nuclear reactions - ii
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DESCRIPTIONNuclear Reactions - II. A. Nucleon-Nucleus Reactions A.1 Spallation A.2 Induced Fission B. Nucleus-Nucleus Reactions B.1 Fragmentation B.2 Multifragmentation B.3 Vaporization. Nuclear Reactions - II Intermediate and Relativistic Energies. Spallation Induced Fission - PowerPoint PPT Presentation
Nuclear Reactions - IIA. Nucleon-Nucleus Reactions
A.1 SpallationA.2 Induced Fission
B. Nucleus-Nucleus Reactions
B.1 FragmentationB.2 MultifragmentationB.3 Vaporization
Nuclear Reactions - IIIntermediate and Relativistic EnergiesSpallation
A-1 Spallation - 0Introduction Generalities
Light particle emissions
The two stage of the spallation process
The Intra-Nuclear Cascade
Production of pions
Comparison with experimental results
Main physical aspects
A.1 Spallation 1GeneralitiesDefinition (Encyclopedia Britannica): high-energy nuclear reaction in which a target nucleus struck by an incident (bombarding) particle of energy greater than about 50 million electron volts (MeV) ejects numerous lighter particles and becomes a product nucleus correspondingly lighter than the original nucleus. The light ejected particles may be neutrons, protons, or various composite particles equivalentprojectile(p, n, p, ...)targetDrawing from the INC model
A.1 Spallation 2Generalities1947: E.O. Lawrence discovers that the number of emitted nucleons, especially neutrons, may be quite large depending upon the conditions of the spallation reaction.
90s: new interest for spallation reactions they are of pivotal importance for the development of powerful neutron sources for various purposes:
hybrid systems, be devoted to energy production or to incineration of nuclear wastes so-called multi-purpose spallation sources devoted to irradiation studies, material structure analysis, future tritium production units
Other interest: the production of isotopes spectroscopy reaction mechanisms astrophysics
Main aspects: energy spectrum and angular distribution of emitted light particles production rate of residues
A.1 Spallation 3Light particle emissionNeutron spectraConclusion: the spallation reaction is a TWO STEP process!
A.1 Spallation 4ResiduesThey are observed in the very late stage of the process: after the cascade, the evaporation, and the possible b-decay of very-short-lived emitters.fission productsspallation residuesvery light fragments: produced by fast ejection (cascade) + evaporation of the remnant
A.1 Spallation 6Residues208Pb + p at 1 AGeV
A.1 Spallation 7The two stages of the spallation processIndividual nucleon-nucleon scatteringThe incident proton loses part of its energy = cascade (~ 1022 s ~ 30 fm/c)2. Evaporation of the residue (~ 1020 - ~10-16 s)projectile(p, n, p, ...)target
A.1 Spallation 8The Intra-Nuclear Cascade1. J. Cugnon, Phys. Rev. C 22(1980)18852. H.W.Bertini Phys.Rev.188(1969)1711 3. Y. Yariv and Z. Fraenkel, Phys. Rev. C 20(1979)2227
A.1 Spallation 9The Intra-Nuclear CascadeLigeBertini/IsabelDpin the nucleusin the nucleusDp
A.1 Spallation 10Production of PionsThe pions are the decay products of the D resonances. They are mesons, exchange particles of the strong interaction between nucleons. The D resonances and pions are coming from inelastic nucleon-nucleon reactions at beam energies above few hundred of AMeV (Mp ~ 140 MeV).C. Hartnack et al., Nucl. Phys. A 495(1989)303Pion cycle ( D Np D ) NN DN hard D production D Np D decay DN NN D absorption Np D soft D production4 sorts of DsD++ 1 (p+p+)D+ 2/3 (p+p0) + 1/3 (n+p+)D0 2/3 (n+p0) + 1/3 (p+p-)D- 1 (n+p-)
A.1 Spallation 11EvaporationThese models are used to simulate the second step of the reaction, i.e. the decay of the excited residue.
Main features: - the life-time of the excited residue is much longer than its formation timeT(residue) = several hundreds of fm/cT(formation) ~ 30 fm/c - the individual properties of the quantum states have a negligible effect at high excitation energy, due to the small distance between the energy levels, in particular in heavy nuclei.
All the states are equiprobable so the deexcitation of the nucleus may be treated in a statistical way.
In other words, a statistical model considers the probabilities of the different deexcitation possibilities with comparable weights, which correspond to individual processes of similar time length.
A.1 Spallation 12EvaporationMost of the current evaporation code describe the residue deexcitation according to the Weisskopf theory which is based on the energy conservation and the assumption of the micro-reversibility of the process.
This assumption is verified for the light particle emissions (n, p, d, t, 3He, 4He), but not for the emission of heavier nuclei or for fission. Note: the average total neutron multiplicity is about 4 times the multiplicity of the neutrons coming from the cascade stage.
A.1 Spallation 13Comparison with experimental results
A.1 Spallation 14Comparison with experimental results
A.1 Spallation 15Main Physical Aspects1. The available incident energy is progressively shared by the incident particle itself, the ejectiles, the pions and the target.
The proton travels trough the target in 10 fm/c (1 fm/c = 3.10-24 s)
A large amount of the energy is removed quickly (~ 20 fm/c) by the emission of a few fast nucleons and some pions.
The nuclear density is depleted when the proton enters the target.
A kind of hole is drilled into the target.
The density becomes more or less uniform before the ejection of the fast particles is over.
The relative energy transfer is maximum for incident energies between 1 and 2 GeV.
On average, the proton loses energy with a rate which is universal.
The number of fast ejected particles peaks around 2 GeV.
A.1 Spallation 16EURISOLEURopean Isotope Separation On-Line radioactive nuclear beam facility http://www.ganil.fr/eurisol/ radioactive beams at very high intensities Applications:
proton & neutron drip-lines changes in shell structure halo structure high-spin physics isospin effects in nuclear matter superheavy nuclei nuclear astrophysics tests of the standard model muons and antiprotons solid state physics
A.1 Spallation 17Spallation sourcesEuropean Spallation Source (ESS) EU http://www.neutron-eu.net/en/index.php????
Spallation Neutron Source (SNS) Japan http://jkj.tokai.jaeri.go.jp/ (2007)
Spallation Neutron Source (SNS) USA http://www.sns.gov (2006!)Applications:
- chemistry complex fluids crystalline materials disordered materials engineering magnetism and superconductivity polymers- .Note: a proton of 1 GeV on Pb target with a thickness of 60 cm produces about 25 neutrons!