Millisecond 23/2+ isomers in the N=79 isotones Xe133

Abstract

Detailed information on isomeric states in A ≈ 135 nuclei is exploited to benchmark shell-model calculations in the region northwest of doubly magic nucleus 132 Sn. The N = 79 isotones 133 Xe and 135 Ba are studied after multinucleon transfer in the 136 Xe + 208 Pb reaction employing the high-resolution Advanced GAmma Tracking Array (AGATA) coupled to the magnetic spectrometer PRISMA at the Laboratori Nazionali di Legnaro, Italy and in a pulsed-beam experiment at the FN tandem accelerator of the University of Cologne, Germany utilizing a 9 Be + 130 Te fusion-evaporation reaction at a beam energy of 40 MeV. Isomeric states are identified via delayed γ-ray spectroscopy. Hitherto tentative excitation energy, spin, and parity assignments of the 2107-keV J π = 23/2 + isomer in 133 Xe are confirmed and a half-life of T 1/2 = 8.64(13) ms is measured. The 2388-keV state in 135 Ba is identified as a J π = 23/2 + isomer with a half-life of 1.06(4) ms. The new results show a smooth onset of isomeric J π = 23/2 + states along the N = 79 isotones and close a gap in the high-spin systematics towards the recently investigated J π = 23/2 + isomer in 139 Nd. The resulting systematics of M2 reduced transition probabilities is discussed within the framework of the nuclear shell model. Latest large-scale shell-model calculations employing the SN100PN, GCN50:82, SN100-KTH, and a realistic effective interaction reproduce the experimental findings generally well and give insight into the structure of the isomers.

Quadrupole deformation of Xe130 measured in a Coulomb-excitation experiment

“…In particular, the GCN50:82 interaction shows good agreement for the neighboring Xe isotopes 131 Xe [48], 132 Xe [49], and 133 Xe [50].…”

“…The calculations were undertaken using the KSHELL program [47]. Both shell-model approaches have successfully reproduced experimental results for a range of isotopes in this mass region, namely 135,136,137 Ba and 131,132,133,135 Xe [31,44,[48][49][50][51].…”

“…Previous studies used larger effective charges for nuclei in this region of the nuclear chart. For example, transition strengths from isomeric decays in 129 Sn, 131 Te, 133 Xe, 135 Ba were described using e π = 1.52e and e ν = 0.81e in both the GCN50:82 and the SN100PN interactions [50]. The same effective charges were used for 133 Te, 135 Xe, and 137 Ba with the SN100PN interaction [52].…”

Low-Energy Coulomb Excitation for the Shell Model

“…In the stable Ge isotopes, the Q 2 invariants extracted for the ground state and the 0 + 2 state via low-energy Coulomb excitation represent one of the strongest signatures of shape coexistence [21,22]. As shown in Figure 3a, the ground-state Q 2 values in [70][71][72][73][74][75][76] Ge are similar, 0.2-0.3 e 2 b 2 , while those of the 0 + 2 states evolve as a function of the neutron number. The 0 + 2 state in 70 Ge is more deformed than the ground state [23], in 72 Ge both states seem to have comparable overall deformations and considerable triaxiality [24], while those for the 0 + 2 states in 74,76 Ge point to nearly spherical shapes [25,26].…”

“…On the neutron-rich side, low-energy Coulomb excitation has provided valuable structure information in the Ge and Zn isotopes. Experiments at ISOLDE identified the first excited 2 + 1 state in 78,80 Zn and yielded the B(E2; 2 + 1 → 0 + g.s ) values in [74][75][76][77][78][79][80] Zn and the B(E2; 4 + 1 → 2 + 1 ) values in 74,76 Zn [42,43]. The obtained B(E2) values hint at the importance of triaxiality also in neutron-rich Zn isotopes, whose ground states were suggested to be rather diffuse in the γ degree of freedom [42].…”