodic Table of Elements and thus its chemical properties. Likewise, stability of nuclear matter is governed by proton-neutron symmetry. The matter surrounding us is stable against nuclear decay, a prerequisite for life on our planet. Stability of nuclei requires a well-balanced composition of protons and neutrons, not necessarily typical for the evolution of matter in the universe. There, nuclei and chemical elements are created in massive stars and supernovae along pathways of nucleosynthesis that often proceed through regions far from nuclear stability.

The challenge in nuclear physics is to understand the structure of nuclei on the basis of the interactions between their constituents. The nucleus is a rather unique many-body system. It is governed by the strong force, forming an intrinsically highly correlated, non-perturbative system. In fact, its mean field and central potential builds from the interactions among all the constituents, a situation which is very different from that in the atom. The atom is built by the long-range electromagnetic force in a central potential predetermined from the charged (and heavy) nucleus and , in consequence, electrons in atoms reveal little correlation (except for trivial charge screening). Atomic nuclei range from few-body systems up to the region of superheavy elements consisting of a few hundreds of nucleons, thus providing an ideal terrain for theoretical many-body methods in a regime where perturbative approaches fail. A consistent theoretical framework that covers the span from few-body correlations in light nuclei up to the many-body features in heavy nuclei is a major challenge.

Nuclear structure research is a well-established research field, already more than 50 years. In the past, however, nuclear research was constrained essentially to a narrow band of nuclei around those of natural occurrence, which have a long lifetime compared to the age of our solar system. Nuclei with a different composition in proton and neutron number than those found on earth may still be ‘bound’, i.e. they do not decay by prompt particle emission but only due to the weak interaction with a certain life span. Their existence, however, requires that their proton-to-neutron ratios stay within certain limits, named the ‘drip lines’. In total, one expects several thousands of such bound nuclei within the drip lines, to be compared to less than 300 nuclides of natural abundance on earth. The discovery of proton decay from the ground state of 151Lu was the first direct proof for having crossed the proton drip line. Recently two nuclides at this borderline, the isotope 48Ni and the heaviest self-conjugate nucleus 100Sn, have been identified. Both are doubly-magic nuclei with filled proton and neutron shells, yet their detailed structure remains to be investigated. On the neutron-rich side, the drip line has been reached only in case of the lightest elements.

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The region of heavier neutron-rich nuclei still forms a wide "terra incognita", just entered with the synthesis of the doubly magic 78Ni as a first landmark. In light neutron-rich nuclei, the observation of unusual nuclear matter distributions, i.e. the formation of neutron halos and skins, has attracted much attention. The disappearance of the N = 8 shell and the appearance of a new N = 16 shell in neutron-rich nuclei as reported recently, is a first confirmation of a conjectured re-organization of nuclear shells in regions far away from stability.

A third borderline of nuclear matter limits the number of the chemical elements. Near uranium, due to the increasing number of protons, the repulsive Coulomb force gradually starts to overcome the nuclear binding forces, the nuclei disintegrate by fission. Beyond this macroscopic fission limit, a new species, the superheavy nuclei are expected to exist, owing their enhanced stability to shell effects. The exploration of the region beyond rutherfordium, Z = 104, gave the first direct proof that the table of elements is extended due to the influence of nuclear shell effects. Element Z = 112 is the heaviest element discovered at GSI. Evidence for elements Z = 114 and Z = 116 has been reported recently by a group in Dubna. Yet, the region of highest stability of the superheavy elements, i.e. the spherical shell closure, is still awaiting discovery.

The installation of exotic nuclear beam facilities during the past decade, allowing in particular unstable nuclei to be used in nuclear reactions, initiated a new and fast growing field of great scientific potential. The discovery of exciting new phenomena, as briefly addressed above, has triggered the presently extraordinarily active field of research with exotic nuclei at a considerable number of competing exotic-nuclear-beam facilities. The promising perspectives of this field have been recognized worldwide. The construction of different types of radioactive beam facilities all over the world, including projects of large-scale advanced facilities, reflect the strong scientific interest in the physics that can be addressed with beams of exotic nuclei. The key issues of the physics program at such facilities were outlined in numerous reports and can be summarized as follows.

Exotic neutron-rich nuclei. At present, the neutron drip line has only been reached up to Z = 8, the proposed facility at GSI will give access up to about Z = 25. In heavier nuclei, though the drip line cannot be reached, nuclei of extreme isospin, i.e. of large neutron-proton ratio can be produced. The proton-neutron asymmetry leads to weak binding. Both effects together bring about a significant modification of nuclear matter properties in comparison with stable nuclei. As already observed in light nuclei, neutron-rich nuclei are likely to exhibit extended surface zones of neutron-enriched, low-density matter. The study of these neutron skins or halos determines the effective interactions in such nuclear environments and thus may help to investigate the equation-of-state of cold neutron matter between saturation and low density. Among the loosely bound valence nucleons, residual interactions such as pairing become enhanced and may produce cluster and molecular-type structures. Proton and neutron matter distributions can be studied by electron and light hadron scattering utilizing the storage rings of the proposed facility.

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Shells and shapes. There are indications that the shell structure changes as one moves towards very neutron-rich nuclei. A detailed understanding of how the shell reorganisation evolves with isospin is definitely needed. Quenching of the known shell gaps and the appearance of new ones goes hand in hand with the evolution of nuclear shapes and deformations. The new facility at GSI will permit detailed studies of nuclear shell effects and nuclear shapes. Extended, yet unexplored regions of neutron rich nuclei, e.g. below lead, will be accessible by uranium fission and fragmentation. The combination of direct mass measurements in a storage ring to explore the nuclear mass surface and detailed spectroscopy of particularly interesting regions with the new generation of efficient detector arrays for particle and γ−spectroscopy will lead to a new and exciting research program.

Proton drip line and N = Z nuclei. Nuclei near the proton drip line could be studied in the past by using reactions between stable beams and targets. Neutron-deficient nuclei are thus much better explored than neutron-rich ones. The heaviest self-conjungate nucleus 100Sn was observed at exotic-nuclear-beam facilities, GSI and GANIL. Nuclei at or near the symmetry line N = Z are of particular interest in many respects. They are the micro-laboratory for high-precision tests of the Standard Model and they are found along the astrophysical rp-process. Nuclear physics interests focus on isospin-symmetry breaking effects in heavy nuclei or on the open question of a proton-neutron pairing phase.

Fundamental symmetries and interactions. Specific exotic nuclei, e.g. N = Z nuclei, allow probing particular aspects of the electro-weak Standard Model at the highest precision. For that purpose, the nuclei need to be confined in storage rings or traps. Violations of fundamental symmetries, such as parity or time-reversal, link the nuclear structure and atomic physics program. The opportunities at the proposed facility in this field will be complementary to those at ISOL facilities in regions of short-lived nuclei and isotopes of refractory elements.

Nuclear astrophysics. Nuclear astrophysics, in general, will strongly profit from the integrated information deduced from nuclear structure data measured for exotic nuclei. Exotic-nuclear-beam facilities can directly contribute, for example, to a better understanding of the nucleosynthesis processes creating the heavy nuclei. Heavy nuclei are produced mostly in explosive stellar events by rapid proton or neutron capture reactions occurring along chains of unstable nuclei. Nuclear masses, weak decay rates, or capture and photo-disintegration cross sections are requested. The main goal, however, is to establish the general nuclear structure trends evolving in nuclei far from stability, e.g. effects such as shell quenching or enhanced capture rates, and thus to place theoretical concepts in nuclear astrophysics on solid grounds as far as exotic nuclei are concerned.

Concepts for exotic-nuclear-beam production

Due to the large research potential of nuclear structure investigations of exotic nuclei and of reactions with them, research programs have been started at a number of laboratories in Europe, including GSI, in North-America and in Japan. The

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tremendous success of these facilities already in a first generation of experiments initiated new large-scale second-generation projects in the USA, in Japan, and in Europe. On an international level, the OECD Megascience Forum developed perspectives for nuclear structure research with radioactive beams [2]. In order to evaluate the prospects for Europe, a working group was installed by the Nuclear Physics European Collaboration Committee (NuPECC), which has recently published a report [1] on this topic.

The next-generation facilities will be based on one of the three principal schemes for exotic-nuclear-beam production:

• The In-flight method employs fragmentation or fission of energetic heavy-ion projectiles followed by in-flight separation of the produced fragments. Typical energies of secondary beams presently range from 30 MeV/u to 1 GeV/u. This scheme, which is being used successfully at several laboratories, including GSI, forms the basis of the present proposal. It has also been adopted for the RIKEN project in Japan which is already in its first phase of implementation.

• The Isotope Separation On-Line (ISOL) approach utilizes reactions induced by intense light ion beams of energies up to about 1 GeV/u or induced by neutrons. The radioactive nuclei are created at rest and then released from a thick target into an ion source system and eventually post-accelerated to energies in the range of 10 MeV/u. The european EURISOL study group [3] is presently exploring the capabilities of the ISOL technique. In Germany, the MAFF project at the Munich research reactor FRM 2 aims at the production and post-acceleration of medium-mass neutron-rich nuclei obtained from neutron-induced fission [4].

• A new hybrid concept was proposed for the RIA project [5] in the USA. In this concept, the exotic species are produced by projectile fragmentation or projectile fission and separated in-flight. They then are stopped in a gas cell, extracted and re-accelerated to energies in the range of tens of MeV/u. Although the feasibility of specific aspects of the concept has already been demonstrated, a complete technical realization needs further research and development work.

In-flight and ISOL facilities are complementary in many respects. As far as secondary beam intensities are concerned, the ISOL method appears to be superior for isotopes of selected elements, i.e. those that are quickly and efficiently released from the target-ion source system. In-flight separation, on the other hand, is independent of chemical properties, and thus yields high secondary beam intensities for all elements. The ISOL method is limited by the release efficiency of the target ion-source system, and is thus in many cases restricted to nuclides with half-lives of seconds or longer. In contrast, the in-flight method gives access to very short-lived species with half-lives down to the sub-microsecond region. The hybrid scheme aims at circumventing the problem of the limited release efficiency of ISOL. The schemes based on re-acceleration of exotic species typically cover energies around the Coulomb barrier up to a few tens of MeV/u for economical reasons. In-flight facilities, in contrast provide secondary beams at energies of several tens of MeV/u up to GeV/u.

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Advantages of in-flight separation

In view of the advantages and drawbacks of each of the three concepts mentioned above, it is not surprising that all three schemes are currently considered for next-generation facilities. This Conceptual Design Report suggests that the european next-generation in-flight facility should be built at GSI. The main advantages of the in-flight separation method can be summarized as follows:

• in-flight separation is chemistry-independent and thus provides secondary beams of all elements;

• separation and transport to experimental devices is limited only by the time of flight through the separator (less than a microsecond) and therefore allows a study of even very short-lived species;

• the concept allows both mono-isotopic beams or mixed beams of isotopes with similar A/Z;

• with a synchrotron as the driver accelerator, quasi-continuous secondary beams or intense short-pulsed beams for injection into storage rings can be provided;

• high energies allow the use of thick secondary-reaction targets and thus high luminosities even for rare exotic species.

During the past decade, GSI has successfully operated the only high-energy in-flight separation facility in the world, consisting of the SIS18 synchrotron as the driver accelerator and the FRS as the projectile-fragment separator. The secondary beams can also be injected into the storage-cooler ring ESR, again a worldwide unique experimental device. The proposed project promises a new quality in the field of exotic nuclear beams. The major goals are:

• a new synchrotron providing maximum beam intensities above 1012 ions/s for all elements up to uranium,

• a high-acceptance fragment separator allows projectile-fission fragments to be separated with efficiencies near 50 % which can presently be achieved only for fragmentation products,

• a new collector ring optimizing the transfer of large-emittance secondary beams to an improved storage-cooler ring,

• novel experimental tools are to be added to those presently used, such as an energy-buncher stage for stopping the separated fast exotic beams and an electron-ion collider that allows experiments that were up to now feasible only for stable species.

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In addition to the next-generation high-energy facility outlined above, GSI will make efforts to maintain its world-leading position in the field of studying superheavy elements produced in low-energy fusion reactions.

The next-generation exotic nuclear-beam-facility at GSI

The key instrument of the exotic-nuclear-beam facility will be a multi-stage Superconducting projectile FRagment Separator (Super-FRS) with large acceptance, see Figure 1.2. It is optimized in particular with respect to an efficient separation of fission fragments from a primary uranium beam which are characterized by a large phase-space volume. The Super-FRS has a large momentum acceptance of ± 2.5% and an angular acceptance of ± 40 (horizontal) and ± 20 (vertical) mrad. A new separator concept with two independent separator stages (pre- and main separator) is necessary to cope with the high count rates and thus to provide an efficient background suppression. About 30%-60% of the produced fission fragments will be transported as spatially separated isotopic beams. The Super-FRS feeds three experimental branches,

• a low-energy branch for decay studies and trapping of ions,

• a high-energy branch for reaction studies,

• a branch into a two-ring system for experiments with stored and cooled exotic nuclei.

The low-energy branch of the Super-FRS will be equipped with an energy-bunching spectrometer and will provide low-energy beams of in-flight separated exotic nuclei with a small energy spread, thus allowing in-beam spectroscopy, decay studies after implantation into detector arrays or, alternatively, efficient stopping in a gas cell. This branch will open up a broad and completely new field of experiments with short-lived isotopes (T1/2 > 100 ns). Furthermore, the whole spectrum of instrumentation and experimental techniques, which have been developed at ISOL facilities, such as Penning traps, Magneto-Optical Traps, or laser spectroscopy can be applied, favourably to short-lived species (T1/2 > 1 ms) and refractory elements, both of which are difficult to access by the ISOL technique.

The high-energy branch of the Super-FRS will include a set-up for structure and reaction studies in inverse kinematics. Performing experiments in complete kinematics will allow the multi-dimensional momentum correlations between reaction products including neutrons to be measured. It will be supplemented by a 4π array with tracking capability for high-resolution in-beam γ-ray spectroscopy.

In order to extend the pioneering experiments performed by the present combination of Fragment Separator (FRS) and Experimental Storage Ring (ESR), an upgraded combination of a Collector Ring (CR) and a New Experimental Storage Ring (NESR) is proposed. The ring combination will be optimized for short-lived projectile fragments, its main features are large acceptance and fast cooling and stacking. The NESR will

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be equipped with an electron cooler and an internal target of high density for reaction studies.

A completely new experimental tool with unprecedented possibilities to explore the structure of exotic nuclei will be available with the eA-Collider, an electron-heavy-ion collider where electrons of energies of several 100 MeV interact with the stored and cooled exotic nuclei.

Competitiveness

Worldwide, high scientific priority is assigned to large-scale exotic-nuclear-beam facilities. The two most outstanding projects are RIA, currently under consideration in the USA, and the RIKEN exotic-nuclear-beam factory which has already entered its first stage of construction. The project proposed here is competitive to those in many respects. The intensities of the exotic beams from in-flight fragmentation are comparable for all three projects. The facility at GSI, however, delivers the highest beam energies of 1.5 GeV/u (RIA: 400 MeV/u; RIKEN: 350 MeV/u), which ensures clean isotopic separation of the heaviest fragment beams up to uranium. In addition, the beam structure from the synchrotron driver accelerator is most appropriate for injection of secondary beams into storage rings. The double storage-cooler ring system provides optimum acceptance and beam quality. The new facility is placed in the forefront of the current global thrust towards exotic-nuclear-beam facilities.

Synergies

The research and technology development program at GSI has benefited widely from its interdisciplinary character. Synergetic efforts of nuclear physics, atomic physics and biological studies, for instance, led to the success of heavy-ion tumor therapy. Both nuclear physics research and related accelerator technologies contributed to the selection of ion beams most suitable for tumor treatments and to the development of the PET diagnostics. Precise experimental results of atomic physics studies of ion penetration through matter now form the basis for exact dose deposition calculations and thus for reliable treatment planning.

Likewise, questions of astrophysical interest are addressed from very different experimental approaches. Energetic heavy-ion collisions investigate the nuclear equation-of-state at increased nuclear density and temperature, whereas exotic nuclei access the equation-of-state at low density and in cold neutron-enriched nuclear matter. Exotic nuclei also deliver the nuclear input for an understanding of the various nucleosynthesis processes. Atomic physics experiments on highly ionised heavy atoms and studies of heavy-ion beam-induced plasmas of macroscopic dimension elucidate processes of electron-ion and electron-nucleus interactions at stellar sites.

Fundamental interactions and their symmetries are the subjects of the proposed antiproton facility. Exotic nuclei, however, provide a laboratory for very specific tests, e.g. of the weak interaction theory, or to probe symmetry violations at a level of

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extraordinary precision. Antiproton beams can also be used as a tool in nuclear spectroscopy, for example to determine the neutron skin thickness with high accuracy [6]. Antiproton beams may be used as well to produce another species of “exotic nuclei”, the hypernuclei, which contain one or more hyperons. Implanting hyperons into different nuclei allows one to trace the influence of the nuclear medium on strange baryons on one hand, and to study specific (short-range) aspects of the nuclear force on the other hand (see Chapter 2).

Synergetic efforts as outlined above are expected also for the future research activities at the proposed new facility. This interdisciplinary character of the research program, together with advanced experimental concepts and technologies, provides very appealing working conditions for young scientists in an environment of international competition and of worldwide scientific exchange.

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1.1.2 Exotic nuclei

Radioactive beams give access to new territories of nuclei, the exploration of their structural and dynamical properties being the subject of this report. Apart from its intrinsic nuclear interest, the study of exotic nuclei has an impact on scientific activities at other frontiers and concerns, in particular, fundamental interactions among elementary particles as well as many astrophysical aspects. The following subsections elucidate the specific role of radioactive ion beams in three main fields of modern physics related to nucleonic matter, fundamental interactions and nuclear astrophysics.

1.1.2.1 Nucleonic matter

Limits of stability

Nuclei are strongly interacting quantum systems of finite size, which are formed of two different types of constituents, protons and neutrons. Stability of atomic matter is governed by charge symmetry favoring charge-neutral systems. Similarly, stability of nucleonic matter is governed by the proton-neutron (“isospin”) symmetry of the strong force, in competition with the isospin-violating Coulomb force. Nuclei which are stable against β decay are located in a very narrow band of proton-to-neutron ratios, see Figure 1.1. Unbalanced proton (Z) and neutron (N) numbers lead to decreasing stability of the nucleus. If, for example, more and more neutrons are added to a nucleus of given proton number, the binding energy of the last neutron drops steadily and at a certain neutron number the nucleus decays instantaneously by neutron emission, i.e. the nucleus becomes “unbound” and the neutron “drip line” is reached. How the binding energy per nucleon in infinite nuclear matter evolves with proton-neutron asymmetry is depicted in Figure 1.3, pure neutron matter is predicted to be unbound (neutron stars are stabilized by the gravitational force).

Figure 1.3: The nuclear equation of state (binding energy per nucleon Eb vs. nucleon density ρ) for neutron-to-proton ratios ranging from symmetric nuclear matter (N = Z) to pure neutron matter (Z = 0). Nuclei that are stable against β decay are close to symmetric matter. At the neutron drip line, extreme asymmetries up to N/Z ∼ 3 are reached as indicated. Predictions for the equation of state in asymmetric nuclear matter differ considerably from each other. The results displayed are from a relativistic approach which determines in-medium interactions and self-energies in Dirac-Brueckner Hartree-Fock theory and uses a realistic nucleon-nucleon potential [7,8]. ρ (fm-3)

Eb

(MeV

)

stable nucleiN/Z = 1 - 1.5

drip-line nucleiN/Z = 2 - 3

neutronmatter

-20

-10

0

10

20

30

0 0.1 0.2 0.3 0.4

exoticnuclei

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In finite nuclei, the exact location of the limits of existence, i.e. the drip lines, is determined by a complex interplay of various effects connected to the proton-neutron symmetry, the Coulomb force, the evolution of shell structures and specific nucleon-nucleon correlations in the nuclear medium. For neutrons, the drip line is much farther away from the valley of stability than that for protons because of the absence of Coulomb repulsion. Actually, the neutron drip line is known only for light elements up to oxygen and can probably not be reached for heavy nuclei above Z ≈ 25 in laboratory experiments. In contrast, it will be possible to access the proton drip line for almost all elements. The third frontier of nuclear matter existence, that of the superheavy elements, is determined by the increasing probability of fission or α -particle emission with increasing nuclear charge.

Physics with exotic nuclei means, in essence, that one can overcome the restriction to the valley of stability with its narrow range in the proton-to-neutron ratio. For many years, experimental nuclear physics was constrained by the fact that in a nuclear reaction both beam and target nuclei needed to be stable. Production of unstable nuclei was essentially limited to neutron-deficient species. Only with the advent of radioactive ion beam facilities, the door has been opened to further advances in nuclear physics. Radioactive nuclei can now be used in reaction measurements, and exotic nuclei on the neutron-rich side of the nuclear chart become accessible. Less than 300 stable or very long-lived nuclei exist in nature. Several thousands of “exotic” nuclear species can be produced and provided as energetic beams for nuclear structure and reaction studies.

The experimental study of nuclei is not just driven by the attempt to reach out towards the most exotic nuclear species but is motivated by the expectation of observing nuclear properties which are very different from those encountered so far. Large neutron excess and weak binding of valence nucleons, open up new regions of the nuclear medium, thus changing symmetries and leading to unusual excitation modes. The subsequent paragraphs of this chapter aim to illuminate the most attractive new nuclear structure and nuclear dynamics phenomena expected to occur in the yet uncharted regions of the nuclear landscape.

At the drip lines: halo nuclei and unbound nuclear systems

Nuclei of the most extreme N/Z ratios are found at small proton and neutron numbers. Among the bound nuclei, 8He has the largest N/Z value reached so far, N/Z = 3. Still larger values, however, may be obtained in forming resonant nuclear systems beyond the drip lines; 5H [9] and 10He [10, 11] were observed as quasi-bound nuclear states. One may hope to study the strong interaction in almost pure neutron matter. Isospin dependent interactions in finite-size nuclear systems can thus be studied in detail. The most unusual nucleonic configurations are expected to occur in light systems. For example, cluster effects should become prominent; chain- and polymer-like spatial structures are predicted.

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6He He

8

Be14

Li11

17B

1

2

3

4

5

6

2

4 6

8

10

12

Z

Be11

16

N

19C

8 B

14

B19

Four−neutron halo Two−neutron halo

One−neutron haloOne−proton halo

Figure 1.4: Halo nuclei known today

One of the most exciting recent discoveries in nuclear structure physics has been the observation of the halo phenomenon, i.e. that some of the nucleons extend far outside the region of their nuclear core. In the archetype halo nucleus 11Li, the two outermost neutrons occupy a volume almost comparable in size to that of the much heavier nucleus 208Pb. It is interesting to recall that the first evidence for the halo effect was gained from experiments with high-energy secondary beams produced in fragmentation reactions [12]. Since then, numerous studies of reactions induced by intermediate-energy and near-relativistic beams of halo nuclei have been carried out in various laboratories including GSI. A view of the landscape of halo nuclei as known today is shown in Figure 1.4 .

The halo phenomenon is now understood as a typical quantum effect, i.e. very weakly bound valence nucleons penetrate into the classically forbidden region beyond the potential barrier. Low angular-momentum orbits contribute preferentially to the formation of extended halos because of their low centrifugal barrier. Proton halos are suppressed by the Coulomb barrier. Measurements at GSI, nevertheless, identified 8B as a one-proton halo nucleus [13]. The single-particle structure is decisive for the formation of nuclear halos. The two halo neutrons in 11Li, for example, exhibit an almost equal occupancy of the (p1/2)2 and (s1/2)2 configurations, as was found in another GSI experiment [14]. This means that the p-shell closure at N = 8 is no longer existent and that pairing correlations are very strong.

Experimental investigations of halo nuclei and resonances in nucleonic systems beyond the drip lines were restricted so far to the mass region A ≤ 20. It is of great interest to gain access to nuclei confining a larger number of neutrons in a halo. 8He is a nucleus which is known to exhibit a significant core + 4 valence neutron component in its ground state and most probably 14Be can be described in a similar manner. Multi-nucleon halos are predicted in heavier nuclei such as the neutron-rich neon

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isotopes [15]. Such nuclei provide the opportunity to study the interaction of multi-neutron systems in an almost proton-free environment. As the halo neutrons are very weakly bound, they are released in reactions even at moderate energy and momentum transfer. A kinematically complete measurement of their momentum correlations thus allows the exploration of the interactions in neutron droplets, an intriguing new prospect. More generally, resonant systems, unbound but of finite lifetime and located beyond the drip lines, can be formed by using reactions with radioactive beams. Nuclei at the transition from “bound” to “unbound” with the characteristics of “open quantum systems” are very challenging also from a theoretical point of view (see below).

Neutron-rich nuclei

While many neutron-deficient nuclei can be produced in nuclear reactions using stable beam-target combinations, the region of neutron-rich nuclei is largely a “terra incognita”. Therefore, exotic nuclei on the neutron-rich side attract most attention in today’s nuclear physics. The neutron drip line lies much farther away from the valley of stability than the proton drip line, since the repulsive electric force does not act on neutrons. With advanced radioactive beam facilities the neutron drip line will at least be reached for elements up to Z = 25. The study of nuclei with large neutron excess does not, however, focus solely on the location of the drip line but is rather motivated by the unusual nuclear structure features that are expected, and in addition, by astrophysical requirements for nuclear data in this uncharted region (see Chapter 1.1.2.3).

In stable nuclei, the strong proton-neutron attraction keeps the volumes occupied by the two species of nucleons almost identical; proton and neutron matter radii differ only marginally and their chemical potentials differ mainly by Coulomb effects. This is no longer the case in nuclei with large neutron excess. The chemical potentials of protons and neutrons shift away from each other and the surface region is predominantly occupied by neutrons. Predictions, although differing from each other considerably, quote a neutron skin thickness of up to about 1 fm for nuclei within reach of experiments. Figure 1.5 presents neutron skin thicknesses for tin isotopes as calculated in different approaches [16]. Two groups of calculated results appear, and the difference can be traced to different asymmetry parameters of the underlying nuclear equation of state. A skin thickness of a size as predicted would mean that the strongly neutron-enriched outer zone comprises up to about half of the entire nuclear volume. Within this surface zone, the composition of the nuclear medium approaches that of pure neutron matter with an average density significantly below the saturation density in stable nuclei. Evidently, this provides an ideal site for probing both the isospin and density dependence of the effective in-medium interactions. The evolution of neutron skins with increasing neutron excess is expected to be a global trend. The first experimental evidence for such a systematic trend was recently observed for sodium isotopes [17].

Along with the spatial distributions, the momentum structure of the loosely bound valence nucleons differs drastically from that of the nucleons in the core; extended

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spatial distributions imply a larger content of low momenta. Hence the valence and core subsystems tend to decouple from each other. New coherent excitation modes arising from oscillations between the two subsystems may be expected, see below.

Figure 1.5: Difference between neutron and proton radii as obtained from relativistic (NL) and non-relativistic (Skyrme forces Sk, SL) mean-field calculations for tin isotopes. From [16].

Proton drip line and nuclei near the N = Z symmetry line

The proton drip line has already been reached for a large number of elements. With the proposed facility the isotopic chains of nearly all elements, at least up to the element bismuth (Z = 83), can be followed to the proton-decay limit of nuclear stability. Beyond the proton drip line, valence protons are still confined due to the Coulomb and centrifugal barriers. Thus, a nucleus beyond the proton drip line forms a quasi-bound state, from which it decays after a certain lifetime via tunneling of protons through the barrier. Proton radioactivity of a nuclear ground state was first observed at GSI [18].

A peculiar case arises when single proton emission from the ground state is energetically forbidden, but the emission of a pair of protons is allowed. The new radioactive beam facility will allow an extension of the search for direct two-proton emission into regions of higher-Z nuclei, where such a decay mode could have better chances to become observable.

The proton drip line crosses the line of N = Z self conjugate nuclei. The heaviest nucleus known, for which the number of protons and neutrons are equal, is 100Sn. This doubly magic isotope of tin was synthesized at GSI for the first time [19]. Nuclei at the N = Z symmetry line provide an ideal terrain for probing the charge independence of the strong force and to study the symmetry breaking effects, which increase with the nuclear charge Z. Studies of energy spectra of mirror pairs near stability have established this approximate symmetry. Fermi β decay provides the appropriate

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signature and it allows, in some favorable cases, high precision tests of the electro-weak Standard Model (see Chapter 1.1.2.2).

In N = Z nuclei, protons and neutrons occupy orbitals of identical quantum numbers. The large spatial overlap of the proton and neutron wave functions may give rise to ‘super-allowed’ α-decay but also to a new pairing phase in nuclei mediated by proton-neutron Cooper pairs. In contrast to n-n or p-p pairs, which couple only to isospin T = 1, and spin S = 0, p-n pairs may also couple to T = 0 and S = 1. Such a pairing phase may contribute to the binding, the so-called Wigner energy, specifically in N = Z nuclei [20]. T = 0 pairing among many proton-neutron pairs in open-shell N = Z nuclei would represent a new phase of superconductivity, which is unique to atomic nuclei.

Superheavy Nuclei

In addition to the proton and neutron drip lines, a third frontier of the existence of nuclei arises with the maximum charge and mass that a nucleus can reach. The front-line position of research at GSI in this field during the past two decades was based on an extremely successful experimental program resulting in the discovery of six new elements with atomic numbers ranging from Z = 107 to Z = 112 [21]. Recently, a research group at Dubna reported convincing evidence for the production of the elements Z = 114 and 116 [22].

The borderline of stability for the heaviest elements, the superheavy elements, evolves from the competition between the attractive nuclear force and the repulsive Coulomb interaction. Superheavy elements are of interest to two research fields, nuclear structure physics and quantum chemistry. The heaviest nuclei have particularly large volumes and are thus most appropriate for a study of bulk properties of nuclear matter. On the other hand, shell effects are quintessential for the existence of superheavy nuclei. The chemist’s interest is attracted by the fact that the electrons close to the nucleus gain high velocities causing large relativistic effects which may result in deviations from the normal periodicity of chemical properties embodied in the Periodic Table of the Elements. Chemical investigations of the elements seaborgium (Z = 106) [23], bohrium (Z = 107) [24] and hassium (Z = 108) have revealed fascinating insights and opened the door for chemical studies of superheavy elements.

Nuclear and chemical studies of superheavy elements will remain a major domain of research at GSI. In order to pursue the program, the key item is to find nuclear reactions that allow the utmost limits to be reached. As far as reactions with low-energy stable beams are concerned, developments are under way to increase the beam intensity. Intense beams of stable isotopes are the prerequisite to proceed further towards new elements and, in particular, to access the region of the spherical superheavy elements, which are predicted, depending on the theoretical model, for proton numbers Z = 114, 120, or 126. A combination of low-energy, unstable, neutron-rich beams with neutron-rich targets may open the opportunity to produce other, more neutron-rich superheavy nuclei.

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Shell Structure

The energies of the single-particle levels in nuclei appear, at a first glance, rather randomly distributed. Pronounced gaps between specific levels, however, give rise to an enhanced stability of nuclei with certain numbers of neutrons or protons, the “magic” numbers. An exploration of shell effects in the new regions of nuclei to be accessed with advanced radioactive beam facilities is a mandatory prerequisite in clarifying structural and dynamical features of unstable nuclei and reactions involving them. Doubly-magic nuclei and their nearest neighbors are thus corner stones in nuclear structure research. Detailed spectroscopic data on their single particle structure are the key pieces of information which are necessary in deriving the effective interactions used in any type of large-scale microscopic calculation. The main ingredients determining the shell structure are the shape of the central potential, the spin-orbit force, and residual interactions, all of which have to be re-investigated for exotic nuclei under the aspect of the exceptional proton-to-neutron ratio.

There are only five stable isotopes with both neutron and proton magicity, i.e. with N and Z being either 2, 8, 20, 28, 50, 82 or 126. By going to unstable nuclei, this number can be doubled. Only in two cases, for 5628Ni and 13250Sn, substantial spectroscopic information is available. Experimental groups at GSI have succeeded to produce 7828Ni

[25] and 10050Sn [19], the nucleus 4828Ni was recently discovered at GANIL [26]. Detailed nuclear structure information, however, is lacking in all three cases.

One of the burning questions to be answered is if the magic numbers, i.e. the shell closures, are generally valid. Specifically in very neutron-rich nuclei, magic numbers may change, with the possibility of new shell sequences appearing and the known shell closures being quenched or even dissolved, see the illustration in Figure 1.6.

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SHELL QUENCHING

20 24 28 32 36 40 44 48 52

8

16

24

N=54

N=52N=50

N=48

Sn100

Ni78

Z

S [ ]MeV2n (a)

32 40 48 56 64 72

6

12

18

N=86

N=84

N=82

N=80

Sn132

Z

S [ ]MeV2n (b)

’’WS’’ ’’HO’’

(c)

50 40

82 70N/Z

126 112

1g9/2

1g7/2

2d5/2

3s1/2

1h11/2

2d3/2

2f7/2

3p3/2

1h9/2

3p1/2

2f5/2

1i13/2

1g9/2

2d5/2

3s1/2

2d3/2

1g7/2

1h11/2

2f7/2

3p3/2

3p1/2

2f5/2

1h9/2(d)

Figure 1.6: With increasing N/Z value and diffuseness of the neutron distribution, the single-particle structure is rearranged. Mean field calculations predict that certain neutron shell gaps ''melt'' with increasing N/Z. This is seen from panels (a,b) showing the Z dependence of the two-neutron separation energies S2n (which serve as a measure for the shell gap) along the N = 50 and 82 magic numbers [27]. The effect is dramatic, e.g. the gap is reduced from 6 MeV for 100Sn to 2 MeV for 78Ni (panel a). It can be traced back to the neutron potential, which follows the neutron distribution, and changes from the Woods-Saxon shape (WS) to the softer harmonic oscillator type (HO), see panel (c). The spin-orbit splitting, which is proportional to the gradient of the potential, is reduced, and the high-l intruder orbitals (g9/2,h11/2,i13/2) move back across the shell gap. This is shown schematically in the single-particle level scheme of panel (d).

Collective modes

Re-arrangements in the single-particle level sequences may lead to a decrease of the usual shell gaps and the opening of new ones, which in turn would be accompanied by the appearance of nuclear deformations in unexpected regions. 32Mg with the magic neutron number N=20 was found in radioactive beam experiments at RIKEN to have

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a rather large quadrupole deformation [28]. In stable nuclei, the deformations of the proton and neutron matter distributions are of similar magnitude. An attractive feature in very proton-neutron asymmetric nuclei is the prediction of rather different deformations for protons and neutrons. Nuclear deformations give rise to collective states of low excitation energy, and characteristic vibrational or rotational band structures up to high spins should be observed.

At higher excitation energies, the giant resonances are found. Giant resonances are basic excitation modes observed with rather regular patterns in stable nuclei. They are understood as coherent states of vibrational type involving a large number of nucleons inside a nucleus. Giant resonances primarily probe bulk properties of nuclear matter. For instance, the giant monopole resonance has delivered the most precise information on the compressibility of nuclear matter, while the properties of the giant dipole resonance are mainly determined by the nuclear symmetry energy.

A large proton-neutron asymmetry in a nucleus has a significant influence on such collective motions. Their spectral distributions are expected to be much more fragmented than in stable nuclei because the particle-hole energies for protons and neutrons differ considerably from each other. In nuclei with large proton or neutron excess, a distinct separation into isoscalar (protons and neutrons vibrate in phase) and isovector (protons and neutrons vibrate out of phase) modes is no longer adequate. This effect is related to the evolution of skins or halos. The nucleons forming the skin are much less bound than the ones remaining in the core. A new type of vibration may evolve, i.e. a vibration of skin nucleons against core nucleons. Clear evidence for such “soft” collective resonances has not yet been obtained. Low-lying dipole strength exhausting a considerable fraction of the total strength, however, has been found in halo nuclei. This dipole strength which appears just above the neutron separation thresholds, is most likely to be assigned to single-particle transitions which are enhanced due to the strong spatial overlap of the asymptotic part of the valence neutron wavefunction with neutron continuum states.

Recently, a measurement at GSI has shown that low-lying dipole strength evolves systematically with the neutron excess in nuclei which do not exhibit a halo; the results for unstable oxygen isotopes [29] are shown in Figure 1.7. If low-lying dipole strength appears systematically in proton-neutron asymmetric nuclei, the corresponding large photo-neutron cross sections near the neutron separation threshold could have a significant influence on the path of the r-process, see Chapter 1.1.2.3 for further discussion. Giant resonances of other multipolarities are of equal interest. The properties of the giant monopole resonance in exotic nuclei, for instance, would allow the determination of the way in which the nuclear compression modulus evolves with increasing neutron excess. This is important information with regard to the equation of state of neutron matter. Magnetic giant resonances in exotic nuclei are equally important to test nuclear models and, in addition, their knowledge is required in astrophysical model calculations; the latter aspect is discussed in Chapter 1.1.2.3.

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σ (γ,x

n) (

mb)

16O

(γ,p)

σ (γ,x

n) (

mb)

20O

(γ,p)

E (MeV)

σ (γ,x

n) (

mb)

22O

(γ,p)

0

10

0

10

0

10

10 20

Figure 1.7: Photo-neutron cross sections measured for the unstable oxygen isotopes 20,22O in comparison to that of the stable isotope 16O. The blue curves show the result of a large-scale shell-model calculation [30]. The (γ,p) thresholds are indicated by arrows. Adapted from [29].

Large-amplitude motion

To further understand the dynamical properties of the two-component nuclear matter, large-amplitude motions can be studied. Nuclear fission is best suited to determine the dynamical properties of cold and moderately excited nuclear matter. At low excitation energies, the onset of dissipation and the influence of quantal effects on large-amplitude collective motion can be studied. Nuclear fission is also a unique tool to explore shell effects at extreme deformations. Fission probabilities yield rather direct information on nuclear level densities. At higher excitation energies, the value of the nuclear viscosity can be extracted from the fission decay width compared to particle evaporation and from other observables of the fission process. First-generation experiments performed at GSI have proven that the use of secondary beams greatly enlarges the prospects of this field. So far, 70 short-lived neutron-deficient nuclei above lead have become available for fission studies, and more than 100 will be accessible in the future.

In addition to its significance for basic research, the general understanding of nuclear fission has become an essential topic in several applications. The characteristics of fission in a wide regime of excitation energies and for a large variety of nuclei are important for the lay-out of spallation neutron sources and of accelerator-driven

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systems designed to incinerate nuclear waste or to produce energy on the basis of 232Th fuel. Spallation or heavy-ion fragmentation reactions followed by fission of the primary reaction products have also proven to be important mechanisms for the production of secondary nuclear beams, e.g. in the proposed facility.

The process of multifragmentation allows the investigation of nuclear matter at low densities and finite temperatures. Calculations in infinite nuclear matter predict a phase transition of first order of the liquid-gas type to occur at such conditions. The multifragmentation phenomenon observed in heavy-ion collisions is understood as a signal of such a phase transition in a finite system, when nuclear matter in the expansion phase enters the region of spinodal instability. However, nuclear matter is a two-component system consisting of neutrons and protons. Such binary systems have more complicated phase diagrams. A qualitative new feature in the liquid-gas phase transition is expected, the onset of chemical instabilities that will show up in a novel nature of the unstable modes, the mixture of density and charge fluctuations leading to an “isospin distillation” effect. Indeed, equilibrium thermodynamics as well as non-equilibrium kinetic considerations predict that an asymmetric system will separate into more symmetric larger fragments ("liquid") and into neutron-rich light fragments ("gas"). Correspondingly, the chemical instability can be investigated experimentally just by measuring the N/Z ratio, or isospin content, of the fragments. The effects mentioned above are driven by the isospin dependent (isovector) part of the nuclear equation-of-state. These interactions are not well known away from saturation density, but their density dependence is very important for reliably calculating, e.g., the structure of the neutron rich, dilute halos of exotic nuclei, or of the structure of neutron stars, as an extreme example. Since the process of fragmentation occurs at low density, information on the isovector forces in this regime will be obtained from multifragmentation studies. The study of collective flow, on the other hand, will provide information for higher densities. To study these effects it is desirable to use heavy-ion collision systems with the widest possible range of N/Z ratios. Therefore heavy-ion collisions with radioactive, in particular neutron-rich beams will strongly increase the sensitivity to these effects and will allow the exploration of a hitherto unknown, important aspect of nuclear matter.

Theoretical concepts

Nuclear structure experiments and the development of theoretical concepts are strongly interrelated and have to progress in a coordinated effort. Theoretical questions have already been touched upon in the previous sections, here the aim is to review new theoretical concepts and methods which will become important for the investigation of exotic nuclei.

The nucleus is a complex many-body system whose quantum spectrum cannot be calculated exactly in most cases. On the basis of the underlying nucleon-nucleon interaction exact solutions can only be obtained for ground states and low-lying levels of the lightest nuclei. Otherwise approximate many-body methods have to be used,

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which were developed extensively in the past and will also be used for exotic nuclei, however, with characteristic differences.

“Ab initio” calculations using modern nucleon-nucleon interactions and exact few-body methods are now possible in light systems up to masses of about 10. They are of great interest since these light systems are among the most exotic ones concerning the proton-to-neutron ratio. On the other hand, light exotic nuclei can often be treated as approximate few-body systems consisting of an inert core and a few halo neutrons. Of particular interest are so-called Borromean systems of a core plus two neutrons, where none of the binary subsystems is bound, and which therefore display strong 3- or few-body effects. In such systems the neutron-core interactions are needed, which often involve unstable nuclei.

For heavier nuclei, many-body methods such as Hartree-Fock, Hartree-Fock-Bogoliubov, RPA, or the shell model, can be used effectively. These techniques are based on the concept of a mean field, in which the particles or quasiparticles move nearly independently and interact via weak residual forces. However, the residual interactions depend on the mean field used and on the model space. There exist theoretical methods to calculate effective interactions from first principles (such as Brueckner theory or renormalization group techniques), but also many empirical parameterizations have been developed. In the past, these methods have mostly focused on nuclear matter or stable nuclei, i.e. around saturation density and for small proton-neutron asymmetry. For the description of exotic nuclei it is essential to determine residual interactions in new regions of nucleonic matter, i.e. for low density and large asymmetries. Such effective interactions have to be developed in close contact between microscopic calculations and detailed comparisons with data of actual nuclei far from stability.

In stable medium-heavy and heavy nuclei it was found that the mean field dominates the quantum spectrum and that the effects of the residual interactions introduce only moderate changes from the independent (quasi)particle picture. For example, the importance of closed shells, i.e. magic numbers, is due to the dominance of the mean field. This is expected to change in exotic nuclei due to the weakness of the mean field for loosely bound nucleons as compared to the strength of the residual interactions. Borromean nuclei are one example, but also nuclei with many-neutron halos, such as 8He. In heavier nuclei theory predicts that the Fermi edge is smeared out and the single-particle orbitals are either no longer fully occupied or empty. This correlation effect becomes increasingly important as one moves away from the valley of stability and it indicates a new dynamical regime. Non-static interactions arising from collective polarization modes become strongly enhanced and eventually the mean-field description is replaced by correlation dynamics. Thus the quasiparticle picture ceases to be valid and shell structures dissolve. To assess such effects quantitatively, large-scale shell model calculations are of great interest. Here Monte-Carlo methods, which are imperative to handle the necessary large model spaces, have been developed and successfully employed recently. Also, there has been a significant advance in the use of effective field theory in nuclear physics in the last couple of years. One constructs

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the effective relativistic quantum field theory for QCD in terms of hadronic coordinates (QHD) and verifies convergence in terms of the ratios of mean fields and Fermi momentum to nucleon mass. The success of relativistic mean field theory is then understood in terms of density functional theory and Kohn-Sham potentials. In this framework relativistic effects such as spin-orbit interactions or momentum and density dependencies emerge naturally and can be treated consistently. QHD contains within it the nuclear shell model and thus provides the natural basis to extrapolate to doubly-magic nuclei far from stability (which, in particular, allows one to probe and calibrate the isovector effective Lagrangian). Modern QHD theories use density-dependent meson-nucleon vertices, either directly or effectively through non-linear interaction terms. Such density dependencies are also suggested by Brueckner theory, however, the exact form remains open. A particular problem is the density dependence of the isovector interaction, which is poorly known but of great importance for the structure of exotic nuclei as well as for neutron matter. Relativistic approaches provide a foundation for the extrapolation of the spin-orbit interaction to exotic nuclei, which is decisive for their structure since they depend on the surface properties of the nucleus.

A characteristic feature of exotic nuclei is the proximity of the continuum. While in stable nuclei the Fermi edge is several MeV away from the continuum threshold, this energy difference may decrease to a few 100 keV in exotic nuclei. Existing many-body methods will have to be adapted to treat these cases efficiently, e.g. through the discretization of the single-particle continuum or continuum shell-model techniques. Here the continuum-continuum interactions will be of significance. Finally it has to be realized that structure information is predominantly obtained through direct nuclear reactions, such as elastic and inelastic scattering, transfer reactions, breakup, etc. Also here the proximity of the continuum requires the development of new direct-reaction methods such as the Continuum Discretized Coupled Channels model, for example. Effective interactions have to be extended to include the effects of breakup channels. The development of reaction models is also important for the extraction of astrophysical S-factors by indirect methods such as Coulomb dissociation or exclusive breakup reactions.

In conclusion, it is imperative to provide new and improved theoretical tools in order to understand and interpret the emerging wealth of data on exotic nuclei. Important challenges include the development of appropriate effective interactions in regions of low density and large isospin, which are solidly based on the most precise nucleon-nucleon interactions and modern techniques in effective field theory as well as quantitative many-body methods capable of accounting dynamically for the weakening of the mean field, the increased role of non-static, long-range correlations and the proximity of the continuum.

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1.1.2.2 Fundamental Interactions

In the Standard Model (SM) of particle physics, that describes the World by interactions between three families of basic quarks and leptons, many terms in the most general form of the interaction Hamiltonian vanish. Nuclear decay properties are specifically sensitive to some of these terms. The large variety of exotic nuclei enables the most selective and most crucial tests, to be performed which may ultimately reveal physics beyond the Standard Model.

Three types of experiments can be distinguished according to the physics to be addressed:

• An accurate measurement of the vector strength of the interaction, characterized by the coupling constant GV, provides a precise test of the conserved vector current (CVC) hypothesis and the ‘three family’ concept. This can be studied in superallowed 0+ → 0+ β decay of nuclei with N = Z, which are found near the proton drip line.

• The specific form of the interaction used in the Standard Model is of vector-axial vector (V-A) structure, a choice that is not mandatory. Beta-ν correlations give access to scalar and tensor terms in the Hamiltonian, with N = Z nuclei again providing an appropriate laboratory.

• Violations of fundamental symmetries such as parity or time-reversal can be studied in precision experiments at the interface of nuclear and atomic physics as they require polarized samples. For a discussion of experiments concerned with symmetry violations, the reader is referred to the Chapter 2.5.

It is the aim of this section to focus on the role of β decay of exotic nuclei in probing the Standard Model. Special emphasis will be paid on experiments using unpolarized samples, i.e. studies of superallowed 0+ → 0+ β decays and of β+-ν correlations in the superallowed β decay of 32Ar, as well as on the pioneering measurements using polarized neutral atoms by storing them in a laser trap. The related experimental program at the new facility at GSI will be sketched in Chapter 1.1.3.4.

Superallowed 0+ →→→→ 0+ nuclear β−β−β−β−decay

The superallowed 0+ → 0+ nuclear β decay provides a direct measure of the coupling constant GV, and can thus be used to test the conservation of the weak vector current (CVC) and the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix [31]. Neutron decay, on the other hand, depends both on the vector coupling GV and the axial vector coupling constant GA, which introduces a larger uncertainty to the determination of GV, aside from experimental difficulties in neutron measurements.

This type of nuclear β decay occurs from the 0+, T=1 ground state of Tz=-1 even-even nuclei, or from the 0+, T=1 ground state or isomeric state of Tz=0 odd-odd nuclei. It has been accurately studied so far for nine cases, i.e. 10C, 14O (Tz=1), 26mAl, 34Cl, 38mK,

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46V, 50Mn, and 54Co (Tz=0). The test of physics beyond the SM requires measurements of the decay energy, the half-life and the 0+ → 0+ branching ratio to high accuracy, i.e. to a few parts in 10000 for all three quantities. The present status of the corresponding data [32] yields, after applying the calculated nucleus-dependent radiative correction δR and the calculated isospin-breaking Coulomb correction δC, the Ft values displayed in Figure 1.8. The least-squares one-parameter fit, displayed in Figure 1.8, corresponds to an average of Ft = 3072.3 ±0.9 s, which supports the CVC prediction at a level of three parts in 10000. This is indeed one of the most accurate confirmations of a fundamental hypothesis in nuclear physics.

Figure 1.8: Ft values for the nine best studied cases of superallowed 0+ → 0+ nuclear β decay from 10C through 54Co, and the result of the least-squares one-parameter fit. Adapted from [31].

When using the average Ft value to determine the effective vector coupling constant (G′V) and the matrix element for the mixing of up and down quarks (Vud), and to test the unitarity of the first row of the CKM matrix (containing |Vud|2), the systematic uncertainties of the corrections δR and δC have to be taken into account, increasing the uncertainty of the average Ft value from 3×10−4 to 7×10−4. This yields [32] Vud = 0.9740 ±0.0005 and |Vud| 2 + |Vus| 2 + |Vub| 2 = 0.9968 ±0.0014. Thus the CKM unitarity test fails by two standard deviations. As seen from Figure 1.9, this provocative result is consistent with the limits deduced from neutron decay data which, however, for reasons given above are considerably less accurate.

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Figure 1.9: Limits on Vud as derived from 0+→0+ β decay (red area) [32] and from neutron decay (yellow area) [33]. The value of Vud obtained from higher quark generation decays is also shown (blue area) [33] assuming unitarity of the CKM matrix.

The demand for a precision of a few parts in 10000 is most challenging for the case of Q-value or mass measurements: A precision of 10−4 for the Q value of interest corresponds to about 1 keV, i.e. 3×10−9 for a mass measurement of mother and daughter nuclei of mass 30. In order to improve the present data set on superallowed 0+→ 0+ decays and to extend the data to heavier nuclei it is desirable to improve the world-record mass accuracy by one order of magnitude (see Chapter 1.1.3.4), which would allow one to determine accurate mass and Q values all the way to 62Ga. Such an extension of precision decay data to heavier members of the series of Tz = -1 even-even and 0+, T=1, Tz=0 odd-odd nuclei is highly desirable as the δC corrections deduced from model calculations increase to a level of 0.8 - 2.0 % for such heavy nuclei, with a considerable spread between the different calculations. Such new data would thus allow for a crucial test of the correction terms.

Measurement of ββββ+-νννν correlations in the superallowed β− β− β− β− decay

The β-delayed protons emitted after the superallowed decay of 32Ar are influenced by the Doppler effect due to the momentum of the recoil nucleus 32Cl. Thus a Doppler broadening originating in the β+-ν correlation characterizes the energy spectrum of these protons [34]. This information could be used to tighten constraints on scalar contributions to the weak interaction [35]. The results obtained from recent ISOLDE experiments [36] are displayed, together with results form the earlier works, in Figure 1.10.

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Figure 1.10: Limits on the time-reversal-even (upper panel) and the time-reversal-odd (lower panel) couplings. CS and CV are coefficients in the β-decay Hamiltonian for scalar and vector interactions, respectively. The annulus (upper panel) and circles (lower panel) represent 32Ar data, whereas limits from other measurements are also included (see [36] and references given therein).

The measurement discussed above was performed by using unpolarized samples. However, the “ideal” β-asymmetry measurement, outlined above, requires polarization. In recent years, such experiments have been initiated at several laboratories by using magneto-optical traps for storing neutral atoms. As shown by the pilot experiments performed at TRIUMF [37] and Los Alamos [38], this approach offers the unique feature of providing a well-polarized, backing-free and localized sample.

Most of the experiments discussed in this section use ISOL systems for the production of the radioactive nuclear species. The in-flight method, however, offers a promising alternative for meeting the challenges involved in precision measurements. This promise holds for experiments on unpolarized as well as polarized samples. Short-lived isotopes, not accessible to measurements so far, will become available in sufficiently high source strength and source purity. The optimum case seen from nuclear structure arguments, can thus be chosen more freely. This will hence allow a considerable improvement of the tests of the Standard Model.

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1.1.2.3 Nuclear Astrophysics

The origin of the elements

Hydrogen, helium and lithium are the only elements that were created in the earliest stage of cosmic evolution, i.e. within the first 1000 seconds after the ‘Big Bang’. All of the heavier elements which we find today in our solar system and in the Universe have been synthesized in stars many millions of years later, by benefiting from this primordial fuel. The creation of heavy nuclei proceeded through sequences of nuclear reactions in a variety of stellar environments, from long-term quiescent burning to violent explosions of stars. The remnants of these nuclear processes, dispersed into the interstellar medium, subsequently condensed to the next generation of stars. In this manner our solar system was born about five billion years ago. But in the Universe, as known from astronomical observations, nucleosynthesis is still continuing today. Understanding not only the origin but also the fate of the universe requires unraveling the relevant nuclear reaction pathways under the conditions of stellar and explosive environments that still are far from our empirical knowledge.

Figure 1.11: A cosmic lightening never seen before has been recorded by the Hubble Space Telescope in the Large Magellan Cloud on February 2, 2000 [39]. The left panel is interpreted as the shock front of the Supernova 1987A reaching, 13 years after its outbreak, the stellar clouds that had been ejected some thousand years ago from the red and blue supergiant precursors of the Supernova (violet ring). Ionized by the impinging débris of the explosion, the ring has lit up spectacularly. The right panel (“difference”) shows the situation immediately before, where the ring still glows faintly in the fading light of matter that had been ionized about five years earlier by the Supernova light flash. Supernovae like the one shown belong to the favored sites of the r-process nucleosynthesis which has created more than 50% of the heavy nuclei in our solar system. The site of the r-process, however, is still under debate.

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The ambitious aims of nuclear astrophysics are to explain the formation, evolution, and final fate of stars, to reveal the mechanisms of their energy supply, and to understand the stellar sites, the pathways and the time scales of the synthesis of the elements. Nuclear astrophysics is the unprecedented adventure to link the smallest with the largest, the elementary building blocks of matter with the structure of stars, galaxies and the Universe itself.

The success of this endeavor was and is remarkable, ever since its ‘birth’ in a seminal paper some forty years ago [40]. Nuclear fusion has been identified as the power station of the stars, and stellar cauldrons as the birth places of all the elements above lithium. Most subtle details of stellar evolution have been figured out on the basis of a better and better understanding of both, strong and weak interaction processes. Whether a star ends up as a white dwarf, a neutron star or as a black hole, can be rather precisely predicted by now, in terms of its mass on the one hand, and of the quantum-mechanical state of its elementary leptonic and baryonic constituents on the other hand.

Though major progress has been achieved, important pieces of the stellar synthesis of matter are still poorly understood. These are, in particular, explosive nucleosynthesis processes which involve extended chains of unstable nuclei. Examples are the 'r-process' (rapid neutron capture) on the neutron-rich, and the 'rp-process' (rapid proton capture) on the neutron-deficient side of the nuclear chart, see Figure 1.12 for illustration. Relevant stellar sites may be exploding stars such as Novae, X-ray bursts, or core-collapse Supernovae.

Role of radioactive beams

A big step towards the exploration of these fields can be expected in the near future from astronomical observations as well as from experiments in terrestrial laboratories. Many space-based telescopes have now been operating for some years and others will be launched soon. They provide images of cosmic objects such as X-ray bursters, accreting neutron stars, or Supernova and Nova explosions, with excellent brightness and covering almost the full radiation wavelength regime; examples are shown in Figure 1.11 and Figure 1.13.

The future radioactive beam facilities will allow the systematic exploration of nuclear structure, nuclear reactions, and weak decays of unstable exotic nuclei located in yet uncharted areas of the nuclear landscape that are relevant for the explosive stellar events. Cross sections for nuclear reactions involving short-lived nuclei, masses, lifetimes, and decay modes of nuclei far from stability form the most important data. Many of these can be determined from experiments with radioactive beams.

It is neither necessary nor possible to perform such measurements for all unstable nuclei involved in astrophysical scenarios. The central issue of research in nuclear astrophysics is rather the careful exploration of the region around a number of key nuclei that are of crucial importance for the process under consideration. The main goal is to establish the general nuclear structure trends evolving far from stability.

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These might include new effects like the formation of large skins, quenching of shell gaps, enhanced (γ,n) strength and others, as already addressed in Chapter 1.1.2.1. The data delivered by radioactive beam experiments at the new GSI facility, or at other upcoming accelerators, will contribute substantially to solving a number of important, long-standing questions of nuclear astrophysics. A more detailed account on selected subjects is provided below.

The r-process of nucleosynthesis

Above iron, nucleosynthesis proceeds on the neutron-rich side of the valley of β stability via a chain of neutron captures and subsequent -decays towards heavier masses and higher-Z nuclei, as shown in Figure 1.12. From the abundances of the stable nuclides in the solar system it was concluded that (at least) two different stellar processes are responsible for the production of the heavy elements above iron. One of them, the 's-process' (slow neutron capture), taking place in red giant stars on a time scale of many years and at moderate neutron flux, primarily generates nuclei in close proximity to the valley of stability. Besides some details, this process is believed to be understood.

.

Figure 1.12: Beta decay half-lives calculated in the framework of the finite-range droplet model (FRDM), and the most likely pathway of r-process nucleosynthesis (purple line). The inset shows the measured solar abundances of nuclei produced in the r-process; their maxima correspond to the neutron shell closures at N = 50, 82 and 126 [41].

In contrast, the true stellar site of the r-process is still under debate. The latter exploits a huge neutron flux, lasting for only a few seconds, which most probably is

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provided by an explosive stellar event, such as the outburst of a Supernova of type II. The r-process creates a wealth of extremely neutron-rich nuclei by successive fast neutron captures and β decays. The capture processes are slowed down at the 'waiting point' nuclei, where the binding energy of the next neutron to be captured becomes comparatively small. There, the nuclei have to 'wait' for decay to reach a nucleus with higher neutron binding energy. This effect, being most pronounced at the magic neutron numbers N= 82 and 126, leads to strong maxima in the mass abundance (see Figure 1.12) at the corresponding neutron shell closures. The observed solar r-process abundances are not yet understood in detail, in particular around the mass numbers A= 120, 180 and 210. It might be that a quenching of the neutron shell gaps at N= 82 and 126 occurs for nuclei far from stability, an effect which could resolve to some extent the discrepancies between the measured and calculated abundances [42]. At the new facility at GSI intense beams of the heaviest unstable nuclei will be generated and, therefore, nuclear structure questions for the N=82 and 126 isotonic chains can be addressed experimentally.

Detailed network calculations of the r-process suggest – despite its short time scale – an equilibrium between radiative neutron-capture reactions and photodissociation reactions. The data most urgently needed for such network calculations are nuclear masses which govern the r-process pathway itself, β-decay half-lives which determine the accumulated abundance pattern along this path, and β-delayed neutron-emission probabilities which lead to modifications of the pure β decay chains.

If (n,γ) and (γ,n) reactions fall out of equilibrium, details of their strength distributions, in particular near the neutron threshold, could have a significant impact on the r-process path. At least towards the end of the r-process in the ‘freeze-out phase’, where temperatures and neutron densities decline, the counteracting (n,γ)- and (γ,n)-reactions fall out of equilibrium. The nucleosynthesis path then depends on the absolute rates of the two reactions, which in turn are determined by the giant dipole resonance strength distributions.

As discussed in Chapter 1.1.2.1, low-lying dipole strength has already been observed in weakly bound neutron-rich nuclei, and their occurrence may be a systematic effect in proton-neutron asymmetric nuclei, which could effect significantly r-process abundances [43].

Present microscopic nuclear model predictions, in particular for heavy unstable nuclei above the N=82 shell, are forced to extrapolate the effective nuclear interaction, as derived from known nuclei close to the valley of β stability, into a region of large proton-neutron asymmetry. Therefore, even small errors in describing the isospin and density dependence of the effective nucleon-nucleon interaction cause large effects, e.g. in calculating the masses of exotic heavy nuclei. Deviations in these calculations of several MeV occur in comparison with measured data. It has to be emphasized in this context that only some of the r-process nuclei have been reached by experiment so far, namely those located at the N=50 and N=82 shell closures [44].

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The rp-process of nucleosynthesis

Neutron-deficient nuclei close to the proton drip line play an important role in a variety of astrophysical scenarios such as Nova explosions, X-ray bursts, or X-ray pulsars. In those scenarios, hydrogen is burnt via a sequence of rapid proton captures and + decays close to the proton drip line ('rp-process'). Recently, exciting pictures from Nova explosions were delivered by the Hubble Space Telescope (see Figure 1.13) in the optical, and by the Chandra satellite in the X-ray regime. However, the most basic questions concerning the rp-process have not yet been answered. There is well-founded hope that intense radioactive beams, like those from the new GSI facility, will pave the way for the following research fields:

Figure 1.13: Optical picture of Nova Cygni 1992, taken by the Hubble Space Telescope 456 days after the outbreak, showing the afterglow (center) and the expanding ejected material [45].

• Novae are, to our present knowledge, explosions of hydrogen-rich material which has been accreted onto the surface of a white dwarf from the envelope of a companion star. Only a reliable nuclear database for rp-nuclei (see Figure 1.14) allows the derivation of narrow constraints on temperature, density and mass of the exploding white dwarf from the observed rp-abundances in Nova ejecta. Because the ejecta probably contain significant amounts of galactic -ray emitters like 22Na or 26Al, an improved understanding of the rp-process would also lead to a more stringent explanation of galactic -ray data, as they will be collected, e.g., from the planned INTEGRAL mission. This will also allow a much better estimate of the contributions of Novae to the observed galactic abundance of 26Al and to isotopic anomalies in meteorites.

• Low-energy (p, ) reaction rates of neutron-deficient nuclei determine the rp-reaction path to a large extent. By using low-energy radioactive beams it is

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very difficult, if not impossible, to measure these rates directly. However, with high-energy radioactive beams some of the crucial reaction yields were already determined by the Coulomb breakup (photodissociation) method. This approach has been successfully applied at RIKEN, GSI, and MSU in exploring the key reaction 7Be(p, )8B via its ‘inverse’ photodissociation 8B(γ,p)7Be. It was shown that the high-energy beams available at GSI represent a significant advantage compared to medium-energy studies [46]. With the beam intensities foreseen for the new facility, reliable reaction rates for other crucial rp-reactions will come within reach, see Figure 1.14.

• In X-ray bursts and X-ray pulsars the nuclear burning takes place on the surface of an accreting neutron star and the rp-process proceeds very close to the proton drip line. Important open questions are the endpoint of the rp-process in X-ray bursts and the composition of the rp-process ashes. This relates to a variety of observational features not yet understood, like very short burst intervals, very long outbursts, and the time structure of the light curve. The composition of the ashes determines also the crust properties and has, therefore, an impact on open issues like the evolution of magnetic fields, or the thermal structure of accreting neutron stars.

• Another open question is whether the rp-process in X-ray bursts can indeed proceed up to the molybdenum-ruthenium region, as calculations based on theoretical data seem to indicate. This would make X-ray bursts a potential production site of the neutron-deficient molybdenum and ruthenium isotopes in the solar system. The origin of these nuclei is a long-standing and yet unsolved problem in nuclear astrophysics. To address this and many other puzzles, improved data on masses, half-lives, and energy levels of the nuclei along the proton drip line up to the element tin are required.

Such a research program has already been initiated at GSI by using both the ISOL and the in-flight technique. The new radioactive beam facility of GSI, however, will bring all of the relevant rp-nuclei within reach. Since for a thorough understanding of the rp-process also proton separation energies of some nuclei beyond the proton drip line are needed, proton-stripping reactions or +-delayed proton decay of very neutron-deficient nuclei could be used to produce and investigate these proton-unbound nuclei for the first time.

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Data needed for the rp-process: Masses ß-decay rates

Key-reaction rates from: Coulomb breakup (γ,p) transfer 2p-captures

0 12

3 45 6

7 8

9 10

111213

14

1516

17181920

2122

2324

25262728

2930

3132

33343536

3738394041

424344

45464748

49505152

535455

56

5758

5960

6162636465666768697071727374

75767778

79808182

n (0) H (1)

He (2)Li (3)

Be (4) B (5) C (6) N (7)

O (8) F (9)

Ne (10)Na (11)

Mg (12)Al (13)Si (14) P (15)

S (16)Cl (17)

Ar (18) K (19)

Ca (20)Sc (21)

Ti (22)V (23)

Cr (24)Mn (25)

Fe (26)Co (27)

Ni (28)Cu (29)

Zn (30)Ga (31)

Ge (32)As (33)

Se (34)Br (35)Kr (36)Rb (37)

Sr (38)Y (39)

Zr (40)Nb (41)

Mo (42)Tc (43)

Ru (44)Rh (45)Pd (46)Ag (47)

Cd (48)In (49)

Sn (50)Sb (51)

Te (52)I (53)

Xe (54)

rp-process

Previous Coulomb-breakup experiments

Figure 1.14: Probable path of the rp-nucleosynthesis in the outbreak of an X-ray burst. Previously performed Coulomb-breakup experiments are marked by red squares.

Gamow-Teller strength distributions

The high energy and intensity of exotic nuclei to be expected at the new facility would also pave the way towards an exploration of Gamow-Teller (GT) strength distributions via charge exchange reactions in inverse kinematics, such as (p,n), (d,2He), (3He,t) or equivalent reactions. Recently, theoretical efforts based on large-scale shell model calculations have been undertaken to derive the GT strength in fp-shell nuclei [47]. The GT transitions, relevant to the core cooling by electron capture in the late evolution of massive stars, concern nuclei with N ≈ Z near the β-stability line but also moderately neutron rich ones, and involve also reactions starting from thermally excited states.

In spite of a noticeable recent progress in measuring GT strength distributions [48], which includes experiments conducted at GSI [49], a sufficient experimental database to reliably test the corresponding predictions is still lacking. A list of the most crucial nuclei in the vanadium-to-nickel region can be found in [50]. At the new GSI facility these isotopes will be produced with rates beyond 108 ions/s. This opens the unprecedented chance to investigate GT reactions on unstable nuclei in their ground states at the NESR (see Chapter 1.1.3.2 and Figure 1.20) and, moreover, in excited states with lifetimes longer than 100 ns at the high-energy reaction setup (see Chapter 1.1.3.3). The latter case, besides being an experimental challenge, would provide, for well selected cases, a crucial test of the assumptions on the GT resonance of excited states (Brink hypothesis) in stellar weak interaction calculations.

GT strength distributions of neutron-rich nuclei are important for the dynamics of the core collapse of a Supernova, as they are related to the ultimate fate of the Supernova, i.e. whether it ends up as a neutron star or as a black hole. Moreover, these data will

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also help in elucidating electron capture in the deep crust of accreting neutron stars. This will lead to a more reliable prediction of the signal, which that the density discontinuity in the electron-capture layer of a rapidly rotating neutron star like Sco X-1 is expected to induce in future gravitational-wave detectors like LIGO or VIRGO. Electron capture in the crust of neutron stars is also a good candidate for the long-searched for heat source of the quiescent emission in transient X-ray bursters.

Neutrino properties

Solar fusion reactions like 3He(α, )7Be or 7Be(p, )8B have to be known with very good precision in order to reliably predict the neutrino fluxes from the Sun and to confine neutrino-oscillation parameters. As indicated already above in the context of the rp-process, high-energy Coulomb breakup can serve as an alternative approach to derive low-energy fusion cross sections. High-precision measurements require, however, much higher intensities of the corresponding radioactive beams than available to date, since very thin targets (≈mg/cm2) have to be used.

Other significant contributions to neutrino physics could come from careful calibrations of terrestrial neutrino detectors. For instance, the nucleus 205Tl transforms slowly by capturing solar (pp) neutrinos into the first excited state (2.3 keV) of 205Pb. The threshold for this reaction is only 60 keV , which is far below the corresponding 240 keV threshold of the GALLEX experiment. From the abundance ratio of 205Pb/205Tl in deep-lying ore the solar (pp) neutrino flux and the solar luminosity, averaged over the last 10 million years, could be determined, provided that the neutrino-capture probability is known. The latter can be delivered from a measurement of the bound-state β decay of 205Tl at the new CR/NESR storage rings, because this decay probes the same weak matrix element as the (pp) neutrino capture into the first excited state of 205Pb.

Stellar conditions in a terrestrial laboratory

A complete understanding of a specific stellar scenario does not only concern nuclear processes but also needs to take the atomic environment into account. The conditions met in hot stellar plasmas lead to highly ionized atoms. The electronic state of an ion, in turn, may significantly influence weak interaction processes in nuclei. The study of such processes in atoms of high charge states can thus be of vital interest with respect to astrophysical applications.

The field of mass and β half-life determination of highly charged atoms has been pioneered during the past few years at GSI. The success was due to the worldwide unique combination of an in-flight fragment separator (FRS) and a storage-cooler ring (ESR). Two highly efficient and precise methods of mass measurements were developed, namely Schottky Mass Spectrometry and Isochronous Mass Spectrometry, see Chapter 1.1.3.2. Moreover, it was the FRS/ESR combination that allowed for the first time the measurement of nuclear β half-lives of highly charged ions, i.e. under conditions close to those in hot stellar plasmas.

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The first detection of bound-state β decay (see Figure 1.15), the time-mirrored twin of orbital electron capture, was indeed an experimental highlight at the ESR [51]. This unusual β-decay branch, being forbidden for neutral atoms due to the lack of inner-shell electron vacancies, can dramatically shorten nuclear lifetimes of highly charged atoms, as was shown in several experiments conducted at the ESR. Perhaps most striking is the case of the 187Re/187Os couple that serves as a ‘clock’ for the age of our galaxy. In this case, the β half-life of 42 billion years of the neutral atom is reduced to only 33 years, if all electrons of 187Re are stripped [52].

Figure 1.15: First direct observation of bound-state β decay. Bare 206Tl nuclei were produced and separated in the fragment separator FRS, then injected into the ESR, where they were stored and electron-cooled. Displayed is the time evolution of three Schottky lines which correspond, from left to right, to bare 206Tl in an isomeric nuclear state, bare 206Tl in the nuclear ground state, and to the bound-state β-decay daughter of bare 206Tl, i.e. hydrogen-like 206Pb 81+.

Time afterinjection

Revolution frequency (30 harmonic)th

There is the well-founded hope to ‘calibrate’ other important r-cosmochronometers, such as 232Th/238U, at the new GSI facilities. Recently absorption lines of thorium and uranium have been detected in metal-poor halo stars of the probably first star generation in our galaxy [53]. But even from the Th/U abundances of these oldest stars no safe constraints for the age of our galaxy can be derived, as long as the r-process production probabilities of 232Th, 238U, and their precursors are unknown. If one succeeds to settle the r-process path in the region of N=126 and beyond, such crucial numbers could be reliably estimated, allowing one to get trustworthy limits for the age of our galaxy and the Universe that do not depend on the rather intricate time scales of the chemical evolution of the stars. This would be a contribution of utmost importance to the newly revitalized dispute about the cosmological constant and the fate of the Universe.

The expertise gained so far at GSI guarantees that at the new facility, comprising the Super-FRS, the collector ring CR, and the new experimental storage ring NESR, masses and half-lives as well as other spectroscopic properties of key nuclei of the r-process can be measured, in particular at the neutron shell closures N=50, N=82, N=126 and even beyond. Moreover, on the neutron-deficient side all relevant rp-nuclei will come into reach.

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1.1.3 Experimental approach

1.1.3.1 Production and separation of exotic nuclear beams

Key features of high-energy exotic nuclear beams

Secondary beams of exotic nuclei of high energy, above a few hundred MeV/u, generally need synchrotrons as a driver accelerator which inherently leads to lower duty factors compared to CW (continous wave) accelerators like cyclotrons or linear accelerators. Nevertheless, the high energies exhibit the following very attractive features that in many respects more than compensate for the lower primary-beam intensities:

• Strong kinematical forward focussing allows the collection of practically all of the projectile fragments produced. With the help of the proposed large-acceptance separator, this will be true also for fission products characterised by a larger emittance.

• Given the pulsed-beam time structure of a synchrotron, high-energy beams can be efficiently transferred to storage-cooler rings for unique precision experiments.

• The high velocities enable in-flight tracking and event-by-event isotope identification. Ambiguities due to various ionic charge states are largely avoided even for the heaviest elements, and thus allows an efficient utilisation of mixed beams composed of several isotopes and elements in a selected magnetic rigidity window.

• For secondary reactions with fast beams, thick targets can be used that increase the luminosity by factors of up to 1000 compared to re-accelerated secondary beams from ISOL-type facilities.

• High-energies enable the efficient and background-free separation of fragment beams in fragment separators like the FRS, operated successfully at GSI over more than a decade. In many cases, the absence of background is crucial for certain experiments and more than compensates for smaller beam intensities.

• The short flight time through the fragment separator of only a few hundred nanoseconds gives access to very short-lived nuclides. Limitations due to the chemical selectivity or long release times for certain elements, which are unavoidable in ISOL-type facilities, do not exist at in-flight facilities.

Production mechanisms for high-energy exotic nuclear beams

At energies around 1 GeV/u, the reaction mechanism is governed by individual nucleon-nucleon interactions of quasi-free nucleons. It can be well described by the intra-nuclear cascade model, or - in the case of heavy-ion collisions - by the abrasion model. Consequently, the neutron-to-proton ratios of the intermediate products

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arising from a high-energy collision are solely governed by the statistical fluctuations of random neutron or proton removals.

These intermediate products form thermalized excited nuclei with excitation energies that may well exceed a few hundred MeV. This excitation energy is subsequently removed by the emission of nucleons or light clusters. For fissile nuclei, fission may also occur during the evaporation cascade. Long chains of nucleon or cluster evaporation tend to populate a universal ridge of most probable fragmentation products, located on the neutron-deficient side of the valley of beta stability. Fission, on the other hand, leads to neutron-rich medium-mass products with large variations in mass but small fluctuations in neutron excess.

An important production mechanism at high energies is the electromagnetic excitation of a high-Z projectile in the Coulomb field of a high-Z target. Since predominantly the giant dipole resonance (GDR) is populated, its decay leads mostly to few-nucleon (in heavier nuclei mainly few-neutron) removal channels which occur with very large cross sections. In the special case of a fissile projectile, however, the fission decay of the GDR produces the most neutron-rich fragments and thus represents the most promising approach to this class of nuclei.

Quantitative predictions

For quantitative predictions of production rates, the above-mentioned reaction mechanisms have been modelled in terms of two-step (abrasion plus evaporation) codes (ABRABLA [54]), with a careful description of fission included [55]. Alternatively, fragmentation cross sections of non-fissile projectiles have been fitted to an empirical formula, EPAX [56]. Both descriptions were extensively compared to the large data base on production cross sections, illustrated in Figure 1.16. Most of them were measured at the FRS during the first decade of its operation (see e.g. [25, 57, 58, 59]). The experimental data can reliably be reproduced, at least within a factor of two by the codes mentioned above.

The rate estimates were obtained by (i) considering all primordial nuclides up to 238U as primary beams, (ii) assuming their intensities to range from 1×1012 per second for 238U to 3×1012 per second for 20Ne, and (iii) using carbon throughout as the production target, with the optimum target thickness chosen (see Chapter 1.2.1.4). The cross sections were calculated with the EPAX formula for all beams up to 209Bi, whereas ABRABLA was used to calculate the production of projectile fragments and fission residues from 238U. In view of the limited accuracy of the cross section predictions, losses due to the finite acceptance of the Super-FRS (50%-100%), the transmission through thick profiled degraders (≥ 50%) and the slowing down of the fast fragment beams for special experiments (yield ≥ 50%) were neglected.

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Figure 1.16: Illustration of the characteristics of the fragment production in relativistic heavy-ion collisions. The clusters in the rectangular box represent fragment production cross sections measured for the reaction 238U + 208Pb at 1 GeV/u. For Z < 28 and for Z > 75, the data are extended by calculations with the ABRABLA and EPAX codes, respectively. The insets show production cross sections obtained for other reactions in specific experiments, exploring the production far from beta stability in asymmetric fission (neutron-rich nickel isotopes [25]), in hot (tin isotopes [57]) and cold (N=118 isotones [59]) fragmentation.

The resulting production rates in units of ions per second are depicted in Figure 1.17 in form of a chart of the nuclides. Apart from high rates up to the order of present day primary beam intensities for most of the presently known isotopes, a large number of nuclides beyond the present limits will become accessible. The most important progress can be expected in the neutron-rich region above the fission fragments, where the N=126 and Z=82 shells can be followed over long chains of isotones and isotopes, respectively. In particular, the r-process path in the vicinity of the N=126 shell comes within reach.

Isotope separation with the Super-FRS

As evident from the introductory remarks above, the quality of a radioactive-beam facility hinges on several key parameters, among which the secondary beam intensities represent only one aspect. Other key parameters are the purity of the

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separated secondary beam after the fragment separator and the instrumentation that allows its fast and efficient tracking event by event.

Figure 1.17: Predicted productions rates (for nuclei with half-lives larger than 100 ns) at the proposed facility. Primordial nuclei, closed shells, and the limits of known nuclei are indicated in black.

The FRS [60] was designed to provide pure secondary beams after high-energy projectile fragmentation of SIS18 beams. The high energies are useful in two aspects: on one hand, they allow the use of very thick targets and energy degraders with areal densities of the order of several g/cm2. The latter, together with a large ion-optical resolution, are indispensable for good isotopic separation. On the other hand, the contamination of a mono-isotopic secondary beam by ionic charge states of neighbouring isotopes can be minimised, even for high-Z fragments. The power of this concept was proven with several remarkable discoveries of extremely rare isotopes, e.g. of 78Ni with about 1 event per 45 hours [25] and of 100Sn with about the same rate, where even correlated β decays over up to three generations were observed [61].

The present FRS scheme has several limitations that could be overcome with a next-generation Super-FRS. The main limitation concerns the transmission of fission fragments after projectile fission of 238U. The intrinsic widths of the fission kinematics are considerably larger than the FRS momentum acceptance of ±1% and its angular acceptance of ±13 mrad. Therefore, only 3-6% of the fission products are transmitted and separated in the FRS. The proposed Super-FRS aims at an order-of-magnitude increase in fission product transmission by an increase in momentum acceptance to

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±2.5% and an increase in horizontal (vertical) angular acceptance to ±40 (±20) mrad, while at the same time the high ion-optical resolution will be maintained.

Another drawback of the present FRS layout is the availability of only two separation stages. This means that at the intermediate dispersive focus, where beam tracking detectors and/or secondary targets have to be placed, the spatial separation occurs only according to magnetic rigidity in the first stage. This leads to very high count rates in the tracking detectors at the central focal plane and to high background due to secondary interactions in the energy degrader. The improved concept of the Super-FRS (see Figure 1.2 for a schematic illustration) foresees two independent separators. The first separator is an achromatic pre-separator with an intermediate energy degrader, which can work without any tracking detectors, but removes the bulk of the unwanted fragments. The second separator is designed like the current FRS, i.e. it consists of four dipole stages and has a dispersive image at the middle and an achromatic one at the final focus. Details of the ion-optical and technical design will be given in Chapter 1.2.1.

Comparison to competing projects

A key statement of the NuPECC report on Radioactive Nuclear Beam Facilities [1] was that the two basic methods to produce radioactive nuclear beams, the ISOL and the in-flight methods, are highly complementary, and that both methods should be pursued. Therefore, the advantages and disadvantages of a high-energy in-flight facility like the one proposed here will not be compared to current or future ISOL-type installations. Competing projects to be considered are medium-energy upgraded facilities like the RIKEN BigRIPS or the Rare-Isotope-Separator project RIA. In both cases, in-flight isotope separation is combined with high-intensity CW beams from a super-conducting cyclotron and linear accelerator, respectively. Owing to this choice of driver accelerators, the maximum energy is limited to 350-400 MeV/u for heavy projectiles. Due to their higher duty factor, CW machines offer primary-beam intensities superior to those from a synchrotron by roughly one order of magnitude. As demonstrated in Figure 1.18, however, even the ambitious RIA project would provide only moderately higher secondary-beam intensities. Even if one disregards the other advantages listed above, a large part of this intensity deficit is already compensated by the useful target thickness and the increased transmission of the Super-FRS.

As stated above, there are other features that make the higher energies desirable, even at the expense of reduced production rates. This is certainly true for high-Z fragments where bare ionic charge states are essential for an unambiguous identification and separation of the selected nuclide. If 216Pb is considered as an example, the Super-FRS will provide an efficient separation of 216Pb at a rate that is 5 times higher than that of the most abundant contaminant. If a similar spectrometer is operated at 350 MeV/u, some of the contaminants will be about 3 orders of magnitude more abundant than the selected isotope. Figure 1.19 shows the situation if in addition those ions are suppressed which have different ionic charge states in the two sections of the separator by using fast tracking detectors. Also in this case, the separation power is drastically improved at the higher beam energy.

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Figure 1.18: Comparison of estimated production rates for the proposed facility denoted as GSI from Figure 1.17 and expected yields of in-flight beams for the rare-isotope accelerator (RIA) project from [5]. Note that the yields as given for RIA (in-flight mode) in [5] have been limited to 109 ions per second in the region close to stability. Additional noticeable differences for certain groups of isotopes mainly reflect different model assumptions in the calculations of the production rates.

Figure 1.19: Separation purity of secondary beams with the Super-FRS optimised to 216Pb fragments from a 238U primary beam of 1500 MeV/u and 350 MeV/u beam energy. The ion-optical separation is enhanced by fast tracking detectors (see text). In this calculation the fully stripped 216Pb ion was the reference fragment for the magnetic rigidity setting. The amount and the composition of the contaminants can be somewhat changed if other charge states are selected. The selected nuclide 216Pb is marked by a circle.

Finally, the injection of separated fragment beams into a storage-cooler ring is one of the most attractive features of both the present SIS18-FRS and the future SIS200-Super-FRS facilities, see Chapter 1.2. This feature can only be effectively achieved with the pulsed-beam time structure of a synchrotron beam.

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1.1.3.2 Experiments with stored beams

Studies of high-energy exotic nuclei in storage rings have unique discovery potential based on three new experimental methods, i.e.

1. precision mass, decay energy, and half-life measurements of bare and few-electron exotic nuclei,

2. direct reaction and scattering studies of radioactive nuclei at intermediate energies in inverse kinematics using internal targets, such as 1H, 2H, 3He, or 4He,

3. elastic and inelastic electron scattering with stored and cooled exotic nuclei.

At the existing storage ring ESR, GSI has developed electron and stochastic cooling techniques which allow the preparation of radioactive beams with the highest possible phase-space densities. The main limitation of the existing set-up for reaction studies is the small acceptance of the ESR for fragment beams in the injection channel. The proposed new two-ring system has been designed to overcome these difficulties. A dedicated Collector Ring (CR) combined with a New Experimental Storage Ring (NESR) will accept the full phase space of the secondary beam delivered by the Super-FRS and will have, in addition, substantially shorter cooling times than the existing ESR. This will yield both an increase in luminosity and access to short-lived nuclei. The main task of the CR is the efficient collection and fast stochastic cooling of the secondary beams to a momentum spread of ∆p/p = 10-4 and a radial emittance of ε = 1π mm mrad with cooling times of 100 ms. These pre-cooled beams can be transferred to the NESR in which electron cooling can further reduce the emittance to ε = 0.1 - 0.01 π mm mrad with a momentum spread of ∆p/p = 10-5 (depending on the number of circulating ions) within another 100 ms.

Atomic masses and lifetimes

The knowledge of the atomic masses and lifetimes is fundamental for the understanding of nuclear matter and the creation of the elements in stars. The study of atomic masses of stable nuclei has lead to the understanding of basic properties of matter, like the existence of isotopes. Today the challenge is to measure the masses and lifetimes of exotic nuclei up to the limits of stability. Such investigations are indispensable for testing the predictive power of nuclear models in unknown territories. Masses and half-lives of exotic nuclei, moreover, govern the pathways of nucleosynthesis processes and the emerging isotopic abundance patterns.

The most accurate and versatile experimental method to determine the masses of exotic nuclei is to measure the repetitive motion of ions stored in special electro-magnetic field arrangements. The use of storage-cooler rings, moreover, provides a novel experimental technique to access lifetimes of bare and few-electron ions in the laboratory, thus under conditions that prevail in hot stellar environments. In pioneering experiments at the fragment separator FRS [60] and the experimental

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storage ring ESR [62], powerful methods for the determination of masses and lifetimes of unstable, highly-charged ions [63, 51, 64, 65, 66] have been developed and improved systematically. These techniques, that combine high efficiency, resolution, and sensitivity in a unique way, can be readily applied at the future collector- and experimental storage rings to presently unknown rare nuclei.

MCPs

anode

ion

B

Esecondary

electrons

C + CsIfoil

TOF Detector

Gas Target and Detector

eHeavy ions

-

- Reaction zone

Figure 1.20: Storage rings and electron-ion collider including the principal experimental equipment.

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Mass measurements of exotic nuclei are hampered because of the low production cross sections and the inherent phase-space enlargement resulting from their stochastic creation processes. A further difficulty arises from the fact that the most interesting nuclei near the drip lines are short-lived and thus limit the preparation and observation times. Two methods have been developed at GSI for accurate mass measurements of stored exotic nuclei: 'Schottky Mass Spectrometry (SMS)' for cooled beams of longer-lived isotopes, and 'Isochronous Mass Spectrometry (IMS)’ for uncooled beams of short-lived fragments. Both methods are based on precise measurements of the revolution frequency f which unambiguously characterizes the mass-over-charge ratio of the circulating ions. By the Schottky technique the revolution frequencies of all ion species stored simultaneously are obtained by a Fourier transformation of their signals induced in pick-up probes at each turn in the ring. In the isochronous mode, the revolution frequency will be measured either by a highly sensitive Schottky pick-up or by a time-of-flight detector mounted in the storage ring aperture. With the SMS a resolving power of 7.5×105 was achieved, while with IMS a resolving power of 1.5×105 was obtained. Both methods have reached the ultimate detection sensitivity to a single stored ion [66].

In SMS the velocity spread of the relativistic hot fragment beams is reduced by electron cooling. The cooling is achieved by merging the heavy-ion beam with a cold electron beam. By elastic collisions the average velocity of the heavy ions approaches that of the cooler electrons, and the velocity spread of the ions is decreased drastically. For example, the relative velocity spread of low-intensity fragments can be reduced to the 10-7 range and the transverse emittance to values below 0.01 π mm mr. A typical Schottky frequency spectrum of cooled bismuth fragments at 370 MeV/u is presented in Figure 1.21. Isotopes with previously known and unknown masses are marked in the spectrum by the blue and red colors, respectively. The energy of the projectile fragments injected into the ESR allows the measurement of the masses in bare, H-like, and He-like charge states, a feature which yields redundancy in particle identification, calibration, and mass measurement. An illustration of the mass resolution achieved and of the ultimate sensitivity down to single ions is presented in the inset in Figure 1.21 where ground and isomeric states of 143Sm ions are clearly resolved.

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150m

,g65+

Dy

150

65+

Tb

143

62+

143m

,g62+

Eu S

m

157

68+

Er

127

55+

Cs

157

68+

Tm

173

75+

166

72+

166

72+

180

78+

Pt

Re

Hf

Ta

152

66+

152

66+

Ho

Dy

159

69+

159

69+

136

59+

Tm

Yb

Pr

W

164

71+

171

74

Lu

164

71+

Hf

145

63+

122

53+

Gd

I

175

76+

161

70+

138

60+161

70+

168

73+

168

73+

Os

Ta

W

Yb

Nd

Lu

149

65+

Tb

156

68+

156

68+

Er

Tm

154

67+

154

67+

HoEr

163

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147

64+

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64+

147

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Dy

Tb

Gd

Lu

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72+

165

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74+

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TaR

e

W

Hf

10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

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7

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nsity

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nsity

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.units

Frequency / Hz

143 62+gSm

143 62+mSm

754 keV

33800 33900 34000 34100 34200 34300 34400 34500

(1 particle)(1 particle)

m/ m 700000 ~~

Figure 1.21: Typical Schottky frequency spectrum of cooled bismuth fragments from the FRS injected into the ESR at 370 MeV/u. The inset shows a Schottky frequency spectrum obtained from only two bare 143Sm ions, one in the ground state, 143gSm, the other one in the isomeric state, 143mSm. The measured excitation energy is (750 ± 50) keV, to be compared to the literature value of 754 keV.

Exotic nuclei with half-lives shorter than the cooling time are investigated by IMS. This novel experimental technique has been successfully applied in recent measurements with uncooled short-lived krypton fragments. As in SMS, nuclides of known and unknown masses are included in the revolution frequency spectra. In the new facility, IMS can be performed on uncooled beams directly in the CR .

The present status of the knowledge of atomic masses and the large area of hitherto unknown masses which will become accessible with the new facility is illustrated in Figure 1.22. The red area covers those short-lived isotopes where mass measurements with IMS will be performed in the CR, without any cooling. For the longer-lived isotopes, depicted in blue, one can afford to stochastically precool the beam in the CR and then transfer it to the NESR, where electron cooling and SMS will be applied. Both methods will profit greatly from the higher primary beam intensities in combination with the order-of-magnitude larger phase-space acceptance of the CR, compared to the present ESR.

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2028

50

82

8

8

20

28

50

82

126

Masses known

Can be measuredwith New ESR

Can be measured withCollector Ring

Figure 1.22: Stable (black squares) and long-lived nuclei (gray area) where nuclear masses are well known. The range of the mass measurements performed presently in the storage ring ESR at GSI is marked by yellow squares. Nuclides with still unknown masses which will become accessible with the new facilities, the double-storage ring system of CR and NESR, are indicated by red and blue colors, respectively.

Spectroscopy of stored exotic nuclei at relativistic energies is a unique tool which allows the study of nuclear lifetimes depending on the atomic charge states. A program of decay studies of bare and few-electron radioactive heavy ions has been initiated with the FRS-ESR facility [64]. An example of recent experimental results for the bound-state ( b) decay of thallium isotopes is presented in Figure 1.15. 207,206Tl nuclei were produced by the fragmentation of 208Pb in the FRS, separated in-flight, and injected into the ESR. This decay mode was observed for the first time in a direct manner since the mass difference of about 1.5 MeV allowed the separatation of the mother and daughter nuclei in the Schottky frequency spectra. Such experiments provide unique information on total and partial b lifetimes, the Q-value, and on the Fermi function of the -b decay. Note that in the past this latter information could only be obtained for +- and orbital electron capture decay.

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Nuclear Reactions with the Internal Target

Reactions induced by light ion beams on stable target nuclei at intermediate and high energies (tens to hundreds of MeV/u) contributed substantially to our present knowledge of nuclear structure. There is a large variety of such reactions, all of which can be used as well for high-precision studies of nuclear structure effects in exotic nuclei by using elastic scattering on protons, deuterons, 3He, 4He etc., inelastic scattering, nucleon transfer, charge exchange and knockout reactions. With secondary beams of exotic nuclei, the experimental challenge is the implementation in reversed scattering kinematics and the quest to cope with low beam intensities. Most reactions of this type are characterized by a rather moderate momentum (and energy) transfer. Thus, only very thin targets can be used in order to be able to observe the recoiling light ions. Reaction rates, in turn, become low. A gain in luminosity, however, can be achieved using internal targets in a storage ring.

Experimental concept. Cooled radioactive beams circulating in a storage ring and crossing a very thin internal target roughly 106 times a second offer the unique opportunity to perform high resolution direct reaction spectroscopy. The high circulating current fNI i ⋅= , with Ni being the number of stored ions and f their

circulation frequency, leads to high luminosities ti NfNL ⋅= even if the number Nt of

target ions per cm2 is rather small such as 1014 cm-2. With only 104 ions circulating in the ring, a luminosity of 1024 cm-2 s-1 can be reached. Equally important, is the fact that most of the scarce radioactive ions can be “used up” by nuclear reactions, although the target is very thin. Reaction studies with internal targets may thus yield the highest efficiency in terms of using the beam; detailed luminosity considerations are provided in Chapter 1.2.2. The price one pays is related to the beam cooling which is needed to avoid degradation of the beam quality (momentum resolution and emittance).

The ions cooled in the CR have an energy of 740 MeV/u. The most effective way to reduce their energy is to slow them down in a degrader placed in the waist position of the transfer beam line, which results in only a modest enlargement of the emittance and the momentum spread (see Chapter 1.2.2 for details). With this procedure, radioactive nuclei can be directly injected with the appropriate energy without a delay for deceleration, as would arise if deceleration was performed using the NESR in the synchrotron mode. The apparatus for experiments at the internal target is discussed in Chapter 1.2.2.2. In essence, it allows high-resolution spectroscopy by means of a kinematical measurement of the recoils from the internal gas target, supplemented by identification of the projectile fragments using the NESR magnets as a spectrometer, see Figure 1.20. The kinematics of inverse light ion scattering is sketched in Figure 1.23. The experimental task is to derive from the kinematical quantities of the recoiling light ion both the angular distribution and the spectral (excitation energy) distribution for the reaction of interest. The angular distribution characterizes the multipolarity of the transition, in most cases it is sufficient to cover the angular range up to center-of-mass angles around θcm = 20o . For a number of measurements it is essential to be sensitive to very small angles near θcm = 0o, see below.

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Figure 1.23: Kinetic energy vs. scattering angle (laboratory frame) of recoiling 1H and 4He target nuclei for elastic and inelastic scattering of a stored 132Sn beam. Center-of-mass scattering angles are indicated.

60 65 70 75 80 85 90 95

θlab (deg)

1

10

100

Ere

coil (

MeV

)θcm=1

o

20o

10o

1H(

132Sn,

132Sn) 740 MeV/uELASTIC

E*=10 MeV

4He(

132Sn,

132Sn

*) 400 MeV/u

E*=15 MeV

Specific experiments. In the following, specific types of reactions are addressed exemplifying the high potential of in-ring experiments with regard to nuclear structure studies.

Elastic scattering. One of the main physics questions to be addressed in light-ion scattering experiments on exotic nuclei concerns the extended density distributions and neutron skins of neutron-rich nuclei as was discussed in Chapter 1.1.2.1.

The nucleon density distributions can be studied in detail by elastic proton scattering. In the high energy regime, reliable reaction models such as the Glauber multiple-scattering theory allow differential cross sections to be related to nuclear matter distributions. Whereas matter distributions in stable nuclei were systematically investigated in the past [67], elastic proton scattering off halo nuclei was performed at GSI recently. The latter experiment used an external target and covered a restricted interval of momentum transfer which is mostly sensitive to the outermost part of the halo wave function [68,69]. A comparison of elastic proton and electron scattering, the latter may be performed with the eA-Collider (see subsequent section), yields both the proton and neutron distributions. Figure 1.24 shows the rates estimated for the doubly magic 132Sn nuclei scattered off the protons of the NESR internal hydrogen target. The sensitivity to small changes in the mean radii is evident from this figure.

Transfer reactions. The single-particle structure of neutron-rich nuclei is best studied by using single-particle transfer reactions. In particular, pick-up reactions of (p,d) and (d,3He) type will be extremely useful to pin down the single-particle neutron and proton structure including the occupation numbers and transition densities of the relevant states. At intermediate energies, such direct reactions transfer large angular momenta, and thus preferably populate high-spin states. This feature will allow the

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direct measurement of the spin-orbit splitting by comparing the energies of close-lying high-spin states, such as the 1 h11/2- and the 1 g7/2- state (see Figure 1.6).

5 10 15 20θc.m. (degree)

103

104

105

106

107

108

CO

UN

TS

per

DA

Y

r = 4.65 fmr = 4.87 fm

Luminosity = 1028

cm−2

s−1

Helm−Model Density Distribution∆θc.m. = 0.2

o

132Sn

Figure 1.24: Yields expected for 132Sn (740 MeV/u) scattered elastically off protons in the NESR. Calculations are shown for two choices of the mean nuclear radius, obtained by using a specific nuclear density distribution. A gas-jet target density of 1014 atoms/cm2

is adopted.

0 2 4 6 8θc.m. (degree)

100

102

104

106

CO

UN

TS

per

DA

Y

134Sn

132Sn

104Sn

Gaint Monopole Resonance

Figure 1.25: Yields expected for inelastic scattering of tin isotopes (400 MeV/u) on a 4He gas target in the NESR. Yields are calculated for excitation of the giant monopole resonance, placed at 15 MeV excitation energy and exhausting 100 % of the sumrule strength. A gas-jet target density of 1014 atoms/cm2 is adopted.

Within the diffuse neutron-enriched surface zone of exotic nuclei, one expects an increase in the neutron pair correlations. This would lead to a corresponding enhancement of two-neutron transfer probabilities which can be studied by using (p,t) reactions. In addition, pair correlations lead to a characteristic smoothing of the population of the Fermi surface which is reflected in single neutron transfer probabilities. There is a very interesting structure phenomenon expected to occur in nuclei along the N=Z line, namely np-pairing in the T=0, S=1 state. Such states should be preferentially populated in (d,α) or (p,3He) reactions on neutron-deficient nuclei.

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Inelastic scattering and charge-exchange reactions. Light-ion scattering allows the evolution of nuclear shapes and collective motion over the entire chart of nuclei to be traced. Low-lying collective states, whose characteristics reflect the particular nuclear deformation, may be excited byproton or α-particle scattering. Of particular interest are giant resonances as they probe the bulk properties of nuclear matter and carry information on the effective nucleon-nucleon interaction in the medium. With a large proton-neutron asymmetry in a nucleus the isospin dependence of giant resonances, which is not well understood so far, may be studied systematically. A number of unusual effects are expected to appear in the strength distributions of giant resonances in exotic nuclei as discussed in Chapter 1.1.2.1.

Inelastic scattering of protons or α-particles are rather selective probes inducing preferentially isoscalar transitions. This is in contrast to electron scattering which does not allow isoscalar and isovector transition to be distinguished. Moreover, for both probes the angular distributions are rather characteristic of the multipolarity of the transition. Proton inelastic scattering played an essential role in the discovery of the isoscalar giant quadrupole resonance, α-scattering is widely used to study the isoscalar giant monopole resonance (GMR). Measurements of the GMR in neutron-rich nuclei will give access to the compressibility of proton-neutron asymmetric matter. The angular distribution of the GMR in inelastic α scattering is known to be strongly peaked at center-of-mass scattering angles around θcm = 0o. It is an essential feature of measurements in inverse kinematics, in conjunction with a gas target in a storage ring, that very small θcm values can be covered, see Figure 1.23 and the discussion in Chapter 1.2.2.

Light-ion induced charge-exchange reactions serve to study spin-isospin excitations. Isobar-analog states, Gamow-Teller (GT) or spin-dipole resonances, for instance, are studied by means of (p,n), (3He,t), or (d,2He) or similar reactions. A measurement of the GT strength for vanadium-to-nickel nuclei is of crucial importance for the calculation of stellar weak-interaction rates in supernovae environments [47, 50] (see Chapter 1.1.2.3). The isovector spin-flip part of the nucleon-nucleon interaction is strongest at nucleon energies of 300 – 400 MeV, thus very appropriate for measurements in the NESR.

Cross sections for giant resonances excited with light ions are typically of the order of 10 mb/sr. Thus, measurements in a storage ring seem feasible for exotic nuclei for which at least 104 ions can be stored. Figure 1.25 shows a yield estimate for GMR excitation in a neutron-rich tin isotopes, based on a realistic experimental scenario. It is essential to note that the specific reaction channel may be selected by triggering with the residual nuclei, emitted in the forward direction and identified in the bending sections of the storage ring. In contrast to conventional experiments, a substantial background suppression of competing reaction channels can thus be achieved.

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Electron scattering at the eA-Collider

Concept

Since the pioneering experiments of Hofstadter [70], electron scattering experiments on nuclei have contributed significantly to reveal the structure of stable nuclei. The electron-nucleus collider (eA-Collider) will give access to structure studies of unstable nuclei, opening up a new era of low-energy electron scattering. Due to the limited luminosities achieved with radioactive beams, first generation experiments will focus on measurements of the radii of the nuclear charge distribution and its diffuseness. In case of light nuclei, studies of the hyperfine structure and isotope shift of atomic states, including in particular laser spectroscopy studies [71], do not give sufficient information on charge radii, and the diffuseness is not accessible at all with these methods. It is interesting to note that information from isotope shifts derived from measured charge radii will already provide a first inside into the elementary magnetic dipole modes for the respective nuclei [72].

Besides the optical experiments, the only electromagnetic probe presently available for structure studies of exotic species are equivalent photons from heavy-ion Coulomb interaction. However, their sensitivity to transition multipolarity is not very pronounced. The challenge is thus to perform detailed nuclear structure investigations by means of inelastic electron scattering by using the eA-Collider. The necessity for an electron-exotic-nucleus collider facility, urgently demanded by the development of contemporary physics, can also be seen from a similar project of an electron-nucleus collider as proposed for the RIKEN radioactive beam factory [73].

The advantages of using electrons are mainly related to the fact that the electron-nucleus interaction is described by quantum electrodynamics, one of the most accurate theories presently existing in physics. Contrary to hadronic probes, the electron-nucleus interaction is relatively weak. For this reason, multiple scattering effects can usually be neglected and for light and medium nuclei the interaction can be well described by one-photon exchange terms. Correspondingly, the scattering dynamics contains terms of energy transfer ω = E-E′ and three-dimensional momentum transfer q = p-p′, the components of the Lorentz-invariant four-momentum transfer Q = (ω,q). The quantities p,p′ and E,E′ denote incoming and outgoing electron momentum and energy, respectively. A certain process can be chosen by a proper selection of transferred energy and momentum. For these processes one can determine form factors. In the case of elastic scattering, they are related to the charge density distributions. For inelastic scattering they are connected with different multipolarities of the excitations and offer a unique way to study collective motion in unstable nuclei. At larger energy and momentum transfer, the mean-field behavior of the nucleus becomes less important and quasielastic scattering on nucleons contributes increasingly. In this process an individual nucleon is knocked out of the target nucleus allowing the single particle structure to be probed.

The proposed eA-Collider, consisting of the new storage-cooler ring (NESR) for unstable nuclei and the electron-storage ring (ER) with an intense beam of electrons,

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will be a unique experimental tool. At the eA-Collider, one can cover electron energies equivalent to a range from 200 to 1600 MeV in the target-rest system by varying the electron and ion beam energies from 200 to 500 MeV and 200 to 740 MeV/u, respectively. For co-propagating beams, this energy could be as low as 30 MeV. Thus, over a wide energy region, specific demands of an experiment can be fulfilled. An essential component of these experiments will be a spectrometer of large acceptance and high resolution to analyze the momenta and scattering angles of the scattered electron in combination with a detector system to analyze the target-like fragments and decay products from the decay of excited states.

The collider technique - being new in the field of nuclear structure research - will allow much cleaner experimental conditions compared to fixed-target measurements. The energetic target nuclei and their possible breakup products are forward peaked and have high energy, and can thus be fully identified with high efficiency. Electron scattering experiments of exclusive type become possible. Moreover, the stored exotic nuclei are bare, and there are hence no interactions of the scattered electrons with target electrons.

Experiments

With the new developments for intense beams of radioactive isotopes and efficient storage and cooling, electron-nucleus collider experiments become feasible for the first time. Electron scattering on a large variety of β−unstable nuclei will become possible. This will considerably extend our knowledge on nuclei and substantially contribute to a conclusive picture of nuclear structure.

Within the one-photon exchange approximation, cross sections are proportional to the Mott cross section which describes the electron scattering on a point-like nucleus. In the common case (where qc < ETS, the electron energy in the target-rest system) the Mott cross section depends essentially on the transferred momentum. Correspondingly, the number of events registered in 2π geometry and in an interval around a selected transferred momentum does not vary significantly either with electron-beam or with ion-beam energy. Therefore, the energies of both beams can be chosen such that an optimal resolution and yield for a given process can be achieved. The resolution required to resolve elastic from inelastic channels depends on the position of the lowest collective excited state in the target nucleus. The currently achievable resolution with a state-of-the art spectrometer results in the colored areas shown in Figure 1.26 which are suited for explorations in electron scattering experiments. Among the tasks, one of the most interesting ones concern the disappearance of the known shell closures and their appearance as magic numbers of a harmonic oscillator model, see Chapter 1.1.2.1. The traces of an opening of a new N=40 subshell can be searched in the charge radii for heavy-isotope sequences crossing subshells, e.g. 48-60Ca , 56-71Ni. The neutron-rich semi-magic calcium, nickel and tin isotopes, and the N=50 and 82 isotones below 78Ni and 132Sn, are the most challenging nuclei to study in order to clarify the evolution of charge radii towards the drip line.

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Figure 1.26: Experimentally known excitation energies for the first excited 2+ states in even-even nuclei. Their dependence on the nuclear charge Z and neutron excess (N-Z) is shown. The circles mark the position for the doubly magic nuclei. The colored area reflects the accessible regions, determined by the achievable energy resolution, for experiments performed with the electron – heavy ion collider.

The resolution in excitation energy for the colliding beams is mainly determined by two effects. First, the electron energy in the center-of-mass (c.m.) system of counter-propagating beams is significantly larger than that in the laboratory system, e.g. the c.m. energy for a 740 MeV/u ion beam and a 500 MeV electron beam amounts to about 1600 MeV. Second, for not too light nuclei, the energy E′ of the elastically scattered electron in the target-rest system (and thus ω) is to a large extent independent of the scattering angle θ, whereas the dependence of E′ on θ is very strong in a measurement with colliding beams. This can be seen from Figure 1.27, where E′(θ) is shown for the different systems. The energy increases with angle in the crossed-beam system and decreases in the merged-beam system. Both of these kinematical effects result in special demands on the resolution of the electron spectrometer. To achieve an excitation energy resolution of about 500 keV, the momentum and angular resolution should not be worse than δp/p≈⋅10-4 and δθ≈1 mrad, respectively.

A minimum value of 20 MeV/c for the transferred momentum q is a reasonable limit. The corresponding minimum scattering angle in the counter-propagating system for the lowest electron-beam energy of 100 MeV is then equal to θmin=10°. The maximum transferred momentum qmax that can be detected in experiments is given by luminosity considerations. A reasonable limit for experiments with elastic, inelastic and quasielastic electron scattering is qmax ≈ 1000 MeV/c which corresponds to a maximum angle of θmax ≈ 90° for counter-propagating beams with an electron energy of 500 MeV and an ion energy of 740 MeV/u.

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Figure 1.27: Dependence of the energy (E’) of the scattered electron on the scattering angle (θ) for elastic scattering on 132Sn for counter-propagating (→←), co-propagating (→→) beams and fixed target experiments (→| ).

θ

The required minimal luminosities for different experiments discussed in the following, are summarized in Table 1.1.

Table 1.1: Required luminosities for different studies. Luminosities are indicated assuming 2π acceptance of the spectrometer. Values given in square brackets indicate required luminosities using two conventional spectrometers of MAMI A type (see Chapter 1.2.2.3). A few of the latter values (given in italics) exceed the achievable luminosities of the proposed eA-Collider.

Reaction Deduced quantity Target nuclei

Luminosity 1/[cm2sec]

Elastic scattering at small q rms charge radius Light 1026 [3×1026]

Light 1028 [1029]

Medium 1026 [7×1026] First minimum in elastic form-factor

Charge-density distribution with 2 parameters.

Heavy 1024 [5×1024]

Medium 1029 [1030] Second minimum in elastic form-factor

Charge-density distribution with 3 parameters. Heavy 1026 [8×1026]

Giant resonance Strength, position, width, decay channels.

Medium and heavy 1028 [7×1028]

Quasi-elastic scattering Momentum distribution, spectral function. Light 1029 [2×1030]

Elastic scattering

Nuclear radii and diffuseness parameters are characteristic properties of nuclear systems. The knowledge of nuclear matter and charge density distributions for very proton- and neutron-rich nuclei is most important in deriving the equation of state for proton-neutron asymmetric nuclear matter. While the measurement based on radioactive beams stored at NESR and gaseous (4He, 1H2) internal targets (see

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preceding Chapter 1.1.3.2) provide the most precise information on nuclear matter density distributions, elastic electron scattering with colliding beams yields charge density distributions. By combining the two methods it will be possible to determine proton and neutron distributions separately for a large number of radioactive isotopes.

The kinematics of elastic (e,e) scattering results in Q2=2Mω, with M being the mass of the target nucleus. Correspondingly, the differential cross section in the framework of the Born approximation can be written as

)(2 qFdd

dd

Mott

Ω=

Ωσσ

,

where )(qF denotes the elastic form factor and Mottd

d

Ωσ the differential cross section

for scattering of an incident electron on a point-like charge. The spatial charge distribution )(rCρ can be obtained as the Fourier transform of the measured form

factor. However, the form factor can only be determined in a restricted range of momentum transfer and with a finite statistical accuracy. Therefore, the density distribution is approximated by an analytical expression. A good description of the experimental data can be obtained by introducing a few parameters, e.g. the three-parameter Fermi-distribution

)1(]/)exp[(1

)( 20 wrdRr

rC +−+

=ρρ .

Its parameters R, d, and w represent the radius, surface width and central-density depression, respectively. The reliability of such a density parameterization has been proven by analyzing experimental electron scattering data (see, for example, [74]).

Covering a wider region of q gives more detailed information on these density distributions, but also requires higher luminosities. In the case of light nuclei it is possible to expand the form factor F(q) for small q in terms of higher moments of the charge distribution. This offers the opportunity to determine the root-mean-square charge radius 2/12 >< r in a model independent way, already at comparatively low luminosities. Moreover, for low momentum transfer the contribution from inelastic scattering is expected to be negligible.

Inelastic scattering

Inelastic electron scattering is an excellent tool for spectroscopy of bound and unbound states in nuclei. Since transitional form factors with different multipolarities have different q dependence, specific multipoles can be excited by varying the momentum transfer, and thus a selectivity in spin is achieved. This can be seen from Figure 1.28, where the squared transition form factors for different multipoles are shown together with the elastic form factor. It is very important to extend the study of

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the dominant collective excitations of nuclei, the giant resonances, to β-unstable nuclei. Large proton or neutron excess significantly influences the collective motions in nuclei, see the discussion in Chapter 1.1.2.1. Giant dipole resonances are of special interest as they give access to the nuclear shape. The dipole strength is predicted to be shifted to lower energies for neutron drip-line nuclei. As the inclusive spectra of inelastically scattered electrons generally show a large background from the radiation tail, measurements with coincident detection of decay products (e,e′X) present a far more powerful tool to selectively study different multipole resonances and their subsequent decay [75, 76].

Figure 1.28: Form factors calculated for elastic scattering, monopole, dipole, quadrupole and octupole transitions for 132Sn. For the sake of clarity, secondary diffraction peaks have been omitted in case of the multipole transitions.

In colliding-beam experiments, the heavy recoils after emission of a few nucleons, can be detected with full acceptance by using a dipole magnet of the NESR as a magnetic analyzer. In electro-fission the angle between fission fragments is about a few degrees. Thus it becomes possible to study the fission process with selected multipole excitation over a large energy range. Such coincidence measurements are not feasible in fixed target experiments as the fission products are normally stopped in the target.

Quasi-Elastic Scattering

In a region where the squared momentum transfer, Q2, is close to 2mω (m denotes the nucleon mass) electron scattering is dominated by the interaction with the individual nucleons inside a nucleus. The process where the incoming electron directly knocks out an individual nucleon from the target nucleus results in a broad peak in the inclusive spectra of the transferred energy. The width of the peak results from the Fermi motion of the nucleons inside the nucleus. Coincidence measurements (e,e′N) can give detailed information on the combined nucleon-hole energy and momentum distribution, the so-called spectral function, in the target nucleus. Thus, not only the binding energies for different shells can be obtained, but also the nucleon momentum distributions can be determined for each shell or subshell. This permits testing of the various extensions of the shell model.

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1.1.3.3 Reactions with high-energy beams

Figure 1.29: Kinematically complete measurements of reactions with high-energy beams. The incoming radioactive ions as well as the fragments emerging after reactions in the target are identified and tracked by means of energy-loss, time-of-flight, and position measurements. The insets show the composition of a mixed secondary beam (upper left) and of fragments of a 20O beam (lower right) from an experiment performed at GSI. The target is surrounded by a γ-ray detector. Neutrons are detected in forward direction.

A highly efficient reaction setup for fast radioactive beams (Figure 1.29) provides unique possibilities for nuclear structure experiments with exotic nuclei. Table 1.2 gives an overview of the reactions that can be studied. The key aspect of the setup is the complete measurement of all projectile reaction products. This includes the detection of heavy fragments, neutrons, light charged particles as well as γ-rays emitted from excited projectile-like fragments. This ensures a kinematically complete reconstruction of the four-momenta, allowing, for example, the determination of the excitation energy of the projectile prior to decay by reconstruction of the invariant mass. In case of scattering on light targets, detection of target-like residues is also foreseen, e.g. of the neutrons in (p,n) reactions, or of recoiling protons of a liquid hydrogen target. The planned setup, which is based on the existing reaction setup at GSI (Figure 1.29), is described in detail in Chapter 1.2.3.

An alternative method of studying reactions at high energies consists of high-resolution magnetic-rigidity measurements by using the Super-FRS as an energy-loss spectrometer. This mode of operation is especially suitable to study knockout reactions when the momentum of the heavy fragment has to be measured with high

projectile charge Z

A/Z

2.5

2.75

4 6 8

22O

21O15C

24F

19N

14B

11Be

2.5

2.75

4 6 8

10

12

14

16

18

20

4 5 6 7 8 9

fragment charge Zf

frag

men

t mas

s A

f

10

12

14

16

18

20

NEUTRONS

CHARGEDPARTICLES

DIPOLE

TARGETDETECTOR(γ, p, n)

EXOTIC

BEAMS

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125

precision. In this case the secondary target is placed in the central focal plane of the separator and is viewed by γ-ray detectors for the detection of excited fragment states. The limited acceptance of the FRS, however, does not allow for complete-kinematics experiments.

The experiments with radioactive beams performed so far have proven that detailed spectroscopic information can be extracted for example from knockout experiments with halo nuclei [77, 14], or from measurements of the electromagnetic excitation [78,29]. The investigation of reactions with high-energy beams has several advantages. One is the possibility of using relatively thick targets of the order of 1023-1024 atoms/cm2. Moreover, full acceptance can be reached even with moderately sized detectors due to the kinematical forward focusing. Thus, high luminosities and detection efficiencies can be obtained even for weak secondary beams. In addition, the quantitative description of high-energy reaction mechanisms developed in the recent years allows one to extract detailed spectroscopic information. As can be seen from Table 1.2, a large variety of reaction and nuclear structure studies can be performed even at modest secondary beam intensities.

Table 1.2: Estimated minimum intensities of high-energy secondary beams, necessary for reaction studies.

Reaction type Physics goals Ions/s

Total Absorption Matter radii 1

Knockout Unbound states, nuclear resonances beyond the driplines

1-10

Single-particle structure 0.1-10

Electromagnetic Low-lying transition strength, single particle structure

0.1-10

Excitation Soft coherent modes 1-100

Giant dipole resonance 100

Giant quadrupole strength 103

B(E2), deformations

Reactions inverse to capture, astrophysical S-factor

1-10

103

(p,n) GT strength, spin-dipole resonance, neutron skin

103 -104

Quasi-free scattering

Single-particle structure 10

Fission Shell structure, dynamical properties 103

Fragmentation High-spin γ-ray spectroscopy, isomeric beams 1-10

Multifragmentation Nuclear equation-of-state, phase transitions 103

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126

Total-absorption measurements

Nuclear matter radii may be inferred from total interaction cross sections derived from total-absorption measurements of radioactive ions in thick targets. This technique requires intensities of the order of only 1 atom/s. Interesting results were obtained by a RIKEN-GSI collaboration by using beams of unstable sodium isotopes. These data provide a first experimental manifestation of neutron skins (see Figure 1.30). The beam intensities available at the new facility will allow systematic studies of this phenomenon in heavy nuclei.

Figure 1.30: Neutron-distribution radii <r>n

and charge radii < r>p of sodium isotopes as a function of the neutron excess, N-Z. Radii of the neutron distribution are deduced from interaction cross section measurements at GSI, radii of the proton distribution from isotope shift measurements at ISOLDE (CERN). Adapted from [17].

N-Z

rms

radi

us (

fm)

1120-32Na

<r>n

<r>p

2.5

3

3.5

0 5 10

Knockout reactions

Breakup reactions induced by high-energy beams of exotic nuclei are an extremely powerful tool which allows the exploration of ground-state configurations and of excited states. Due to limitations in beam intensity for heavier nuclei, this method has been restricted mainly to light halo nuclei so far. Single-nucleon knockout reactions and Coulomb-induced breakup are particularly promising types of reactions. At high bombarding energies, the knockout mechanism is a sudden process, which essentially leaves the internal nucleon motions undisturbed. It is thus a very clean probe for ground-state configurations. Knockout reactions have been used in particular to map the momentum distributions of halo nucleons, from which their spatial distribution is derived via Fourier transformation. They also yield spectroscopic factors as well as spectral information on excited states. As an example, Figure 1.31 shows data for one-neutron knockout from the two-neutron halo nucleus 11Li, from which the occupancy of the (s1/2)2 and (p1/2)2 configurations of the two halo neutrons in the 11Li ground state were deduced.

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127

10 Li

px (MeV=c)

d=dpx

(mb=(MeV=c))

-200 -100 0 100 200

1

0.1

0p1=2

1s1=2

-0.5 0.5

400

600

800

1000

cos(nf)

d=dcos( nf)

n

nknock

9Li nf

Figure 1.31: Left panel: Momentum distribution of the unbound 10Li after one-neutron knockout from 11Li. From the fit of the calculated distributions to the data, the occupancy of the (s1/2)2 and (p1/2)2 components in the ground state of 11Li are determined. Right panel: Neutron-fragment angular distribution for one-neutron knockout in 11Li and subsequent decay of 10Li. The asymmetric distribution reflects the interference between s- and p-wave components. Adapted from [14].

The beam intensities available at the proposed facility open up the possibility to study heavy neutron-rich nuclei, produced by in-flight fission. Given the high sensitivity of knockout reactions together with a high detection efficiency, secondary beam intensities in the order of one atom/s are sufficient to extract detailed spectroscopic information, thus making nuclear structure studies possible even very far from stability (see intensities estimated in Chapter 1.1.3.1). The availability of new efficient high-resolution γ–ray spectrometers, like the proposed AGATA array, gives additional new prospects to these studies. A coincident measurement of γ-rays of the knockout fragments gives access to spectroscopic factors of ‘core-excited’ states in loosely-bound exotic nuclei. First measurements of this type were performed at MSU [79, 80], see Chapter 1.1.3.5. Another promising field is the study of resonant states in nuclei beyond the drip lines, the configuration of neutron droplets being particularly appealing.

Electromagnetic Excitation

Electromagnetic processes in heavy-ion interactions at energies far above the Coulomb barrier give access to a wealth of nuclear structure information on exotic nuclei. At energies of the order of 1 GeV/u, collective nuclear states at low and at high excitation energies are excited in peripheral heavy-ion collisions with large cross sections. Due to Lorentz contraction, the mutual electromagnetic field contains high frequencies up to several tens of MeV/•, and is of transversal nature. Rotational states, surface vibrations and giant resonances can be studied even with moderate beam intensities. The short interaction time, however, suppresses multi-step excitations, so that only the lowest members of a rotational band are excited. Figure 1.32 gives typical cross sections for the excitation of collective nuclear states in exotic nuclei as a

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128

function of beam energy. The cross sections allow experiments with minimum beam intensities of 1 to 1000 atoms/s, provided efficient devices for γ-ray detection (discrete states) or particle detection (giant resonances) are implemented.

Figure 1.32: Integrated cross sections for collective states in exotic nuclei. The cross sections are calculated for the electromagnetic excitation on a lead target as a function of the bombarding energy. Typical nuclear structure parameters for a medium-mass nucleus were adopted.

E (GeV/nucleon)

σ (m

b)

GDR

2-ph GDR

3-ph GDR

GQR (is)

2+

1

10

10 2

10 3

10-1

1 10

Electromagnetic excitation of the giant dipole resonance induced by high-energy beams on targets of high nuclear charge is an extremely powerful tool. It was pioneered at GSI in exploring the multi-phonon states of the dipole resonance [81]. This method can readily be extended to secondary beams of exotic nuclei. Recently, a first systematic investigation of low-lying dipole modes was performed at GSI for the chain of neutron-rich oxygen isotopes [29] utilizing this technique. The resulting photo-neutron cross sections are shown in Figure 1.7 of Chapter 1.1.2.1. From these data, it is evident that low-lying strength appears for neutron-rich oxygen isotopes and that the usual pattern of the dipole resonance strength distribution dissolves. With the proposed new facility, the measurement of the dipole strength of neutron-rich nuclei relevant for the r-process will be feasible. In the region of the N=82 closed shell, for instance, the giant dipole strength can be deduced even beyond 132Sn. The higher beam intensities also allow the study of giant quadrupole strength. Compared to dipole excitations, the required beam intensities are an order of magnitude larger. Giant resonance studies, in particular monopole and quadrupole excitations, will also be investigated at the CR and NESR (see Chapter 1.1.3.2).

Besides resonant excitations, direct non-resonant transitions to the continuum occur for weakly bound nuclei. This ‘threshold strength’ is characteristic for the single-particle structure, being extremely sensitive to the spatial distribution of the valence nucleons. Similar to knockout, the l-value of the removed nucleon and spectroscopic factors can be deduced. For a halo-like structure, cross sections become very large, and spectroscopic information can be obtained with beam intensities down to 0.1 atoms/s

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129

The experimental technique discussed here also allows the extraction of (p,γ) reaction rates, which essentially determine the astrophysical rp reaction-path (see Chapter 1.1.2.3). While the direct measurement of these rates is very difficult, the (γ,p) reaction can be measured by electromagnetic excitation using high-energy secondary beams. Thereby, the high secondary-beam energies available at GSI are of significant advantage compared to the low and intermediate energies available at other facilities, as demonstrated in a recent GSI experiment exploring the important 7Be(p,γ)8B reaction [46]. With beam intensities as foreseen for the new facility, a reliable determination of reaction yields for the most crucial rp bottle-necks, e.g. 22Mg(p,γ)23Al, is possible.

Light-ion Scattering and Charge Exchange

Scattering off light target ions was discussed in Chapter 1.1.3.2 in the context of measurements with internal targets in a storage ring which provide the optimum experimental conditions for reactions involving charged recoils of low momentum. For the (p,n) reaction, however, a better performance can be achieved with a comparatively thick ( ~ 1 g/cm2) external liquid hydrogen target. Studies of the GT strength are of particular astrophysical interest, see Chapter 1.1.2.3. In this context, a very attractive feature is that the GT strength of extremely short-lived nuclei and, moreover, for excited states with half-lives down to 100 ns can be accessed. The latter arise from the fact that fragmentation reactions produce isomeric beams with considerable isomeric ratios, for a more detailed discussion see Chapter 1.1.3.5. This enables a test of an assumption crucial for stellar weak interaction rate calculations for supernovae, namely the Brink-hypothesis [47,82], stating that the GT strength distributions on ground and excited states are similar. For this test, odd-odd and odd-neutron vanadium-to-nickel nuclei are most important [50] (see Chapter 1.1.2.3).

Cross sections of the spin-dipole giant resonance excited in (p,n) reactions are rather directly related to the neutron skin thickness, corresponding measurements were performed so far only for stable isotopes [83, 84]. With exotic nuclear beams, such measurements require secondary beam intensities of the order of 1000 ions/s, and thus a systematic study of neutron skins will be feasible by a method alternative to that consisting of a combination of proton and electron scattering data, which was discussed earlier.

Fission

First-generation experiments performed at GSI have proven that the use of secondary beams indeed opens exciting new prospects for studies of nuclear fission [85]. More than 100 short-lived neutron-deficient nuclei will become available for such investigations. Figure 1.33 displays part of the Z distributions of the fission products measured after electromagnetic excitation of the respective fissile nuclei.

The quality of experimental information to be obtained for a large number of fissile nuclei at the future facility at GSI by using dedicated experimental equipment has never been available from conventional experiments. The use of inelastic excitation in

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130

the eA-Collider will allow the energy and the multipolarity of the excitation to be controlled. By simultaneously measuring the proton and neutron numbers of both fission fragments as well as their kinetic energies, the influences of neutron and proton shells and of pairing correlations on fission dynamics may be deduced. The full isotopic distribution of the fission fragments is a sensitive signature of the excitation energy at which fission occurs in the statistical de-excitation cascade. Observation of the neutrons and γ-rays emitted in the fission process allows the determination of the excitation energies of the final products.

Figure 1.33: Fission-fragment element distributions in the range from Z = 24 to Z = 65 after electromagnetic excitation of 28 short-lived neutron-deficient nuclei between 221Ac and 234U are shown on a chart of the nuclides. The experiment was performed at the FRS by using secondary beams from the fragmentation of 238U. The strongly different structures found in the distributions reflect the influence of shell effects at extreme deformation.

Fragmentation

Coulomb excitation (see Chapter 1.1.3.4) as well as one-nucleon knockout give access only to the first excited states with low angular momenta. Fragmentation reactions on exotic beams additionally populate higher-lying excited states in the residual fragment, including those of high spin. High-resolution γ-ray detection devices open the field of high-spin γ-ray-spectroscopy of heavy exotic nuclei. A more detailed discussion of this subject will be presented in Chapter 1.1.3.5.

Multifragmentation

Multifragmentation offers the possibility to probe nuclear matter at low densities and, more generally, modes of disintegration of dynamically unstable systems. In particular, the expected link with the nuclear liquid-gas phase transition provides a continuing motivation for studying multifragmentation. Very valuable results have already been obtained from studies employing stable isotopes [86]. However, secondary beams of exotic nuclei will be indispensable for continuing the exploration of isospin effects in multifragmentation. Such experiments using the existing GSI facilities are in preparation. While the exploration of the nuclear phase diagram seems more speculative at present, the new possibilities to probe the reaction mechanism by exploiting isotopic effects have a wide potential of application. Only

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131

with a very detailed knowledge of the reaction mechanism can one hope to relate multifragmentation phenomena to properties at low density and to the nuclear-matter equation of state.

Reactions with stable beams

During the past decade, experiments at GSI succeeded to identify the two-phonon giant dipole resonance (GDR) excited by virtual-photon absorption in peripheral heavy-ion collisions. The discovery of higher-phonon vibrations remains a challenge. The availability of higher beam energies, as foreseen for the new facility at GSI, will provide the possibility of choosing optimal conditions for such investigations. For the electromagnetic excitation of a four-phonon GDR state in a medium heavy nucleus e.g., the optimal beam energy is around 3.5 GeV/u, see Figure 1.32. Additionally, at beam energies around 20 GeV/u and above, virtual photons with energies higher than the pion production threshold also allow electromagnetic excitations of delta resonances.

Since the quality of isotopic separation of an in-flight separator is directly proportional to the ion-optical resolving power, the Super-FRS is an ideal tool for precise Q-value measurements in nuclear reactions. In such experiments at the FRS, deeply bound pionic states in heavy nuclei were observed for the first time employing the (d,3He) reaction on lead targets [87]. This field of pionic atoms is an interdisciplinary one, having a strong impact on both atomic and nuclear physics.

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132

1.1.3.4 Low-Energy and Stopped Beams

An energy-bunching stage in combination with a gas-filled or solid-state ion catcher behind the Super-FRS [88] will be the key instruments for advanced spectroscopy experiments with exotic nuclear beams of low energy and for precision experiments with stopped beams, in particular for those in ion and atom traps. This branch of the Super-FRS will combine advantages of the in-flight separation method and the ISOL concept. All components are either already well established or presently under development, and can thus be provided by using existing technology, without any principal limitations being expected.

Principle of the method

The central part of the low-energy branch is an energy buncher, which basically consists of a dispersive magnetic dipole stage combined with a mono-energetic degrader (see Figure 1.34). The latter is a specially shaped energy degrader of variable thickness, which is made of high-quality aluminum and has extremely small shape and surface tolerances.

With this combination, the separated fragment beams can be slowed down and their large momentum spread of up to 2.5% can be reduced drastically. This leads to narrow range distributions and to the possibility to implant the isotopes into thin stoppers of small areal weight, which is advantageous for spectroscopy experiments. In particular, it allows for the possibility to stop the ions in a gas-filled cell. From there, the isotopes can be re-extracted and delivered to various experimental set-ups for ‘ISOL-type’ experiments. In this respect, the gas catcher behind the Super-FRS will combine the intrinsic advantages of the in-flight technique of fast and chemistry-independent production and separation, with those of the ISOL technique delivering high-quality, low-energy beams. With this experimental scenario, the emphasis is on providing exotic nuclei which cannot be produced at ISOL facilities with sufficient intensities, e.g. short-lived isotopes and refractory elements. The ions are stopped and implanted into solid-state detectors with a delay time of less than 100 ns, in case of high-precision experiments in ion and atom traps the overall delay time amounts to about 10 ms.

The transport of relativistic heavy ions through matter within ion-optical systems [89, 90] has been pioneered and studied in great detail at the FRS. The corresponding degrader techniques and applications are well established and are in routine operation for most of the present FRS experiments. The nuclear and atomic interaction of heavy ions slowed down in matter have been the subject of detailed investigation [56, 91, 92, 93, 94]. The experimental results and their theoretical description, which has been developed in close collaboration with theoreticians, have yielded a reliable data set. This has been implemented in various simulation programs [95, 96] serving for the design of the Super-FRS and the low-energy branch. Gas cells behind in-flight separators are under development in RIKEN, MSU, ANL, and GSI. The present performance of these devices has not yet fully reached the design goals, but the principles have successfully been proven, validating the

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133

simulations. At the SHIPTRAP facility of GSI, first tests with the gas cell and extraction system behind SHIP have been initiated [97]. At the FRS, an experimental program has recently been started [98] in a collaboration of groups from RIKEN, ANL, and european institutes with the goal to develop, build, and to characterize a gas-catcher device for the FRS. Both projects will be further developed within the european RTD network ION CATCHER.

Details of the design goals, technical specifications, and expected performance of the energy buncher are found in Chapter 1.2.4.

Trap system

1m

Extraction RFQ

Cooler trap

Precision trap

Detector

LASER spectroscopy

Decay spectroscopy

.γ-ray spectroscopy

AGATAp

+ pδ

- pδ

p’

p’

p’

Si-Detectors

NaI - Crystals

Super-CloverGe-Detector

NaI - Crystals

Figure 1.34: Schematic view of the energy-buncher stage in the low-energy branch of the Super-FRS. The swift exotic nuclei delivered by the Super-FRS are spatially separated according to their momentum and then slowed down in the mono-energetic degrader to identical final mean momentum yielding low-energy radioactive beams. The principal experimental equipment is sketched.

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134

Experimental opportunities

At the low-energy branch, there are three ways to access and investigate the exotic nuclei. The energy-focused, low-energy beams can either be studied while in flight or implanted into a silicon-detector array or stopped in and re-extracted from a gas catcher for transfer into a trap system. The experimental program is presented in Table 1.3. High-precision spectroscopy of stopped exotic nuclei is addressed below in more detail, whereas γ−ray spectroscopy with stopped or slowed-down secondary beams is presented in Chapter 1.1.3.5. Atomic physics techniques applied to nuclear physics, such as laser spectroscopy, hyperfine structure studies etc., as well as applications in solid-state physics, medicine, and life science are discussed in a separate contribution to this Report.

Table 1.3: Experimental opportunities with the low-energy branch of the Super-FRS

Research field Experimental method Subject Minimum Intensity request (ions/s)

Nuclear structure and nuclear astrophysics

Decay spectroscopy Heavy-ion reactions near the Coulomb barrier

Half-lifes, spins, nuclear moments, GT strength, isomer decay B(E2), lifetime, moments, spins, high-spin states, spectroscopic factors

10-3...105

105...107

Atomic physics methods applied to nuclear structure

Laser spectroscopy

Nuclear charge radii Magnetic dipole and electric quadrupole moments

102…104 103

Precision experiments

Tests of the Standard Model

Penning traps Magneto-optical traps (MOT) Solid-state detector array

Nuclear binding energies (mass measurements, Q-values) Half-life measurements β-decay branching ratios β-ν correlations

1...103

1...103

1...103

103

Decay spectroscopy of stopped ions

The possibility to implant in-flight separated isotopes into arrays of thin silicon detectors allows for precision - and -decay measurements and offers several advantages over experiments at ISOL systems. Very short lived isotopes of all elements and isomeric states down to lifetimes in the 100 ns region can be investigated, an isotopic separation and a full identification of the implanted ion is

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135

possible and the number of implanted ions for the nuclear species of interest can be counted accurately. These advantages form the ideal basis for the determination of accurate -decay half-lives and branching ratios. On the other hand, the technique is ideally suited for exploratory measurements at the limits of known nuclei because the measurements can be carried out at ultimate sensitivity, i.e. with only a few ions of interest per week.

The pioneering work in this research field traces back to the identification of heavy elements and their -decay spectroscopy at the velocity filter SHIP of GSI, where superheavy elements have been identified by observing the decay sequence of the isotopes implanted into position-sensitive silicon detector arrays. Utilizing a similar technique at the FRS, half-life measurements of the doubly magic 100Sn were carried out with as few as seven atoms in one week [19]. Further proof for the expertise at GSI concerns (i) -decay measurements of 36,37Ca [99] yielding a relative accuracy of 6×10-3 for the half-life, (ii) a series of successful -decay studies of nuclei in the sd and pf shell (see e.g. [99, 100, 101]) as well as near 100Sn, with the GT resonance being experimentally identified in the latter region of nuclei [49]. The potential of the in-flight method for such experiments has been recognized worldwide in recent years. At MSU, β-decay branching ratios were recently measured for 32Ar (T1/2=98ms) with unprecedented accuracy [102]. At the in-flight separators LISE of GANIL and FRS of GSI, the 0+→0+ decays of the short lived (T1/2≈50ms) isotopes 78Y, 82Nb, 86Tc, 90Rh, 94Ag, and 98In were observed for the first time [103, 104]. Precision experiments along the N=Z line and the proton drip line aim at the test of the Standard model (see next section and Chapter 1.1.2.2), isospin mixing and proton-neutron pairing. As the release efficiencies are too small, short-lived isotopes of these “chemically difficult” elements can hardly be produced at ISOL facilities with sufficient source strength. Correspondingly, the in-flight method at the proposed facility at GSI would provide a superior alternative, as production rates of more than 105 ions/s are foreseen for N = Z nuclei up to 82Nb. Further applications in high-resolution γ-ray spectroscopy of extremely neutron-rich nuclei are discussed in Chapter 1.1.3.5.

Tests of the Standard Model and trap systems

Weak-interaction experiments aiming at the test of the conserved vector current (CVC) hypothesis and the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix were discussed in Chapter 1.1.2.2. For related tests of the Standard Model the determination of ft-values is required with a precision of a few times 10-4. This is achieved by measuring lifetimes, non-analog branching ratios and QEC values. The task of precision β-decay experiments can be illustrated by choosing the superallowed 0+→0+ decay of 62Ga as an example. Concerning the half-life of 62Ga, it is expected that the required accuracy can be achieved in the near future. For the observation of the non-analog branches of 62Ga, however, much higher intensities will be required. At the Super-FRS, intensities of 107 ions/s for the parent nucleus 62Ga and 108 ion/s for the daughter nucleus 62Ge are expected. These intensities may be compared to about 1200 ions/s available presently at the ISOL facility of GSI. For the QEC value of about 10 MeV, the required accuracy for the ft value of the order of 10-4 corresponds

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to an accuracy of the mass measurement of the mother and daughter isotope of δm/m ≈ 3×10-8 . Such extraordinary precisions can be obtained from high-accuracy mass measurements in a Penning trap mass spectrometer, as proposed at the low-energy stage of the Super-FRS. The ions will be energy-focused by the above mentioned energy-buncher stage, stopped in the gas cell, extracted within a few milliseconds, and transferred via the RFQ beam buncher to the Penning trap system. The layout will be similar to the ISOLTRAP experiment operated by GSI at ISOLDE/CERN. It consists of two Penning traps, a cooler trap for mass-selective buffer gas cooling, and a precision trap for high-accuracy mass measurements [105, 106]. At ISOLTRAP a relative mass accuracy of 1×10-8 is obtained for nuclei with half-lives of 1s, and in a recent experiment about 1×10-7 was reached for 33Ar [107] which has a half-life of only 174 ms. With stable nuclei, accuracies of 1×10-10 have been achieved [108]. The principal limitation for accuracy and resolving power is due to the Fourier limit and to statistics, i. e. the number N of detected ions. The statistical accuracy can be estimated as δm/m = R-1N–1/2. The resolving power R= ωc /∆ωc depends on the line width ∆ωc = 0.9/∆tobs of the cyclotron resonance, which is determined by the “observation” time ∆tobs of the ion motion in the trap, which in turn is limited by the nuclear half-life. In practice, a ∆tobs value of up to three times the nuclear half-life is appropriate. With B=10 T and a N value of the order of 104, this estimate yields a relative mass accuracy of δm/m = 4×10-9 even for short-lived nuclei like 62Ga. Thus the QEC value (about 9200 keV) can be determined with an uncertainty of about 0.3 keV, corresponding to a contribution of 1.6×10-4 to the uncertainty of the ft-value. With further improvements and developments like the use of magnets with higher field strength, excitation of higher harmonics of ωc etc., a still better performance will be possible. These are the goals of the european RTD network NIPNET.

Advanced tests of the Standard Model can be performed by means β−ν correlation measurements in a magnet-optical trap (MOT) (see Chapter 5). The experiments on short-lived atoms stored in a MOT, performed at TRIUMF [37] and Los Alamos [38], have succeeded in showing that by measuring β−ν correlations a 1% limit for scalar interactions can be obtained. At the future GSI facility, MOT experiments will allow one to considerably improve this limit by carefully checking the systematic errors of such experiments. Due to the element independent, free choice of the mother nucleus and the availability of new possible candidates, it is expected that these opportunities will yield an important contribution to this exciting research field.

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1.1.3.5 High-Resolution γγγγ-Ray Spectroscopy

High-resolution γ-ray spectroscopy has contributed most essentially to our present knowledge of the structure of nuclei. During the past decade, the field of γ-ray spectroscopy was boosted due to the installation of large arrays of high-resolution, Compton-suppressed germanium detectors of extraordinary efficiency. Arrays, such as EUROBALL [109], a european joint venture, and its counterpart, the GAMMASPHERE [110] in the USA, may be named explicitly though a number of other multi-detector devices exist all over the world. EUROBALL, for instance, was operated at various accelerator facilities in Europe, demonstrating its high flexibility and capabilities in nuclear structure and related studies. Yet, substantial improvements in terms of detection efficiency, background suppression and other features are still feasible, a fact which led to the presently ongoing developments of ’γ-ray tracking’ devices. They are supposed to provide full-energy detection efficiencies almost approaching unity by means of a completely new technique.

Evidently, high γ-detection efficiency is an important issue in conjunction with secondary exotic nuclear beams. Throughout the preceding Chapter 1.1.3.3 and 1.1.3.4 it became evident that γ−ray spectroscopy is indispensable. This chapter comprises the potential applications of high-resolution γ-ray spectroscopy as foreseen for the proposed exotic nuclear beam facility. The unique properties of the new-generation detector arrays can be exploited over the full range of beam energies provided by the proposed facility:

• Stopped secondary beams allow for β-decay studies, isomer studies, and measurements of nuclear moments by means of implantation techniques, taking advantage of the Super-FRS energy-bunching stage (see Chapter 1.1.3.4). Exotic nuclei obtained at production rates even down to 10-3 ions/s and with (isomeric) lifetimes down to about 100 ns can be investigated.

• In-beam studies at secondary beam energies from several tens to several hundreds of MeV/u provide nuclear structure information on single-particle properties, nuclear deformations and high-spin states by means of knockout reactions, relativistic Coulomb excitation, and secondary fragmentation, respectively. The use of thick targets ensures high luminosities, so that such reaction studies can be performed at beam intensities as low as 1 – 1000 ions/s depending on the specific reaction.

• In-beam studies at beam energies around the Coulomb barrier are, in principle, feasible with slowed-down beams, utilizing the energy-bunching stage of the Super-FRS. At such energies, multiple Coulomb excitation, heavy-ion fusion, and direct reactions are of interest. Comparatively high secondary beam intensities of at least 105 ions/s are required. The energy domain around the Coulomb barrier certainly is a field to be addressed in the first place by advanced ISOL facilities equipped with a post-acceleration stage. Nevertheless, as discussed earlier, certain regions of the chart of nuclides, in particular very short-lived isotopes of “difficult” elements, cannot be accessed by ISOL techniques.

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The feasibility of such experiments requires optimization of the following design goals of an advanced γ-spectrometer: Maximum photo-peak efficiency and excellent spectral response, as measured by the peak-to-total ratio; very good angular resolution for the emission direction of the γ rays to enable Doppler correction even at the highest ion velocities; high granularity and high rate capability in order to cope with background radiation; a versatile geometry allowing for combination with ancillary detection systems to increase selectivity for exotic species.

To meet these requirements, an Advanced Gamma-ray Tracking Array, AGATA, for high-resolution γ-ray spectroscopy has been proposed by an european collaboration. A detailed technical description of this device is found in the appendix to this Report, its basic properties are summarized in Chapter 1.2.5. The GRETA detector, presently under development in the USA, follows a similar concept [111]. Selected aspects of γ-ray spectroscopy are addressed below.

Decay studies and isomeric beams

Studies of β-decay or decay from isomeric states, populated in the fragmentation reaction producing the exotic nuclear beam itself, take advantage of the energy-focusing property of the Super-FRS energy-buncher. This technique allows a controlled implantation into an active detector, providing a unique identification of the stopped nuclei of interest. Using detectors such as AGATA, γ-ray spectroscopy following β- or isomer decay may be performed for isotopes produced with low intensity ( < 1 ion/s) and for nuclear states with half-lives down to 100 ns. A prime example for such an experiment at the proposed facility is 78Ni, with N/Z =1.8 the most neutron-rich of the accessible doubly-magic nuclei, which was first observed at GSI [25]. It is situated close to a key point of the astrophysical r-process path and in close vicinity to a region where a significant shell quenching is predicted (see Figure 1.6). Signatures for a persisting shell gap are provided by seniority and spin-gap isomers, while quenching of shell gaps goes along with disappearing isomerism and increasing collectivity. At the proposed facility, the expected intensity of 78Ni amounts to 10 ions/s thus allowing for detailed γ-ray spectroscopy subsequent to isomer- and β-decay. The feasibility of isomer decay studies is demonstrated in Figure 1.35 for 78Zn, the nucleus closest to 78Ni studied so far. The γ−ray spectrum was obtained in a fragmentation experiment at GANIL [112] with a 78Zn intensity of about 1 ion/s. Secondary beam intensities in the range of 1 – 1000 ions/s will also be obtained for nuclei e.g. around 100Sn and for neutron-rich isotopes with N/Z ~ 1.8 between the Z=50 and Z=82 magic lines, see Figure 1.17. The decay of a high-spin K-isomer of the heavy neutron-rich (N/Z = 1.56) nucleus 190W, produced at the present GSI FRS facility with an intensity of only 0.2 ions/s [113], is shown in Figure 1.35 to indicate the future potential. Implantation techniques, moreover, allow nuclear spins and moments to be measured by means of perturbed γ-ray angular distribution studies, as was demonstrated in pioneering experiments at GANIL [114].

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0+

(2 )+

(4 )+

(6 )+(8 )+

0

730

1620

25282673

730

890

908

145

T1/2=0.32(1) µs

30 48

coun

ts

E [keV]γ

78 Zn

Figure 1.35: Isomeric γ-decay of 78Zn [112] (left panel); Isomeric γ-decay of 190W [113] (right panel).

In-beam spectroscopy at intermediate energies

Detailed spectroscopic information can be deduced with γ-ray spectroscopy in reactions of secondary beams at intermediate energies of several tens to hundreds MeV/u. Thick targets, of the order of g/cm2, can be used, the relevant cross sections being of the order of 0.1 – 1 barn. Thus, experiments are feasible with very modest beam intensities. This was demonstrated in measurements performed at the in-flight facilities of GANIL, MSU, RIKEN, and GSI. Based on this experience, the potential of in-beam spectroscopy at intermediate energies is evaluated below. Such experiments would be performed at the high-energy branch of the Super-FRS using the apparatus described in Chapter 1.1.3.3 with the γ-detection system arranged in 4π geometry around the target.

Single-particle structure. Single- or few-nucleon knockout and Coulomb breakup reactions allow the orbital momentum quantum numbers to be determined and precise spectroscopic factors to be deduced. For this purpose, γ rays from the excited states of the residual nuclei need to be measured. The method was applied so far to nuclei near the neutron drip line in the mass range A = 10 – 20 (see e.g. [80]). As an example, a measurement performed at MSU [79] is shown in Figure 1.36, where the γ spectrum is displayed as measured in coincidence with 10Be fragments after 1n knockout from 11Be. Spectroscopic factors were deduced for the ground and excited states. This method can readily be extended to heavier nuclei. Of particular interest

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are neutron-rich nuclei near the shell closures N = 20, 28 and N=50, 82, the latter ones being produced from 238U projectile fission.

Figure 1.36: Gamma decay of 10Be after a one-neutron knockout reaction of 11Be, measured at MSU. From the partial 1n knockout cross–sections, determined for various 10Be states, the associated spectroscopic factors were deduced. Adapted from [79].

Energy (keV)

Cou

nts

/ 40

keV

2+ → 0+

2- → 2+

1- → 2+

1- → 0+

Irel

0.09

0.07

0.06

0.78

2- 6.26

1- 5.96

2+ 3.37

0+ 0

10

10 2

10 3

2000 4000 6000

Deformations. Projectile Coulomb excitation in heavy-ion collisions at beam energies far above the Coulomb barrier is an excellent tool to measure accurate B(E2) values. An early result was the confirmation of the sudden onset of quadrupole deformation in the N=20 nucleus, 32Mg [28], obtained from γ−ray spectroscopy of the 2+ state. Similar measurements were performed for heavier neutron-rich nuclei, e.g. the chain of sulfur isotopes, see [115] for a review. Experiments of this type allow in general to map the evolution of nuclear deformations over the full range of structural symmetries between the U(5), SU(3) and O(6) limits. Islands of octupole deformations or of other multipolarity can be accessed by the same method. At the new facility it will be possible to enter the region of nuclei with large neutron excess between the N = 50 and N = 126 shells, a region not accessible at existing facilities. Experimental experience shows that one has to cope with a large Bremsstrahlung background in such measurements, the discrimination of which requires a Ge detector array of high granularity.

High-spin states. The population of high-spin states in high-energy fragmentation reactions is a novel concept that, at present, still needs further exploration. The process of multi-nucleon knockout in a fragmentation reaction leaves the residual nucleus in a highly excited states with considerable angular momentum. The characteristics of the ‘entry line’ prior to particle evaporation are very similar to those obtained in a low-energy heavy-ion fusion reaction. On the basis of abrasion-ablation models, quantitative predictions for spin populations were obtained [116]. Figure 1.37 shows the calculated spin distributions for residual nuclei produced by fragmentation

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of a 208Pb beam at 1 GeV/u. A qualitative experimental confirmation was obtained by the observation of isomeric states with spin values up to 35/2 after fragmentation, examples are shown in Figure 1.35. Likewise, short-lived high-spin states should be populated, the γ-decay of which could be observed in in-beam experiments in coincidence with the decaying fragment. The apparatus described in Chapter 1.2.3. would serve to identify the latter one.

Figure 1.37: Spin distribution of nuclei produced in fragmentation of a 208Pb projectile at 1 GeV/u [116].

Nuclear moments. Another unique feature of high-energy reactions concerns the atomic charge state of the fragments. Choosing the appropriate kinetic energy, hydrogen-like and few-electron ions will emerge from the reaction target. In heavy ions extraordinary strong hyperfine fields will thus arise. These can be utilized for the measurement of nuclear moments of short-lived excited states, which enables to determine specific nucleonic (proton/neutron) components of the nuclear wave functions as well as nuclear shapes. The technique used for lifetimes in the range of 10-12 s is projectile Coulomb excitation in combination with strong transient magnetic hyperfine fields and electric field gradients. Field strengths of the order of 103 T and 1021 V/cm occur when the highly charged ions move through ferromagnetic materials [117]. Precessions of the nuclear moments are observed in measurements of particle-γ angular correlations using γ detectors at fixed polar angles in the reaction plane and an annular particle detector with azimuthal position sensitivity. For the detection of quadrupole precessions, sufficient polarization of the nuclear state is provided by the Coulomb excitation process. Under these conditions, measurements were successfully performed on several nuclei at energies of ~10 MeV/u [117]. At much higher projectile energies (~50-100 MeV/u) the strong Lorentz boost of the γ-intensity distribution requires position-sensitive γ detectors, such as the AGATA array, at forward angles.

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In-Beam spectroscopy with decelerated beams

A wealth of nuclear structure information has been derived from heavy-ion induced reactions in conjunction with high-resolution γ-ray spectroscopy. Inelastic nuclear scattering, multiple Coulomb excitation, direct reactions and fusion-evaporation at energies around or slightly above the Coulomb barrier are standard tools. It is intriguing to transfer the experience collected in nuclear structure investigations with stable-isotope beams to beams of exotic nuclei. Clearly, an ISOL scheme for the production of unstable nuclei, equipped with a post-acceleration stage seems most appropriate for that purpose. ISOL facilities with post-accelerators are already available today, and more advanced ones are under construction or consideration, as has been mentioned in the introductory chapter. Depending on chemical properties, however, some elements are difficult to extract from the production target; the time scales for extraction and re-acceleration, in addition, do not allow isotopes with half lives considerably below 1 second to be treated. Therefore, the option of secondary low-energy fragment beams will be briefly addressed here.

In principle, the energy-buncher stage of the Super-FRS can decelerate secondary fragment beams to any energy with negligible time delay (< 100 ns) and can direct the beam onto a target without significant loss in intensity. Although the beam quality in terms of spread in kinetic energy and emittance is rather poor, see details in Chapter 1.1.3.4 and Chapter 1.2.4, it imposes no principal limitations.

In contrast to experiments at high energies, only thin targets of the order of mg/cm2 can be used. Integrated cross sections for multiple Coulomb excitation or fusion-evaporation reactions are of the order of barns. Assuming a cross section of 1 barn, a 4π solid-angle coverage for γ-rays, and a photo-peak efficiency of 20%, as would be provided by an instrument such as AGATA, one arrives at an observed γ-ray rate of the order 10−6 per incident exotic nucleus. It appears that γ spectroscopy becomes feasible for exotic nuclear beams with intensities of at least 105 ions/s. Though very exotic nuclear species are excluded from such measurements, the presently known part of the chart of nuclides is covered, while for nuclei with Z > 50 even regions beyond can be reached, as can seen from Figure 1.17. This is important as, in particular on the neutron-rich side, spectroscopic information is scarce aside from ground-state and decay properties.

High-spin states in neutron-rich lanthanide nuclei, just in order to provide one specific example, can be produced in fusion-evaporation reactions with e.g. a 132Sn secondary beam on a neutron-rich target such as 48Ca. 132Sn would be delivered at the proposed facility with a rate of about 108 ions/s. Neutron-rich lanthanide isotopes are predicted to be stable against fission up to very high angular momentum ( ~ 80 • ). Such an extension of the accessible spin range may lead to the observation of exotic nuclear shapes, in particular it would open up a new possibility to search for hyper-deformed states.

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1.2 Instrumentation and Detectors

1.2.1 The Fragment Separator

To effectively pursue the diverse research program with exotic nuclear beams, as described in the previous chapter, dedicated instrumentation will be developed. This includes, first of all, a new powerful fragment separator which provides spatially separated isotopic beams. The proposed separator, the Super-FRS, is superior to its predecessor, the FRS, due to the incorporation of more separation stages with large-aperture superconducting magnets. The Super-FRS is based on techniques and methods which were pioneered and developed for relativistic heavy ions at the existing FRS facility. The beams of separated rare isotopes will be transported to three principal experimental areas. The first branch of the Super-FRS is dedicated to experiments requiring uranium fragments of high energy up to 1.5 GeV/u. The second branch allows experiments with low-energy and stopped fragments and the third one combines the fragment separator with an ion storage and cooler ring system (CR, NESR) to perform precision mass and lifetime measurements and moreover enables a whole new generation of scattering experiments. Each of these experimental areas includes several target stations and sophisticated tracking, identification and measurement equipment. The coupling of a storage-cooler ring with a fragment separator was pioneered at GSI, realized in the present FRS-ESR system. The Super-FRS can also be used as a high-resolution magnetic spectrometer, a feature which led to important discoveries with the present FRS.

This chapter describes the proposed Super-FRS and the associated instrumentation and discusses the research and development program which is necessary to cost-effectively optimize construction and performance. The aim is to provide a world class in-flight facility in Europe for the international research community.

The proposed Superconducting projectile FRagment Separator, Super-FRS, builds and expands upon the considerable experience made with the present, first-generation system. The existing FRagment Separator, FRS [60], at GSI provides high-energy, spatially separated monoisotopic beams of exotic nuclei of all elements up to uranium. The accessible isotopic lifetimes, given by the time of flight through the ion optical system, range from the sub-microsecond domain upwards. The FRS is the key instrument of the exotic beam facility at GSI. It has proved to be an extremely versatile instrument for fundamental nuclear and atomic studies, as well as for experiments in applied physics. More than 150 new isotopes have been discovered and studied directly at the FRS, including the doubly magic nuclei 100Sn [19] and 78Ni [25]. Additionally, it delivers beams for decay and reaction studies to the ALADIN-LAND setup [118, 119] and the KAOS spectrometer [120], and for precision mass and lifetime measurements to the experimental storage-cooler ring ESR [62]. Moreover, the FRS has been used as a high-resolution magnetic spectrometer. Precise momentum measurements lead to the discovery of new halo properties [13, 121], the first observation of deeply bound pionic states in heavy nuclei [87], and several fundamental properties in atomic collision physics [91, 92, 94, 122]. In several fields of

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applied physics important studies were performed, such as the first optimizations of heavy-ion beams and PET diagnostics [123] for the GSI cancer therapy project. Other contributions to applied physics include the calibration of detectors for NASA spacecraft missions.

In pioneering experiments studying fission of 238U projectiles, it became clear that projectile fission at relativistic velocities is a rich source of very neutron-rich nuclei of medium mass. About 120 new neutron-rich isotopes were discovered. However, due to the relatively large spread in total kinetic energy of the fission fragments, their transmission through the FRS is typically only a few percent compared to projectile fragments which have a transmission of 70% or more. A first important improvement with the new separator will be to substantially increase the acceptance for all fragments.

Based on the experience gained with the FRS and the related experiments, the new facility at GSI is in an ideal position to fully profit from the advantages of a high-energy in-flight facility [124]. To access the rarest exotic species, which become available with the increased primary beam intensities, a clean separation is absolutely necessary. This requires the production of fully stripped ions to avoid ambiguities from ionic charge states as well as multiple separation steps to achieve the required background rejection. From these considerations, design parameters for the next generation in-flight facility in Europe were determined at an international NuPECC workshop [88].

The proposed Super-FRS facility allows experiments with fast secondary beams up to a magnetic rigidity of 20 Tm ("High-Energy Branch") and experiments with low-energy and stopped exotic nuclei ("Low-Energy Branch"). The latter branch, which delivers beams with magnetic rigidities up to 10 Tm, includes a special dispersive separator stage. This acts in combination with several profiled energy degraders, including a monoenergetic degrader [89], to reduce the energy spread of the fragments. Additionally, bunched secondary beams are injected into the Collector Ring CR with rigidities up to 13 Tm (“Ring Branch”). The corresponding experimental areas at the final focal planes of the different branches are indicated in Figure 1.38.

The high ion-optical resolving power of 1500, at an emittance of 40 π mm mrad, simultaneously satisfies the requirement of clean isotopic separation and that of a high-resolution spectrometer, a combination of properties which has already been proven to be beneficial during FRS operation. In this respect, the Super-FRS is even more flexible due to the additional pre-separator section and the larger phase-space acceptance at the same ion-optical resolving power.

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Pre-Separator Main-Separator

SIS-200

Low-Energy BranchAG

ATA

50 m

Ring Branch

10 / s12

1.5 GeV/u

Proton - UraniumProjectiles

Figure 1.38: Schematic layout of the proposed Super-FRS exotic nuclear beam facility, shown to scale. The three main branches of experimental areas are indicated.

1.2.1.1 Operating Domain

An essential requirement for the performance of the Bρ-∆E-Bρ separation method [60] (Bρ: magnetic rigidity, ∆E: energy loss in shaped degraders) is that the primary beams and the fragments should be fully stripped with only a negligible contribution from other charge states. This can only be achieved if the velocities of the heavy ions are sufficiently high whilst penetrating the layers of matter within the Super-FRS, i.e. the production target, energy degraders, and detectors. On the other hand, the thickness of matter has to be optimized to prevent substantial losses due to nuclear reaction (nuclear absorption).

The two main design criteria, the charge-state population and the nuclear absorption, are shown in Figure 1.39 as a function of the atomic number Z of the separated fragments. The energies shown are for where 50% nuclear reactions in a niobium degrader is reached, with a thickness of half the atomic range. In addition, the energies are given where the equilibrium charge state population reaches 80, 90, and 95% fully stripped fragments.

The conclusion from an optimization of the conditions mentioned above and from the experience with present experiments is that the Super-FRS should accept beams up to a maximum magnetic rigidity of 20 Tm corresponding to 238U92+ ions of 1566 MeV/u. With this layout the Super-FRS will be unique worldwide particularly with respect to the isotopic separation of heavy exotic nuclei.

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Nb degrader

Figure 1.39: Operating domain of the Bρ-∆E-Bρ separation method calculated for a Nb degrader.

1.2.1.2 Design Goals and Performance

The FRS was primarily designed for the separation of projectile fragments. To make use of the powerful production method of uranium fission, it is necessary to drastically increase the phase space acceptance. The design of the Super-FRS has been based upon this criterion. In Table 1.4, the main parameters of the FRS and the Super-FRS are compared. Though the phase-space acceptance has been greatly increased for the Super-FRS, the ion-optical resolving power has been preserved in order to guarantee the separation quality and also the momentum resolution for the spectrometer option. The resulting larger apertures in the magnets require larger fields, which can only be achieved in a cost-effective way by means of superconducting magnets.

At the higher energies available at the new facility, it is possible to use much thicker production targets, resulting in greatly increased production rates. Additionally, the transmission is increased as the fragment beams are strongly forward focused due to reaction kinematics at relativistic energies.

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Table 1.4: Momentum and angular acceptance of the present FRS and the proposed Super-FRS

Facility ∆p/p ∆Φx ∆Φy Resolving Power

FRS (Bρmax = 18 Tm)

± 1 % ± 13 mrad ± 13 mrad 1500 for20 π mm mrad

Super-FRS (Bρmax = 20 Tm)

± 2.5 % ± 40 mrad ± 20 mrad 1500 for40 π mm mrad

The benefits due to the increased production rates and transmission at high energies is demonstrated for two examples in Table 1.5 and Table 1.6. The rates of 132Sn isotopes produced via fission of 238U projectiles and 24O from the fragmentation of 48Ca, separated in flight by a separator system, with the same ion-optical parameters as the Super-FRS, were calculated. In this example, the incident energy of the primary beam was varied between 100 MeV/u and 1500 MeV/u. An improvement of a factor of 150 and 40 can be seen for the fission fragments and the fragmentation, respectively. These examples are generally representative of all fission fragments and exotic nuclei produced via projectile fragmentation in the case where the fragment drastically differs in mass and atomic number from the primary beam. Therefore, it is desirable to perform experiments at energies as high as possible, depending on the specific requirements of a given experiment. The chosen fixed acceptance of the assumed ion-optical system is a reasonable upper limit to guarantee a good isotopic separation performance.

Table 1.5: Gain in rate of 132Sn nuclei, after separation, with increasing kinetic energy

Energy of 238U [MeV/u]

Optimum target thickness [g/cm2]

Rate of 132Sn produced with 1012 /s 238U ions [1/s]

100 0.1 2×105

400 1.0 9×106

1000 3.0 3×107

1500 3.5 3×107

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Table 1.6: Gain in rate of 24O nuclei, after separation, with increasing kinetic energy

Energy of 48Ca [MeV/u]

Optimum target thickness [g/cm2]

Rate of 24O produced with 1012 /s 48Ca ions [1/s]

100 0.43 300

400 2.8 5800

1000 6.5 12000

1500 8.5 12000

A comparison of the improved transmission for uranium fission products as a function of the element number of the fragments is illustrated in Figure 1.40. In addition to the gain in transmission, it will also be possible to profit from the high-current performance of SIS-200 in providing heavy primary beams, such as lead and uranium ions, with a factor of 104 higher intensities than the present SIS-18. For experiments with the proposed new storage ring system we win an additional factor of 50 in the injection system compared to the FRS-ESR facility.

T

Figure 1.40: Comparison of transmission through the Super-FRS and the FRS for uranium fission fragments. The gain in transmission with the planned Super-FRS in the region of 78Ni is a factor of 30.

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SIS 200: 2/s

SIS 200: 65/h

SIS 200: 1 10 /s9

SIS 200: 8/s

SIS 200: 1 10 /s8

Figure 1.41: Chart of the nuclei, high-lighting the doubly magic nuclei far from stability. The doubly magic nuclei 78Ni and 100Sn, which were discovered at the FRS are shown with the rate with which they were produced. The expected rates, directly after the production target, with the new facility are also given.

Thus, in total, the rate will be up to a factor of 105 higher than those presently available at the FRS. This is illustrated in Figure 1.41 by a rate comparison and prediction for the rarest doubly magic nuclei.

The ion-optical layout of the Super-FRS and its imaging conditions are presented in Figure 1.42. The envelopes and the dispersion line are plotted for 40 π mm mrad and 2.5%, respectively. The system consists of the pre-separator and the main-separator, each equipped with an energy degrader stage. Each of the dipole magnets bend the beam by 28º. The pre- and main-separator are achromatic systems, hence the complete system is also achromatic. The achromatism allows an image at the final focal plane which is independent of the momentum spread of the fragments at the entrance of the system. The ion-optical plot shows the image conditions in the x-direction, the dispersive plane, and in the perpendicular coordinate, y. The quadrupole magnets are necessary to guide the fragment beam, to achieve the optimum resolution by proper illumination of the dipole magnets, and to focus the beam at the different image planes. The sextupole and octupole magnets are essential to correct the image aberrations which would otherwise ruin the necessary separation performance. Due to the much larger phase-space acceptance, the optical aberrations in the Super-FRS are much more critical than in the present FRS, where only sextupole correction fields are applied.

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Length / m

0

10

20

30

10

20

30

y /

cm

x / c

m

dipole quadrupole + octupole sextupole magnetPre-Separator Main-Separator

Figure 1.42: Ion-optical elements, beam envelopes (green lines), and the dispersion line for 2.5% momentum deviation (black line) are shown in the lattice of the Super-FRS. Only the scheme of the high-energy branch is plotted here.

The separation method of the Super-FRS is in principle similar to that of the FRS, i.e. a two-fold magnetic rigidity analysis before and after a thick energy degrader. The combination of atomic energy loss and magnetic rigidity analysis, the Βρ−∆Ε−Βρ separation, provides spatially separated monoisotopic fragment beams. However, special measures have to be applied due to the higher phase-space acceptance and the expected much higher projectile beam intensities. The planned intensity upgrade of beams from the accelerator will lead not only to larger rates for primary fragments, but also to larger rates for contaminants compared to the present situation at the FRS. However, by combining two separator stages, a pre-separator and a main-separator, the separation at the final focal plane will be as good as, or superior to that at the present FRS.

In Figure 1.43 the separation performance of the Super-FRS is illustrated for a very challenging example. A 1.5 GeV/u 238U primary beam is focused onto a 4 g/cm2 carbon target with the goal of providing spatially separated 132Sn ions. The area of the isotopes in the N-Z plane represents the corresponding intensities at the different focal planes. These intensities were obtained, using the Monte-Carlo simulation program MOCADI, by convoluting production probability and transmission at each element of the separator. The remaining contamination of the spatially separated fragments at the final focal plane can be eliminated using event-by-event particle tracking combined with time-of-flight and energy-deposition measurements.

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Z

N

Sn132

Pre-Separator Main-Separator

Sn132Sn132

primary beam

SIS-200

Figure 1.43: Projection of the Super-FRS in the dispersive plane showing the ion-optical elements and the separation of ions at each stage. It is essential that the primary beam is removed prior to the first degrader as indicated at the first dipole stage. For simplicity, only the envelopes of the ions with the mean reference magnetic rigidity at the optical axis are shown. The red envelopes denote the selected fragment, 132Sn in our example, along with contaminants as shown in the chart of nuclei shown below.

1.2.1.3 Production Targets

The requirements for the production targets are determined by the parameters of the primary beam from the proposed synchrotron SIS-200. Intensity, energy, time structure, and phase space of the projectiles are the main characteristics, which are summarized for the most crucial projectile, 238U ions, in Table 1.7.

The proposed new synchrotron, SIS-200 requires new technical developments for the production targets to use it to its full. The challenge for target technology is that the intensity of the primary beam will be increased to more than 1012 ions /s. The optimum target thickness for the production and separation of exotic nuclei is strongly dependent on the reaction mechanisms employed, on the projectile and its energy, as well as on the specific experimental conditions. Some general criteria for the production targets are outlined below. Using projectile fragmentation and projectile fission, the optimum target thicknesses range from a few g/cm2 up to 10 g/cm2 depending on the Z of the projectile and the energy selected. The key parameter for target design is the power density deposited by the primary beam and the fragments. Since only ions lighter than the projectile are created, it is safe to consider only the incident beam parameters.

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Table 1.7: Key parameters of the primary beam for the Super-FRS provided by SIS-200 for 238U ions

Primary Beam Parameters Slow-Extraction Mode Fast-Extraction Mode

Intensity 1012 /s 1012 /spill

Spill length DC 50 ns

Max. Energy (238U) 1500 MeV/u 1500 MeV/u

Emittance (εx, εy ) (13, 5 ) π mm mrad (26, 10 ) π mm mrad

Beam spot 1 mm 1 mm

Momentum Spread ± 0.07 % ± 1 %

The accelerator parameters, listed in Table 1.7 above, clearly demand two different types of targets depending on the extraction mode. The experiments performed in the low- and high-energy branch of the Super-FRS require slow extraction from SIS-200, whereas in combination with the CR and NESR, only the fast extraction mode provides an efficient injection of secondary beams into the storage rings.

Targets for Experiments Using Slow Beam Extraction

Using slow extraction, the intensity of the primary beam can be distributed over several seconds, as it is the case with the present SIS-18. When using magnetic rigidities up to 100 Tm, the two-ring structure of SIS-200 also allows a quasi-continuous (DC) beam. Since the size of the beam spot in the direction of the dispersive coordinate (x) is crucial for the ion-optical resolution of the Super-FRS and thus for the separation performance, it is not possible to simply reduce the power density in the target by increasing the beam-spot size. Standard deviations of σx =1-2 mm will be aimed for. Therefore, effective cooling and the use of rotating-wheel targets are two measures to cope with the power density in the targets.

In an example, we calculate the temperature of a carbon target under the bombardment of a slowly extracted uranium beam at an intensity of 1012/s. Since uranium has the highest energy deposition, all other primary beams will be less critical with respect to target damage. For slow extraction, the temperature evolution in solid target material depends on the deposited energy, the time structure of the beam, its spot size, and cooling conditions due to radiation and conductivity, which can be calculated by classical heat conduction theory. Rotating the target leads to a significant increase of the limits in energy deposition as the irradiated area is increased. We foresee a target wheel with 10 cm inner radius (Rc) mounted on a tube with a liquid coolant with an equilibrium temperature of 450 K.

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Ion beamRt Rb

Rc

RtRbRc

10 11 12 13 14 15300

400

500

600

700

800

1 g/cm2

6 g/cm2T

[K]

R [cm]

10 cm

Figure 1.44: Temperature distribution along the radius of two carbon targets with 1 and 6 g/cm2 thickness, bombarded at radius Rb with 1 GeV/u 1012/s uranium ions. The target wheel is mounted on a tube with radius Rc. The total target radius is Rt.

In Figure 1.44, the temperature distribution along the radius of two carbon targets is calculated assuming incident uranium ions at a rate of 1012 /s. The target wheel is assumed to rotate with a frequency of 1 Hz. The incident beam energy was assumed to be 1 GeV/u, the beam intensity to be constant over time, and the spatial distribution to be of Gaussian shape with σx = σ y = 1 mm and centered at the radius Rb. The calculated maximum temperature increase is much smaller than the critical sublimation temperature of 3377 K. The target handling for slow extraction is therefore feasible with current technology, however further research and development into the realization is required.

Targets for Experiments Using Fast Beam Extraction

The characteristic target properties for fast extraction have been simulated in two-dimensional hydrodynamical calculations which describe the heavy-ion target interaction [125]. The primary goal is to find the conditions for minimum hydrodynamical expansion of the target material in the beam heated region. If the target were to expand, its density would decrease drastically and a large fraction of the projectiles would penetrate the target with considerably fewer atomic and nuclear interactions. This would not only cause a substantial loss in the optimum production rate of exotic nuclei but also would result in an increased energy spread and thus transmission losses.

Representative results are shown in Figure 1.45, again for the most crucial case of 1012 uranium projectiles extracted in a single bunch of variable time duration. The atomic energy loss of the emerging uranium ions provides a sensitive monitor for the target conditions during irradiation. In this calculation we have chosen targets of 5.5 g/cm2 lead and 3.4 g/cm2 lithium. It can be seen that the fractional energy loss in the

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target increases with extraction time. The variation in production rate and energy straggling due to the change in thickness is comparable with the width of the inevitable statistical distribution for the fragment production, for extraction times of 50 ns and below. However, the enormously large power deposition results in a temperature increase (>104 K) which exceeds the melting point of any material by several orders of magnitude under the required condition that the beam spot has a circular size of σ = 1 mm. This can be combated in two ways, namely the beam spot size can be increased, or the target can be replaced after each spill.

Using a beam spot of σ = 4 mm causes a decrease of the deposited power density and allows the heat transfer mechanism to be described by classical heat conduction theory. For example, the maximum temperature developed in a slowly rotating 2 g/cm2 carbon target remains below the sublimation temperature. This larger beam spot can be compensated for by using the two storage rings, in combination with a degrader stage in between, to provide the required fragment separation.

0 10050 200 3000.74

0.76

0.78

0.80

0.82

Pb

Li

E out

/E in

Extraction time / ns150 250

Figure 1.45: Fractional energy loss of 1 GeV/u 1012 incident uranium ions in 5.5 g/cm2 lead or 3.4 g/cm2 lithium as a function of bunch duration and for a beam spot of σx = σy = 1 mm. The calculations show that for the range below 50 ns there is practically no hydrodynamic expansion of the target material during irradiation.

The principal result of these studies is that the duration of the extracted beam bunch must be 50 ns or shorter, a condition which is also required for bunch rotation and successive stochastic cooling in the CR. Using a 1mm beam spot, the target has to be replaced after each high-intensity 50 ns pulse. A possible technical solution could be a windowless liquid lithium target, a technology which is presently under investigation worldwide.

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Radiation Shielding and Remote Target Handling

Radiation shielding and remote target handling with a robot are already successfully applied at the present FRS for the maximum beam intensities currently available at SIS18, e.g., a few times 1011 /s of 2H or 18O projectiles. It is foreseen that both the present knowledge and equipment will be transferred to the Super-FRS.

1.2.1.4 Secondary-Beam Intensities

Of key importance to the new facility are the secondary-beam intensities, which are determined by the intensity of the primary beam, the production cross section, the target thickness, and the transmission through the Super-FRS as well as the separation performance.

The optimum values for target thickness and incident-beam energy are chosen according to the following arguments:

• The optimum target thickness dopt is defined as the thickness giving the maximum yield for fragments near to the primary beam in mass and nuclear charge at the exit of the target. The value of dopt is almost independent of beam energy.

• The optimum beam energy is defined by the condition that the range of the projectile is 3 times the optimum target thickness, this assures a relatively small emittance of the secondary beam.

Figure 1.46 illustrates that primary-beam energies of the order of 1 to 1.5 GeV/u are required for optimum production conditions. If needed, these high-energy secondary beams can be slowed down in matter, or even stopped with only moderate losses in a thick tantalum stopper.

Expected rates for isotopic chains of some representative elements are presented in Figure 1.47. Projectile fragmentation and fission of 238U are the two reaction mechanisms applied in this rate prediction [56, 54].

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E [MeV/u]

stop

P2

Rang

e [g

/cm

]

E [MeV/u]

Figure 1.46: Parameters relevant for the primary beam energy selection for an in-flight facility. Upper part: Ranges of 86Kr (left) and 238U (right) ions in carbon (solid black line) as a function of the incident beam energy. In addition, the optimum target thickness dopt , which yields maximum production, is shown. The arrows indicate the optimum energies, 1.2 GeV/u for 86Kr and 1.5 GeV/u for 238U which assure a good secondary-beam quality. Lower part: Nuclear survival probability Psurvival for stopping projectile fragments in tantalum without nuclear reactions. The calculated properties are valid for fragments which are close in mass and nuclear charge to the primary beam.

Figure 1.47: Predicted rates for isotopes of elements from Ar to Fr. The primary beam intensity is assumed to be 1012 ions/s and the optimum target thickness dopt, as described in the text, was chosen. Note that in this prediction the stable nuclei, marked in black color, are also produced as fragments.

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1.2.2 Experiments in the Storage-Cooler Rings

The proposed heavy-ion storage and cooling facilities will provide excellent research opportunities based on both new and well established experimental techniques. The main goals are high-precision mass, lifetime and in-ring reaction studies. In addition, a completely new field will be opened with electron scattering off exotic nuclei.

100

m

Figure 1.48: Instrumentation for experiments with stored beams. Beams of short-lived nuclei from the Super-FRS are collected and stochastically cooled in the Collector Ring (CR). For experiments which require the highest phase-space density, the fragment beam is then transferred to the New Experimental Storage Ring (NESR) where they are cooled further by electron cooling. For the shortest-lived isotopes (T1/2 ≤ 0.5 s down to the µs range) experiments can be performed in the CR. The NESR is equipped with an internal gas target for reaction studies and a small electron storage ring for structure studies with scattered electrons.

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The ESR is the first and presently only experimental storage ring for heavy ions of several hundred MeV/u energy. It has been designed primarily for the storage of low-emittance primary beams from the synchrotron for atomic physics experiments. Consequently, the injection efficiency for large-emittance fragment beams from the FRS is rather small, of the order of a few percent. Nevertheless, a number of pioneering nuclear physics experiments, requiring only low beam intensities, such as mass and half-life measurements were performed and have demonstrated the large research potential [63, 64, 52, 65]. Nuclear reaction studies with secondary beams using the internal target are, to date, not possible. Therefore, the proposed ring system in combination with the Super-FRS will give access to this new area of research. The design optimization for the new storage-cooler ring system has grown out of the experience with stored heavy-ion beams gained at GSI. Its performance will be world leading.

The phase-space acceptance of the injection channel of the present ESR is a limiting factor for experiments with projectile fragment beams. Moreover, beam preparation, e.g. stacking, cooling, and experiments, presently have to be performed in the same ring. The separation of these functions into two dedicated rings will significantly improve the efficiency and experimental conditions. Hence, it will be possible to attain sufficient luminosities, even with exotic beams, which, will permit reaction studies with hitherto unattainable precision.

In the proposed scheme, the dedicated Collector Ring (CR), combined with the New Experimental Storage Ring (NESR), is designed for fragment beams and will accept the full phase space of the secondary beams delivered by the Super-FRS. The efficient storage of fission fragments will give access in particular to rare neutron-rich nuclei. An important feature of the new ring system is the short cooling time of about 0.5 s for all fragment species. This will give access to nuclei with shorter lifetimes, not accessible today for storage-ring experiments.

A property of all in-flight separated beams is their large emittance. This is generally solved in reaction studies, by measuring the incident beam parameters, on an event-by-event basis. New techniques under development will allow the separation of cooled beams from isobaric contaminants and, as an exciting new option, the preparation of isomeric beams. Cooled beams from the NESR will, by nature, have a much lower emittance. The advantage for reaction studies will be the availability of rare isotope beams, and even isomeric beams, with a well defined beam spot and trajectory, removing the necessity for using tracking detectors for the incident beam.

Here, it should be noted that half-lives of EC-unstable species will be prolonged considerably, as nuclei in the ring are either fully stripped or carry only a few electrons. Fast stacking in the new CR will provide an enhancement of the intensity of stored exotic beams for isotopes with half-lives above 1 s.

In summary, the new ring system will prepare high intensity exotic nuclear beams with high phase space density and excellent purity allowing monoisotopic and even pure isomeric beams. The new ring system will be equipped with an internal gas

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target in the NESR for reaction studies and a small electron ring. The NESR can be operated in conjunction with the electron ring in colliding mode, to measure electron scattering off exotic nuclei for the first time.

The following subsection illuminates the experimental aspects concerned with the storage rings and the electron collider; for more detailed technical specifications of the rings see the accelerator part of this proposal.

1.2.2.1 The Collector Ring (CR)

The main task of the CR is the efficient collection and fast cooling of secondary beams to a relative momentum spread of 10-4 and an emittance of ε = 1 π mm mrad before the exotic nuclei are transferred to the NESR. This high beam quality is achieved by bunch rotation and subsequent stochastic cooling within 100 ms, for a typical fragment beam characterized by an initial momentum spread of ±2.5%. The fact that the beam has been pre-cooled in the CR will considerably reduce the time for electron cooling in the NESR. With this pre-cooling it will be possible to reduce the emittance by electron cooling to ε < 0.1 π mm mrad with a relative momentum spread of ∆p/p < 10-4 for an intensity of 108 fully stripped 238U ions within another 100 ms. These parameters are an upper limit and considerably lower values for the emittance and momentum spread will be achieved for lighter ions.

Experiments involving nuclei with very short half-lives (<0.5s down to the µs range) can be performed directly in the CR. For this purpose the CR has been designed to also allow operation in the isochronous mode. This mode is characterized by the property that the revolution frequency is dependent only on the mass over charge ratio of the isotope and is independent of the velocity of each individual ion. The length of the trajectories through the ring are varied according to the velocity of the ion, such that faster ions follow a longer path than slower ones of the same species. Thus, precise mass and lifetime measurements for short-lived nuclei can be performed. This technique was pioneered at the ESR [66]. In addition to conventional diagnostic detectors, the experimental equipment in the CR will include a time-of-flight detector and a very sensitive resonant Schottky probe. An independent measure of the lifetime can be achieved by measuring the daughter nucleus of a decay in particle-identification detectors placed near the closed orbit of the stored beam [63].

The CR can be used as an efficient high resolution separator stage, allowing contaminants in the selected isotopic beam to be removed prior to its injection into the NESR. This can be achieved with the mass selective stochastic cooling in combination with additional rf excitation.

1.2.2.2 The New Experimental Storage Ring (NESR)

The pre-cooled ions stored in the CR can be transferred into the NESR. For technical reasons, the stochastic cooling is fixed to a velocity corresponding to 740 MeV/u which may not be an appropriate energy for an experiment in the NESR. One possibility to change the energy is to decelerate inside the NESR using rf cavities. As this

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procedure will result in a strong increase in emittance it must be accompanied by electron cooling. In principle, this will allow a reduction in energy down to 20 MeV/u. However, for short-lived nuclei the combined cooling and deceleration via rf power will be too slow. For example, to reduce the beam energy from the initial 740 MeV/u to 100 MeV/u will take about 10 s. An alternative is the application of slowing down in matter, which occurs on a picosecond range. In this case, the slowing down material is placed at a position between the two storage rings where the ion-optical conditions have been optimized such that the phase-space ellipsoid is focused upright. These atomic interactions at relativistic energies result in only modest enlargements of the emittance. Thus, fragments can be injected directly into the NESR at the required energy. In fact, this method is capable of slowing down beams from 740 MeV/u to 250 MeV/u without severe losses in optical transmission. Using copper as the degrader material, one can reduce the losses due to nuclear absorption compared to materials with lower atomic numbers, without significantly increasing the angular and energy straggling. Figure 1.49 shows the increase in relative momentum spread and emittance in a copper degrader.

214Pb 132Sn 50Ca 19C

100 200 300 400 500 600 7000

10

20

30

40

ε [π

mm

mra

d]

E [MeV/u]

214Pb 132Sn 50Ca 19C

100 200 300 400 500 600 7000.000

0.005

0.010

0.015

0.020

2 σ p/p

0

E [MeV/u]

Figure 1.49: Slowing down in a copper degrader from an initial energy of 740 MeV/u to a selected energy E, during the transfer from CR to NESR for 214Pb, 132Sn, 50Ca, and 19C-beams. Left panel: increase of relative momentum spread shown in units of two standard deviations. Right panel: corresponding increase in emittance. The acceptance of the NESR is indicated by the dotted lines.

A hydrogen cluster target will provide the best conditions for in-ring nuclear physics reaction experiments. The areal density of the gas jet aimed for is 1015 protons/cm2 which can be achieved by cooling the gas to liquid nitrogen temperature [126, 127]. The reaction rate in the internal target is enhanced by the circulation frequency of the stored beam which is more than 1 MHz. Thus, for a storage time of one second, the effective thickness of the internal target corresponds to about 1021 atoms/cm2, equivalent to 1 mg/cm2 of hydrogen. An important option to increase the luminosity for internal–target experiments is fast beam stacking. Table 1.8 gives some representative examples. The only limitations which arise are from the nuclear

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lifetime and the losses in the storage ring. The latter is determined by atomic charge-changing processes in the target, with the rest gas, and in the electron cooler.

Table 1.8:Expected luminosities at 740 MeV/u for an internal target of 1015 protons/cm2.

Nucleus

Rate after prod. target

[1/s]

Lifetime incl. losses in storage

ring [s] Luminosity

[cm-2 s-1]

11Be 2.2×109 35 1.2×1029 46Ar 6.4×108 20 1.2×1029 55Ni 7.9×107 0.52 5.1×1027 72Ni 9.0×106 4.1 1.9×1028 82Ge 3.8×107 12 1.2×1029 92Kr 1.5×108 4.7 6.9×1028 104Sn 1.2×106 38 2.7×1028 132Sn 1.1×108 58 1.2×1029 134Sn 7.9×105 2.7 3.1×1026 187Pb 1.2×107 12 6.9×1028 207Fr 3.9×107 9.7 1.1×1029 227U 1.6×106 9.8 8.4×1027

A design of a detection system appropriate for measurements of proton (or helium) scattering in the storage ring is shown schematically in Figure 1.50. As discussed in Chapter 1.1.3.2, both the kinetic energy and scattering angle of the light ion recoiling out of the gas target must be measured in order to achieve a complete kinematical characterization of the scattering event. The measurement can be performed with position sensitive Si diodes, e.g. double-sided strip or pixel detectors. Depending on the scattering angle, the kinetic energies may be rather low, even below 1 MeV. For recoil angles close to 90o, the energy is too low to allow tracking of the ions. The size of the beam-target interaction zone, of the order of a few millimeter, together with the required angular resolution of about 5 – 10 mrad, determines the target-detector distance (~ 40 cm). At more forward angles, the light ions can be tracked by means of two subsequently traversed Si detectors. At large angles, the ions may be stopped in Si diodes (~ 1mm thick), thus delivering the information on kinetic energy. For larger recoil energies, the (thinner, 0.1mm) Si diodes are operated as ∆E-E telescopes together with thick inorganic scintillators. The Si detectors will be placed inside the ring vacuum in order to avoid atomic interactions before the coordinates of interest to the reaction dynamics are measured. The scintillators may be placed outside the vacuum so that they may also be used to detect γ-rays or neutrons emitted from excited projectiles or projectile fragments. The detection system also gives information on the flight-time relative to that of the projectile fragments observed in coincidence.

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40 cm

0.1 mm Si

1mm Si

1mm Si CsI

CsI

Gas jet

p

Heavy ions

Figure 1.50: A detector system for reaction studies at the internal gas jet target of the NESR. Proton measurement, in coincidence with heavy fragment detection, is realized in position sensitive ∆E-E telescopes with tracking capabilities for larger energies.

The heavy-ion detection system will detect fragments from breakup reactions emitted in the forward direction. The system consists of a forward detector setup to determine the nuclear charge and mass of the heavy fragment. The heavy fragments will be analyzed by the NESR magnets, as shown in Figure 1.50, a technique already routinely used in the present ESR experiments.

1.2.2.3 The Electron – Heavy Ion Collider

The electron-heavy ion collider (eA-Collider) will be used to scatter electrons off nuclei far from stability. From the technical and experimental point of view, this part of the proposal is the most challenging. The eA-Collider consists of the NESR to store and cool the exotic nuclei and the electron-storage ring, both operated in collider mode. The basic parameters of the collider are listed in Table 1.9, a detailed technical description is given in the accelerator section.

Table 1.9: Key parameters of the electron-heavy ion collider.

Parameters Electron ring Ion ring (NESR)

Circumference 45.215 m 187.717 m

Energy 200-500 MeV 200-740 MeV/u

Number of bunches 6-8 36-42

e- / ions per bunch 5×1010 ~107

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Figure 1.51: Calculated luminosities as a function of the energy, EI of 14C and 132Sn ion beams stored in the NESR (left panel), and the corresponding length of the reaction zone (right panel). The arrows (→→) and (→←) indicate co-propagating and contra-propagating beams, respectively.

In the following, we will present examples for scattering experiments being feasible with current technology. However, new developments in cooling and spectrometer design will certainly extend the range of investigated nuclei towards the drip lines.

To a large extent, the maximum number of ions which can be accumulated in a single bunch of the NESR is limited only by space-charge effects. This results in a maximum luminosity of L≈1029 cm-2sec-1 for an ion-beam energy of 740 MeV/u. The luminosity does not significantly depend on the electron energy E, within the range E = 200 to 500 MeV. However, it increases with increasing ion-beam energy (EI) as shown in Figure 1.51 (left panel).

The proposed eA-Collider will be operated with contra-propagating (head-on collisions) electron-ion beams. The possibility of co-propagation (merging beams) may be considered in the future. Here, the spectral energy resolution could be improved in the low momentum transfer region, however, the size of the reaction zone is enlarged considerably. The corresponding lengths of the reaction zone are presented as a function of ion energy in Figure 1.51 (right panel) for co- and contra-propagating beams.

The circumferences of the rings were designed such that, within one turn, about 40 NESR bunches collide with one of the eight bunches circulating in the electron ring. As previously mentioned, the maximum number of ions which can be accumulated in a single bunch of the NESR, and consequently the maximum luminosity, is limited by space-charge effects, and therefore decreases from light to heavy bare ions. The expected number of radioactive ions per bunch and their corresponding luminosities are listed in Table 1.10 for several reference nuclei.

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Table 1.10: Number of ions per bunch and luminosity of the eA-Collider for several reference nuclei. Production cross sections, separation efficiency and space-charge limits for stored beams are included.

Nucleus Number of ions/bunch

Luminosity,cm-2s-1

238U92+ 0.9×107 1.0×1028

56Ni28+ 2.8×107 3.3×1028 69Ni28+ 2.8×107 2.4×1028 71Ni28+ 6.6×106 1.1×1027 104Sn50+ 6.0×105 7.0×1026 132Sn50+ 1.6×107 1.8×1028 133Sn50+ 2.1×106 1.8×1026

The luminosities are highly favorable when compared with the MUSES project at RIKEN, Japan. The GSI project, based on pulsed beams, is optimized for storage-ring operation whereas the RIKEN project relies on intense DC beams.

The technological challenge for the eA-collider is a result of the combined criteria for large acceptance and high momentum resolution. From the point of view of the laws ion optics, these criteria are contradictory. The required momentum and angular resolution of the electron spectrometer should be 10-4 and 1 mrad, respectively. In addition, the spectrometer should allow to trace the position of the reaction vertex inside the reaction zone. Existing magnetic spectrometers only partially fulfill these specifications. For instance, the electron spectrometers at the universities of Darmstadt [75] and Mainz [128] and at the research centers TJNAF [129] and NIKHEF meet the requirements with respect to momentum and angular resolution. They can handle an extended reaction zone up to 10 cm, but have only a moderate acceptance of ∆Ω <40 mrad. Existing toroidal and solenoidal spectrometers, e.g. HADES [130], BLAST [131] and BELLE [132], that cover 2π in azimuthal angle, provide the required acceptance, but have only modest resolution. The main limitations for the resolution arise from energy and angular straggling of electrons in the tracking detectors. A large-acceptance spectrometer has advantages, but further research and development are needed for a suitable design, which can satisfy both experimental requirements. Due to the fact that differential cross sections for electron scattering decrease rapidly with the angle of the scattered electron, an ideal electron spectrometer should cover 2π in azimuthal angle but only requires a moderate acceptance in polar angle of about 10°-20°.

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Table 1.11: Different types of existing spectrometers for electron scattering. The MAMI-A spectrometer [128], printed in bold, is used as a reference for calculations which are discussed further in the text.

Large acceptance spectrometers

Small acceptance spectrometers

Parameters CLAS (TJNAF)

HADES (GSI) MAMI-A QCLAM (TUD)

Solid Angle 8°-142° 8°-145° 28 msr 36 msr

Momentum Acceptance

200–4000 MeV/c

100-1500 MeV/c

20% Pmax=735 MeV/c

20% Pmax=250 MeV/c

Momentum Resolution

0.5% 1% 0.01% 0.01%

Effective target length

10 cm 10 cm 5 cm ------

Angular resolution

2 mrad 10 mrad 1 mrad 1 mrad

Costs 40 MDM 12 MDM 4 MDM 1 MDM

Simulations were made under the assumption that a MAMI-A type spectrometer is used for elastic scattering measurements. Two energies, 740 MeV/u and 200 MeV/u, for a 132Sn ion beam in the NESR have been considered. The result is shown in Figure 1.52. The transferred-momentum region from 0.02 GeV2/c2 < Q2 < 0.2 GeV2/c2 can be covered by three settings of the spectrometer. The achievable resolution in excitation energy E* depends on the ion energy and on the scattering angle of the detected electrons. At 740 MeV/u one obtains an energy resolution of ∆E*≈1 MeV (FWHM). The resolution can be improved to about 0.5 MeV for 200 MeV/u, as can be seen in Figure 1.52. However, the luminosity in the latter case decreases significantly as shown in Figure 1.51. The expected number of counts collected per 10 MeV/c momentum bin during a one week experiment for a 740 MeV/u beam energy using two MAMI-A spectrometers is shown in Figure 1.53.

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Figure 1.52: Results of simulations for electron-nucleus scattering at ion energies in the NESR of 740 MeV/u (left) and 200 MeV/u (right). In the calculation, the ion-optical parameters of the MAMI-A electron spectrometer were used. The upper panels show the square of the transferred momentum (Q) versus the energy of the scattered electrons (E'). The two groups of loci represent the acceptance of the MAMI-A spectrometer, positioned at the laboratory angles of 25° and 40°. The lower panel shows the resolution in excitation energy (E*) for the same angular settings, 25° (red histogram) and 40° (green histogram).

The expected yields for two different rms charge radii of 4.9 and 4.7 fm are shown by the blue and red curves. A shift in the maxima and minima of the yields is clearly perceptible. It can be concluded that measurements are feasible even with existing spectrometers with a small solid angle. However, the goal remains to develop a spectrometer with larger acceptance without degrading the resolution.

The electrons will be detected by position sensitive detectors such that their energies and trajectories can be measured simultaneously. Cerenkov counters can be used for the identification of electrons and scintillation counters for the timing information required for coincidence measurements with heavy ions. This coincidence requires the

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development of a new detection system. Target-like fragments and their decay products are emitted into a narrow cone in the laboratory system and can therefore be detected far outside the reaction zone. The reaction products, including neutrons, can be fully identified with respect to mass, charge and momentum, in a similar way to the experiments performed in the high energy branch. Magnetic rigidity analysis with the dipole magnets of the storage ring will be used for identification of the heavy ions from the reaction, as with the experiments with the internal target.

Figure 1.53: Yield simulation of the counts per 10 MeV/c momentum bin expected for elastic electron scattering off 132Sn nuclei using two MAMI-A type spectrometers to cover a larger acceptance angle without degrading the resolution. The blue and red curves reflect the expected yields for assumed rms radii of 4.9 and 4.7 fm, respectively. The ion energy chosen was 740 MeV/u.

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1.2.3 Reactions with High-Energy Exotic Nuclei

The combination of fast radioactive beams and a highly efficient reaction setup provides unique possibilities for nuclear structure experiments with exotic nuclei. The experiments to be performed cover a large variety of reaction types such as knockout reactions, electromagnetic and nuclear excitations, fission studies, in-beam gamma-ray spectroscopy, multifragmentation, as well as charge-exchange reactions, see the discussion in Chapter 1.1.3.3. The planned experimental configuration is based on a concept similar to the existing reaction setup at GSI. This consists of the large-gap dipole magnet ALADIN [133], tracking detectors for detecting the ions, a CsI array for γ-ray measurements, and the Large Area Neutron Detector LAND [134]. This combination is presently unique worldwide in terms of acceptance and efficiency. For certain experiments, however, the present setup has severe limitations, in particular due to the limited bending power of the dipole magnet ALADIN. A new superconducting large-gap dipole magnet is currently being designed, which, in combination with high-resolution tracking detectors, will provide a much better momentum resolution. This will allow unique fragment mass determination even for heavy nuclei, where a resolution in magnetic rigidity of the order of 1/1000 is required. This requires, in addition, a measurement of the fragment velocity v with a precision of about ∆v/v = 10-3. The higher bending power also allows the use of the highest beam energy available and thus provides the highest transmission. In addition, the resolution in momentum enables an accurate measurement of the longitudinal momentum distribution in halo-break-up reactions. The experimental setup is shown schematically in Figure 1.54. A design study and a partial implementation of an advanced experimental reaction setup is the subject of the research and technical development project R3B (Reaction studies with Relativistic Radioactive ion Beams) funded by the European Community, which started in April 2000. The main goals of the R3B project are to define the technical requirements for the advanced reaction setup.

p’

γ

magnet

FP

N

Figure 1.54: Schematic drawing of the experimental setup (not to scale). The incoming secondary beam is identified by measuring magnetic rigidity, energy loss dE and time-of-flight ToF. Behind the target, the kinematically forward focused reaction products are identified and momentum analyzed. The distance between target and neutron detector is typically 12 m.

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Large-acceptance superconducting dipole magnet. A zero-degree superconducting dipole magnet is presently being designed within the R3B project. The main parameters of the spectrometer are: (i) A large vertical gap allowing a cone with an angular range of ± 80 mrad for neutrons to be covered. (ii) A horizontal acceptance which allows a maximum bending angle of 35°. This ensures an acceptance close to 100% even for experiments involving proton emission, or in general for experiments with very different magnetic rigidities for the beam and fragments. (iii) A high field integral of 5 Tm is required, which allows a bending angle of 18º for a 15 Tm beam (e.g. 1 GeV/u 132Sn). The required momentum resolution of up to 10-3 can be achieved by tracking the particles with high resolution. The layout of the magnet which is presently under consideration is shown in Figure 1.55. The design includes four super-conducting coils, which are tilted to match the required acceptance angle for the particles of interest. The side coils are optimized to reduce the fringe field, and guarantee a low magnetic field in the target region, where detectors have to be placed.

Figure 1.55: Superconducting dipole magnet. Left: Layout of the four superconducting coils. The side coils are optimized to reduce the fringe fields. The field integral amounts to about 5 Tm. Right: Three dimensional cut-away drawing of the whole dipole magnet system, adapted from [135].

Tracking detectors. Three position measurements with sub-mm resolution are needed to track the particles through the magnetic field and to obtain the required resolution in momentum. Several types of detectors are considered, these include silicon strip detectors, large scintillating fiber-arrays and large drift chambers. For specific experiments, for example in fission studies, the tracking detectors also need multi-hit capability. A large multi-sample ionization chamber MUSIC will be used for charge identification of the fragments.

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Velocity measurements. The velocities of fragments and light charged particles can be determined by a time-of-flight measurement. With the presently used equipment a relative velocity resolution of 10-2 can be reached. To ensure a unique identification of the heavy fragments up to mass A=200, a precise velocity determination is required in combination with the momentum measurement using the magnetic spectrometer. Therefore, a ring-imaging Cerenkov detector will be developed which aims at a velocity resolution of better than 10-3. Such a detector has also multi-hit capability and is thus very well suited also for fission experiments [136]. First successful tests have been performed at the FRS.

Neutron measurement. The existing LAND detector has multi-hit capability and measures neutrons with an efficiency larger than 90% (single neutron) for energies above 300 MeV. With its neutron identification and energy resolution it is presently the most powerful neutron detector for high energy neutrons worldwide. It is feasible to improve the presently achieved neutron momentum resolution of 1% by almost an order of magnitude with an upgrade of the time-of-flight detectors. Additional possibilities aiming at further enhancement of the multi-hit resolving power will be investigated. This will be especially useful for fission studies where large neutron multiplicities are expected and also for experiments investigating multi-neutron correlations (e.g. neutron droplets).

Detection of gamma-rays and light particles. An in-beam gamma-ray measurement at incident ion beam energies of several hundred MeV/u requires good angular resolution to minimize Doppler broadening. The present setup contains a CsI array consisting of 144 crystals covering the forward hemisphere. For many experiments, the detection efficiency is one of the most important issues. Therefore, a full solid angle (4π), 100%-absorption gamma-detection device with moderate resolution is required. The angular resolution, given by the granularity of the detector, will match the intrinsic resolution of the crystals (e.g. CsI or NaI). Low-energy neutrons from the target, e.g. produced in charge-exchange (p,n) reactions, are detected in the crystals as well and separated from the gamma-rays by time-of-flight. Charged particles scattered to large angles, for example scattered protons from a hydrogen target in (p,p) or (p,p’) experiments, but also knocked-out charged particles, can be detected by position-sensitive detectors placed in the forward direction (typically covering an angular range between 20º and 90º) inside the 4π γ-spectrometer. For high-resolution in-beam γ-spectroscopy experiments the AGATA Ge-array can be utilized.

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1.2.4 Experiments with Low-Energy Exotic Nuclei

The low-energy experimental area behind the Super-FRS will open up a broad new field for experiments with slowed-down and stopped secondary beams. The key instrument for these new experiments is an energy-focusing device [88], which reduces the energy spread of the secondary beams delivered by the Super-FRS.

The common requirement of these experiments is the need to slow down the separated ion beams quickly and efficiently and to minimize their energy spread, and thus, also their range distribution when stopped in matter. The fastest way to reduce the energy of heavy ions is by slowing them down in matter. The stochastic processes in the production reactions of projectile-fragmentation and projectile-fission inherently result in a large phase-space volume; the momentum spread of the fragments can reach several percent. This leads to range distributions in helium gas of several atmosphere-meters, as can be seen from the left panel in Figure 1.56. Since such lengths are not practicable, a substantial reduction of the energy spread is required.

σ R [c

m]

Ein [MeV/u]0 100 200 300 400 500 600 700 800

0

100

200

300

400

500

0 100 200 300 400 500 600 700 8000

1000

2000

3000

4000

5000

6000

σp/p = 0.1%σp/p = 1.6%Ar

Ar

Figure 1.56: Standard deviation σR of the range distribution for various fragments (Ar, Kr, Xe, U) as a function of the incident kinetic energy in helium gas at room temperature and pressure. The left panel shows σR for non-bunched beams, giving a momentum spread of σp /p=1.6% at the final focal plane of the Super-FRS, the right panel shows σR for a relative momentum spread of an energy-bunched beam, such that the momentum spread is σp/p=0.1%. The dashed orange line in the right panel corresponds to a standard deviation of one meter in the planned gas-filled stopping cell.

This can be realized with a specially shaped energy degrader system placed at a dispersive focal plane of an ion-optical spectrometer [137]. With such an energy-buncher stage one can, for example, compress the relative momentum spread of a 300 MeV/u fragment beam to values as small as σp/p = 10-3 and that of a 6 MeV/u beam to σp/p = 0.35. This will drastically reduce the range distributions of stopped fragments to approximately 1 to 4 atmosphere-meters of helium gas for ions from 100 to 700 MeV/u respectively, as can be seen in the right hand panel of Figure 1.56.

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The momentum spread of the fragment beam can be reduced if the resolving power guarantees that the spot size at the degrader is dominated by the dispersion and not by object magnification and aberrations. Since the emittances of the fragment beams are inevitably large, a large-aperture spectrometer with the necessary high resolving power is needed to compensate for the energy spread. In total the momentum resolving power of the dipole stage and the energy-loss straggling in a monoenergetic degrader determine how well the energy distribution can be bunched. A monoenergetic degrader bunches the energy spread of an incident dispersed beam such that the mean energy of the ions is the same, independent of the position they emerge from the monoenergetic degrader. This is shown in Figure 1.57 for different fragment beams compared to ideal monoenergetic beams. The area indicated in green represents the working range of the proposed energy-buncher.

σR0

0

in A

l

σR

Resolving power

Figure 1.57: Achievable range straggling σR of 300 MeV/u 78Ni (blue solid curve) and 132Sn (red solid curve) fragments in an aluminum stopper as a function of the momentum resolving power of the energy buncher. The dotted lines indicate the range straggling σR0 of ideal monoenergetic beams of the same isotopes. The resolving-power domain which can be reached with the proposed energy buncher is marked in the figure.

Figure 1.58 illustrates the ion-optical layout of the energy buncher. The spectrometer consists of a dispersive ion-optical stage with a large dipole magnet, quadrupole (red) and sextupole magnets (blue). The magnetic quadrupole doublet in front of the dipole magnet is needed to properly illuminate the field volume of the dipole magnet to reach the required resolving power and to focus the secondary beam onto a monoenergetic degrader. The quadrupole triplet behind the dipole magnet guides the exotic nuclei

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into the gas cell or any other detector array. In Table 1.12 the key parameters of the energy buncher are listed.

1000

mm

1000

mm

Beam

Beam

Monoenergetic degrader

Gas cell

5 m

Figure 1.58: Ion-optical layout of the energy buncher.

Table 1.12: Ion-optical parameters of the energy buncher.

Magnetic Rigidity (Bρmax) 10 Tm

Momentum Resolving Power 600

Momentum Acceptance ± 2.5% (FWHM)

Acceptance in x-direction

y-direction

15mm * 20mrad

10mm * 20mrad

When the ions are stopped in a gas catcher filled with high-purity helium at atmospheric pressure, the majority of the ions will be singly ionized after thermalization because of the high ionization potential of helium. They can be extracted from the cell within a few milliseconds with a combination of electric fields and gas flow. After extraction from the cell, the ions will be transported with a gas-filled rf-quadrupole cooler and ion-guide structure through various differential pumping sections to a high-vacuum region. From there the ions will be distributed and transported with energies of a few keV to the various experimental setups.

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The low-energy branch of the Super-FRS provides energy-bunched exotic nuclei for nuclear physics experiments in the energy range below 350 MeV/u. These are ideal conditions for high-resolution gamma-ray spectroscopy using the Advanced GAmma-Tracking Array AGATA, as described in the following section.

1.2.5 High Resolution Gamma-Ray Spectroscopy

Requirements and Features

Any detector array designed for gamma-ray spectroscopy of very exotic nuclei must combine the highest possible efficiency, selectivity and energy resolution with a simultaneous capability to handle high gamma-ray multiplicities of up to Mγ=30. The instrument must also accept a wide range of beam velocities (from stopped to β = v/c ≈50%) and must function in high gamma and particle background levels. These features can only be achieved with a closely packed arrangement of Ge-detectors, which allows the "tracking” [138] of the detected gamma-quanta, i.e. the possibility to detect the energy, time and position of the interactions of individual gamma-quanta throughout the detector volume. This requires a large solid angle (4π) “shell” of highly segmented Ge crystals, with properties such as listed in Table 1.13.

Table 1.13: Defining properties of AGATA

Photopeak efficiency (Eγ=1 MeV, Mγ=1, β<50%) 50%

(Eγ=1 MeV, Mγ=30, β<50%) 25%

(Eγ=10 MeV, Mγ=1) 10%

Peak/Total ratio (Mγ=1) > 60%

Angular resolution ∆θγ (∆Eγ./ Eγ < 1%) < 1º

max. event rate (Mγ=1) 3 MHz

(Mγ=30) 0.3 MHz

With the help of sophisticated computer tracking algorithms [139, 140] to disentangle the event information, the resultant increase in effective detector “granularity” allows a precise reconstruction of the gamma event including emission angle. Apart from the superior ability for geometric characterization of the events (e.g. Doppler corrections) leading to improvements in energy resolution, the maximum counting rates of the detector system is also greatly increased.

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AGATA Set-Up and Design

The on-going research into gamma-ray detection arrays [141] has led to the proposal of AGATA, the Advanced GAmma Tracking Array. This instrument is being designed to overcome the severe limitations of all current or near-future arrays. AGATA will be built by a collaboration of the leading european γ-spectroscopy groups. To maximize the physics output the instrument has to run permanently and with all kinds of exotic and stable beams. Complementary physics campaigns at other host facilities will also be pursued. A full description of AGATA is given in a separate proposal supplementing the present document. A brief summary is given here.

As shown in Figure 1.59, the geometrical structure of AGATA is based on the geodesic tiling of a sphere (inner diameter 17 cm) with 12 regular pentagons and 180 slightly irregular hexagons. The total number of segments in the array is 6780, which provides the optimum position sensitivity. The total solid angle covered by germanium material is close to 80% of 4 π. Realistic simulations of the tracking performance indicate photo peak efficiencies of 50% for individual transitions, and of 25% for a cascade of 30 γ-rays.

Figure 1.59: Left panel: An artist’s view of AGATA. The three slightly different sized hexagonal crystal shapes are shown in red, green and blue. Three crystals are packed in cryostats, as indicated by the gray aluminum walls. The total number of such triple cluster detectors is 60. Two of the 12 (30-fold segmented) pentagonal detectors are shown in light blue color. Right panel: An AGATA Detector Module consisting of (1) three 36-fold segmented Ge detectors, (2) preamplifier, (3) frame support, (4) digital pulse processing electronics, (5) fiber-optics read-out, and (6) LN2–dewar. The source/target position is indicated by (7).

An AGATA detector module as shown in the right panel of Figure 1.59 contains three 36-fold segmented Ge detectors of hexagonal, tapered shape. To improve reliability, each individual Ge detector is encapsulated in an aluminum can – a new technology

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developed in the framework of the Euroball [109] and Miniball [142] projects. The preamplifiers consist of a cold part including the FETs mounted inside the cryostat and a warm part behind the Ge detectors. Highly integrated digital pulse processing electronics is mounted in a second layer behind the preamplifiers. The data are transferred via a fiber-optic channel for further analysis. The Ge detectors are cooled with liquid nitrogen contained in conical dewars.

A key feature of AGATA is to determine the emission direction of the detected gamma-quanta with a precision of <1° corresponding to an effective granularity of >5×104. This ensures an energy resolution of better than 0.5% for transitions emitted by nuclei recoiling at velocities as high as 50% of the speed of light. This value is only a factor of two larger than the intrinsic resolution of Ge detectors, and is comparable to the values currently obtained at 10 times smaller recoil velocity. Figure 1.60 demonstrates the large gain in sensitivity and selectivity obtained by AGATA.

The AGATA electronics will be based on digital signal processing, by which the preamplifier output is sampled and digitized with fast ADCs to record the time evolution (“shape”) of the signals. From the signal detected in a segment and the mirror charges observed in its neighbors the interaction position of a γ-ray can be determined with an accuracy of 1-2 mm. Digital processing electronics, placed directly adjacent to the detectors, will extract energy, timing and interaction positions from the sampled signals. Each data item will be time stamped, allowing later event reconstruction as well as the construction of delayed coincidences without dead-time problems. In addition, software triggering will be implemented, providing an easily configurable and flexible event “filter”, especially important for rare events.

From the front-end detector electronics, the preprocessed data packets will be transferred in parallel by high bandwidth fiber links to a central event builder. This will perform all necessary functions of time-ordering, data-merging and gain-matching in order to fully re-construct the γ-ray interaction sequence by using tracking algorithms.

Besides the Ge detectors the AGATA set-up will have particle tracking detectors. Common to all experiments is that the exotic nuclei are identified event-by-event with respect to mass and atomic number (A, Z) prior to the reaction target the catcher, respectively. Moreover, the energy and trajectory of each ion is measured. This requires new particle detectors capable of very high rates. At high beam energies identification and tracking of the beam-like particle behind the target is foreseen. With slow beams a variety of ancillary detectors for heavy and light particle, electron and neutron detection will be available like with Euroball and stable beams nowadays. Finally for stopped beams an active catcher is anticipated to detect the implanted nuclei as well as light particles and radiation from their decay.

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Figure 1.60: Simulated gamma-spectra comparing the response of AGATA (top) with that of a conventional germanium detector array (bottom) assuming a fragmentation reaction at 100 MeV/u.

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Acknowledgments

This chapter, like all of the other chapters of this document, was prepared with the help of many individuals. The appendix lists the names of all the individuals involved in the CDR document, from the contributions to the science discussions, workshops and working groups to the research and development, the detailed scientific and technical planning activities, and, last not least, the drafting of this Conceptual Design Report.

The concepts and presentations of the science and technical plans in this chapter were in particular developed by the following authors:

Thomas Aumann, Fritz Bosch, Leonid Chulkov, Peter Egelhof, Hans Emling, Hans Feldmeier, Hans Geissel, Jürgen Gerl, Hubert Grawe, Walter Henning, Fritz Hessberger, Sigurd Hofmann, Kate Jones, Paul Kienle, Horst Lenske, Gottfried Münzenberg, Ernst Roeckl, Hendrik Schatz, Matthias Schädel, Christoph Scheidenberger, Karl-Heinz Schmidt, Haik Simon, Gerhard Schrieder, Klaus Sümmerer, Helmut Weick, Martin Winkler, Wolfgang Trautmann, Hermann Wolter

(Coordination: H. Emling, H. Geissel, G. Münzenberg)

Collaborating Institutes:

Aarhus Universitet (Denmark), Argonne National Lab (USA), Hahn-Meitner-Institut Berlin (Germany), LBL Berkely (USA), University of Berkely (USA), Ruhr-Universität Bochum (Germany), Universität Bonn (Germany), CEN Bordeaux (France), Bordeaux-Gradignan (France), CIP Bukarest (Rumania), Comenius University Bratislava (Slovakia), Università di Catania (Italy), CIRIL, Caen (France), Rutherford Appleton Laboratory, Chilton (Great Britain), Commissariat à l'Energie Atomique Saclay (France), University of Conneticut (USA), CERN (Switzerland), CRNL Chalk River (Canada), CCLRC Daresbury Laboratory (Great Britain), Technische Universität Darmstadt (Germany), Technische Universität Dresden (Germany), Universität Erlangen (Germany), Johann Wolfgang Goethe-Universität Frankfurt (Germany), GANIL Caen (France), Institute of Nuclear Research Debrecen, (Hungary), Justus-Liebig-Universität Giessen (Germany), Chalmers Tekniska Högskola Göteborg (Sweden), Universität Göttingen (Germany), KVI Groningen (The Netherlands), Max-Planck-Institut für Kernphysik, Heidelberg (Germany), Universität Heidelberg (Germany), JAERI, Takashi (Japan), JINR Dubna (Russia), JYFL Jyväskyla (Finland), Forschungszentrum Jülich (Germany), Forschungszentrum Karlsruhe (Germany), Universität Kassel (Germany), University Kopenhagen (Denmark), INP Krakow (Poland), Uniwersytet Jagellonski Krakow (Poland), Kurchatov Institute, Moscow (Russia), Institute of Modern Physics, Lanzhou (China), LNL Legnaro (Italy), K.U. Leuven (Belgium), University of Liverpool (Great Britain), LANL Los Alamos (USA), Uniwersytet Lublin (Poland), CSIC Madrid (Spain), University of Manchester (Great Britain), Philipps-Universität Marburg,

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Johannes-Gutenberg-Universität Mainz (Germany), Michigan State University (USA), Università di Milano (Italy), Ludwig-Maximilian Universität München (Germany), Technische Universität München (Germany), Niigata University (Japan), University of Notre Dame (USA), ORNL, Oak Ridge (USA), IPN Orsay (France), Osaka University (Japan), INFN Sezione Padova (Italy), Università di Padova (Italy), Universität Regensburg (Germany), RIKEN (Japan), Universidade do Rio de Janeiro (Brazil), Forschungszentrum Rossendorf (Germany), Universidade de Santiago de Compostela (Spain), Universidade de São Paulo (Brazil), CTA São José dos Campos (Brazil), CRN Strasbourg (France), University of Surrey (Great Britain), Institute of Nuclear Studies, Swiek (Poland), PNPI St. Petersburg (Russia), University Tennessee Knoxville (USA), Tokyo Institute of Technology (Japan), Universty of Tokyo (Japan), Università di Torino (Italy), PSI Villigen (Switzerland), Uniwersytet Warszawski (Poland)

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