Lifetime measurements in N=50 nucleus 87Rb

Spokesperson(s): D. Deleanu

D. Deleanu1, N. Marginean1, D. Bucurescu1, G. Cata-Danil1,2, I. Cata-Danil1, D. Filipescu1, D.G. Ghita1, N. Florea1, I. Gheorghe1, T. Glodariu1, M. Ivascu1, R. Lica1, , R. Marginean1, C. Mihai1, I.O. Mitu1,2, A. Negret1, A. Olacel1, S. Pascu1, T. Sava1, L. Stroe1 , G. Suliman1 ,

R. Suvaila1, S. Toma1, N.V. Zamfir1 1- ”Horia Hulubei” National Institute for Physics and Nuclear Engineering , Bucharest, Romania

2Department of Physics, “Politehnica” University of Bucharest

Patrick Regan3, Zs. Podolyák3, Robert Carroll3, Terver Daniel3, Laila Gurgi3, Callum Shand3, Richard Britton3

3Department of Physics , University of Surrey, Guildford, GU2 7XH, UK

Alison Bruce4, Oliver Roberts4, Frank Browne4, Chantal Nobs4 4School of Computing Engineering and Mathematics, University of Brighton, Brighton, BN2 4GJ, UK

Steven Judge5, John Keightly5, Cyrus Larijani5 5Radioactivity Group, National Physical Laboratory, Teddington, UK

Abstract

The configuration of a series of isomeric states in nuclei near to the N=50 neutron shell closure shows that the two-proton (

, ) configuration is energetically

favored in this region. Isomers attributed to the two-proton ( ,

)

configurations were previously identified in this region: in 86Kr and 85Kr, in the last an additional g9/2 neutron hole being involved, in 84Kr the isomeric 12+ yrast level at 5373 keV. Further manifestations of this two-proton configuration that should lead to an isomeric state are expected in 87Rb. Previous works propose the level at 3644 keV in 87Rb with a two-proton (

, ) configuration and also issue the

hypothesis of exciting the neutron core. We propose to measure the lifetime of the state with this configuration for the N=50 87Rb nucleus using the fast timing method with LaBr3(Ce) and RoSphere HPGe detectors. Lifetime measurements provide physical quantities more sensitive to the microscopic structure of the state. The 87Rb nucleus will be populated by bombarding a 82Se target with a 7Li beam at 20 MeV and through evaporation of two neutrons from the compound nucleus.

I. SCIENTIFIC MOTIVATION

87Rb lies close to the semi-doubly magic nucleus 88Sr. Having one proton less than 88Sr, a relatively limited number of configurations is available for excitations in 87Rb. Bellow ~4 MeV, according to the systematic behavior observed in the neighboring 85Kr and 89Y, the states are given by the coupling of the 1p3/2, 1p1/2 and 0f5/2 proton orbitals to an even-even core. Above 4 MeV, the breaking of the neutron core should start to be involved in the excitations. Two spectroscopic studies of the high-spin states of neutron-rich 87Rb50 were previously made. The most detailed level scheme was built with GAMMASPHERE array by prompt -ray spectroscopy of fragments following the fission in three different heavy-ion-induced reactions [1]. A less detailed level scheme but with more robust spin-parity assignments was obtained with GASP -array after deep-inelastic reactions [2]. Spin/parity assignments in the GAMMASPHERE study was mainly based on comparisons to similar states in the neighboring 85Kr and 89Y isotopes and is also guided by shell-model calculations from Ref. [3]. In GASP study, the angular distribution ratios from oriented states support the spin-parity assignment. A brief discussion of the differences between the results of these two works reveals our goals within the proposed experiment.

In both studies new states were built above the 9/2+ isomeric state at 1578 keV. The spin and parity of this state was fixed in (3He,d) and (d, 3He) reactions [4 ] and the half-life of 6(1) ns was reported in Ref. [5]. For the wave function of the 9/2+ isomeric state,

the theory predicts a main component from the (g9/20+) coupling and a weaker

component from the (g9/22+) coupling Ref [5]. The observation of a weak E3 transition

of 1578 keV, from the 9/2+ isomer to the ground state [1], indicates a very small but

probable admixture of the (p3/23−) configuration. ( E3

- ~ 3.1 MeV for 86Kr and ~2.7 MeV

for 88Sr).

There is good agreement between the level schemes built on top of the 9/2+ isomeric state in both studies (Fig 1). ADO ratios allow fixing the spins for six yrast states, up to 23/2(+) ( ~5481 keV), within the GASP level scheme [2], while the systematics were useful in GAMMASPHERE study [1] to tentatively indicate the spin and parity for only two states above the 9/2+ isomeric state.

states with tentative spin/parity (11/2+) and (13/2+): In Ref. [1], based only on the resemblance with the sequences from the odd-mass 85Kr and 89Y, tentative spin and parity (11/2+) and (13/2+) were assigned for the first two excited states at ~3002 and ~3409 keV above the 9/2+ isomer in 87Rb. The configuration of these states in 85Kr and 89Y is described as a nucleon coupled to the lowest 2+ states of the 86Kr (at~1.5 MeV) and 88Sr cores, although this simple coupling is not supported for 13/2+ by the comparison of the B(E2) values from 85Kr and 86Kr [7]. According to Ref. [7], the 13/2+ level in 85Kr also contains the 4+ proton excitation in addition to the 2+ proton excitation, an assumption supported by the difference between the excitation energies of the 13/2+ and 17/2+ levels of only 60 keV. The energy difference between these states increases in 87Rb to 235 keV but the sequence is kept similar suggesting the same

configuration for the 13/2+ state in 87Rb (Fig.2) – (13/2+ state in 85Kr and 2+ state in 86Kr have T1/2 approx. 0.3 ps).

In Ref. [2] an ADO ratio of ~1.31 for the ~1831 keV transition from the (13/2+) state to the 9/2+ is found smaller than the expected value for an E2 stretched transition (~1.4). This is a consequence of an alignment reduction and it is seen as a clue for the existence of higher lying isomeric states [2]. Spin and parity 13/2+ are fixed for the 3409 keV state in this work and, similar to Ref.[1], spin (11/2+) is tentatively assigned for the ~3002 keV state based only on systematics.

states with tentative spin/parity 17/2+ , 15/2+ assignment : In Ref. [2] the ADO ratios of the upper ~235 and 507 keV transitions indicate the 15/2(+) and the 17/2(+) assignments respectively for the ~3644 and 4151 keV levels. In Ref. [1] the level at ~ 3644 keV is proposed as a candidate for the 17/2+ state, although not excluding the 15/2+ assignment, based on the comparison with levels ordering for the odd-mass 85Kr and 89Y. The hypothesis of the coupling of the neutron g9/2 hole to the lowest 4+ state of the cores to form the 15/2+ and 17/2+ states is issued in Ref. [1]. In Ref. [2], the assumption of pure proton configurations involving the coupling of the odd g9/2 proton to the first excited states in the core, shows better agreement with the shell-model calculations with no particle-hole excitations across the N=50 core.

(In 85Kr and 89Y the breaking of the neutron core is found above 4 MeV, and in 88Sr states described as neutron particle-hole excitations g9/2,

) configurations appear

above 4.5 MeV – for states with particle-hole excitation across the shell the core-coupling picture fails to describe the excited states.)

Keeping the core-coupling picture, the coupling of the odd g9/2 proton to the lowest 2+ or 4+ states of the proton core can form 11/2+, 13/2+, 15/2+ and 17/2+ states in 87Rb. The coupling of the odd g9/2 neutron hole to the 4+-proton core excitation gives the 17/2+ states in 85Kr.

Shell model calculations with pure proton configuration, (excluding possible neutron-core excitations), predicts, in good agreement with experimental data, a two-proton configuration (

, ) for yrast 4+ state at ~2.2 MeV in 86Kr [8]. (Calculations in the

same Ref. [8] show that an extent of the configuration space with neutron-core excitations cause the 4+ state to shift slightly to higher energy.)

The yrast 4+ state 86Kr proton core is an isomeric state with a half-life of 3.1(6) ns [6,7]. This value implies a very small B(E2) value for the transition to the 2+ state – explained by the fact the 4 is the maximum spin for two-proton (

, ) excitation.

The yrast 17/2+ state at ~1.9 MeV in 85Kr is an isomeric state with a half-life of 1.2 s [6, 7]. This value implies a much higher B(E2) value for the transition to the 13/2+ state than similar E2 transition in 86Kr, and as it was already stated this is coming from the configuration of the 13/2+ level in 85Kr which contains also the 4+ proton excitation in addition to the 2+ proton excitation. Based on similar yrast sequences up to ~ 2.2 MeV observed for 87Rb, 85Kr and 86Kr - proton core (Fig. 2), isomeric yrast states above 9/2+ state in 87Rb are expected, the values of half-life’s depending on the microscopic structure of the states. In case when

235 keV transition in 87Rb has an E2 character similar half-life with 4+ state in 86Kr is expected, else an M1 character will be indicated by a much faster transition.

Lifetime measurements will provide physical quantities more sensitive to the microscopic structure of the states. Firm spin/parity assignment through DCO ratios measurements and comparison with shell model calculations would clarify the structure of the states in this spin region for 87Rb nucleus.

SUMMARY

High-spin states in the N=50 nucleus 87Rb will be investigated via the 82Se(7Li,2n) reaction. As previously stated, the spins/parities of the states above the 9/2+ state are tentative: assigned either based on systematics or from ADO measured in deep inelastic reactions. Spin/parity of the states above 9/2+ will be fixed based on the DCO ratios from HPGe coincidence events. The main goal of the present experiment will be to search for isomeric states above 13/2+ state in 87Rb. The best candidate for isomerism is the (15/2+,17/2+) state, expected as a manifestation of the predicted distinct configurations of the 13/2+ and (17/2+,15/2+) states. In-beam FEST method will be used with 11 LaBr3:Ce detectors available. The half-life of the (15/2+,17/2+) state will follow from the measurement of the time difference between 506 and 234 keV transitions in the LaBr3:(Ce) detectors

(Eγ-Eγ ) matrix with both anticipated and delayed HPGe timing gates for transitions below and above the I π = 9/2+ isomer of 6 ns. The gates will be selected from the Energy-Time matrix constructed for the HPGe detectors where the I π = 9/2+, T1/2 = 6 ns isomeric state will provide discrete anticipated (1831, 704, and probably 171 keV) and delayed (402 and 1175 keV) reference energy peaks.

Fig. 1 Level schemes from Fotiades et al [1] (left) and Zhang et al [2] (right).

The cross section for producing the 86Kr via the 82Se(7Li,p2n) reaction is only a small percent of the total cross section but an attempt to increase the accuracy of the half-life measurement of the 4+ yrast state in 86Kr, currently 3.1(6) ns, will be made.

Fig. 2

II. THE EXPERIMENT

High-spin states in the N=50 nucleus 87Rb will be investigated via the 82Se(7Li,2n) reaction. We calculated the reaction with the COMPA code, and for an incident energy of about 20 MeV, we obtained a cross section of about 22 mb for producing 87Rb. Assuming a 5 mg/cm2 82Se target, 5 particle-nA 7Li beam, 20 mb 87Rb production cross-section, 5.8 x 104 87Rb nuclei per second will be produced. Assuming a 60 % population of the (15/2+, 17/2+) yrast state and a photopeak efficiency of about 0.5 % (at 1.8 MeV) for HPGe detectors and about 2% (at 500 keV) for LaBr3 detectors, 10000 useful HPGe-LaBr3-LaBr3 coincidences per 5 day are expected. With an additional day for set-up the experiment we request a total of 6 day’s beam time. A much larger number of coincidences detected in HPGe detectors would be acquired during this time, sufficient for DCO measurements and finally spin assignment.

Reaction: 82Se(7Li,2n)

Target thickness: 5 mg/cm2

Beam current: 5 particle-nA 7Li

6 day’s beam time

Fig. 3 Cross sections for 10 strongest channels. Ch200 – 87Rb

Cross sections. Only for asked channels in compa.inp

E Ch200 (2n) Ch300 (3n) Ch 400 (4n)

18 16.1 141 1.72

20 22.2 301 17.3

22 21.8 470 81.2

24 16.5 545 216

26 11.1 542 403

28 8.14 471 600

30 5.06 382 788

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Kostova, H. Prade, R. Schwengner, and G. Winter, Z. Phys. A 352, 127 (1995). 6. B. Singh, Nuclear Data Sheets 94, 1 (2001) 7. G. Winter, J. Döring, LFunke, L. Käubler, R.Schwengner, and H. Prade, Z. Phys. A

332, 33 (1989). 8. G. Winter et al. , Phys. Rev. C 48 (3), 1010 (1993).