Shell-model states with seniorityν=3, 5, and 7 in odd-Aneutron-rich Sn isotopes

Abstract

Excited states with seniority ν = 3, 5, and 7 have been investigated in odd neutron-rich 119,121,123,125 Sn isotopes produced by fusion-fission of 6.9-MeV/A 48 Ca beams with 208 Pb and 238 U targets and by fission of an 238 U target bombarded with 6.7-MeV/A 64 Ni beams. Level schemes have been established up to high spin and excitation energies in excess of 6 MeV, based on multifold gamma-ray coincidence relationships measured with the Gammasphere array. In the analysis, the presence of isomers was exploited to identify gamma rays and propose transition placements using prompt and delayed coincidence techniques. 121 Sn, has now been confirmed. In both cases, isomeric half-lives were determined with good precision. In the range of ν = 3 excitations, the observed transitions linking the various states enabled one to propose with confidence spin-parity assignments for all the observed states. Above the 27/2 − isomers, an elaborate structure of negative-parity levels was established reaching the (39/2 − ), ν = 7 states, with tentative spin-parity assignments based on the observed deexcitation paths as well as on general yrast population arguments. In all the isotopes under investigation, strongly populated sequences of positive-parity (35/2 + ), (31/2 + ), and (27/2 + ) states were established, feeding the 23/2 + isomers via cascades of three transitions. In the 121,123 Sn isotopes, these sequences also enabled the delineation of higher-lying levels, up to (43/2 + ) states. In 123 Sn, a short half-life was determined for the (35/2 + ) state. Shell-model calculations were carried out for all the odd Sn isotopes, from 129 Sn down to 119 Sn, and the results were found to reproduce the experimental level energies rather well. Nevertheless, some systematic deviations between calculated and experimental energies, especially for positive-parity states, point to the need to improve some of the two-body interactions used in calculations. The computed wave-function amplitudes provide for a fairly transparent interpretation of the observed level structures. The systematics of level energies over the broad A = 117 -129 range of Sn isotopes displays a smooth decrease with mass A, and the observed regularity confirms most of the proposed spin-parity assignments. The systematics of the B(E2) reduced transition probabilities extracted for the 23/2 + and 19/2 + isomers is discussed with an emphasis on the close similarity of the observed A dependence with that of the E2 transition rates established for other ν = 2, 3, and 4 isomers in the Sn isotopic chain.

Effects of one valence proton on seniority and angular momentum of neutrons in neutron-rich Sb51122–131 isotopes

“…and νh −3 11/2 configurations, respectively [18,20,21], an additional hole in the νh 11/2 orbital coupled to the counterparts, 7 − and 10 + isomers in even-A Sn, respectively. The experimental data from these nuclei, due to proton shell closure, probe the neutron-neutron (νν) interaction.…”

“…17. [20,21], (f) 13 + → 11 + in the even-A 126−130 Sb (see text), and (g) 27/2 − → 23/2 − in odd-A 119-129 Sn isotopes [20,21,49]. The shell-model calculations using the full interaction SM2 (red solid line), SM2 restricted to only one broken neutron pair (green dash-dotted line) and SM2 with e π = 0 (purple dotted line) are also shown.…”

“…Case I: (a) 10 + → 8 + in even-A[118][119][120][121][122][123][124][125][126][127][128][129][130] Sn[16,21,48], (b) 23/2 + → 19/2 + in odd-A 123-131 Sb (see text); Case II: (c) 7 − → 5 − in even-A 118−126 Sn [21], (d) 19/2 − → 15/2 − in odd-A 123-131 Sb (see text); Case III: (e) 23/2 + → 19/2 + in odd-A 119-129 Sn [20,21], (f) 13 + → 11 + states in the even-A 126−130 Sb (see text); and Case IV: (g) 27/2 − → 23/2 − in odd-A 119-129 Sn isotopes [20,21,49].The full calculations are shown by red solid lines, those restricted to one broken pair are shown by dash-dotted ν )/P(υ ν +2) P(I ν )/P(I ν -2)(h) P(I ν )/P(I ν -1) P(I ν )/P(I ν -2) ν )/P(υ ν +2) P(υ ν )/P(υ ν +2) The experimental (Expt.) B(E 2) values (black squares) for the (a) 10 + → 8 + in even-A 118-130 Sn [16,21,48], (b) 23/2 + → 19/2 + in odd-A 123-131 Sb (see text), (c) 7 − → 5 − in even-A 118−126 Sn [21], (d) 19/2 − → 15/2 − transitions in odd-A 123-131 Sb (see text), (e) 23/2 + → 19/2 + in odd-A 119-129 Sn…”

Development of a liquid scintillator based active fission target for FIPPS

“…Ref. [32] for details) is used at HPGe-detector arrays for the investigation of isomeric states with half-lives in the tens of ns to μs range. In this kind of investigations, the γ-ray energy spectra obtained with different time conditions are analyzed, considering as time reference the "trigger" of the events.…”

Effects of one valence proton on seniority and angular momentum of neutrons in neutron-rich $^{122-131}$Sb$_{51}$ isotopes

“…Even-A 118−130 Sn isotopes possess 7 − and 10 + isomers, with dominant neutron νh −1 11/2 d −1 3/2 and νh −2 11/2 configurations, respectively [16][17][18][19]. Similarly, odd-A 119−129 Sn isotopes display 23/2 + and 27/2 − isomers with dominant νh −2 11/2 d −1 3/2 and νh −3 11/2 configurations, respectively [18,20,21], an additional hole in the νh 11/2 orbital coupled to the counterparts, 7 − and 10 + isomers in even-A Sn, respectively. The experimental data from these nuclei, due to proton shell closure, probe the neutronneutron (νν) interaction.…”