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

Abstract

The neutron-rich ${}^{122-131}$Sb isotopes were produced as fission fragments in the reaction ${}^9$Be(${}^{238}$U,~f) with 6.2 MeV/u beam energy. An unique setup, consisting of AGATA, VAMOS++ and EXOGAM detectors, was used which enabled the prompt-delayed gamma-ray (${\hat{I}}^3$) spectroscopy of fission fragments in the time range of 100 ns - 200 $\hat{I}¼$s. New isomers, prompt and delayed transitions were established in the even-A ${}^{122-130}$Sb isotopes. In the odd-A ${}^{123-131}$Sb isotopes, new prompt and delayed ${\hat{I}}^3$-ray transitions were identified, in addition to the confirmation of the previously known isomers. The half-lives of the isomeric states and the $B(E2)$ transition probabilities of the observed transitions depopulating these isomers were extracted. The experimental data was compared with the theoretical results obtained in the framework of Large-Scale Shell-Model (LSSM) calculations in a restricted model space. Modifications of several components of the shell model interaction were introduced to obtain a consistent agreement with the excitation energies and the $B(E2)$ transition probabilities in neutron-rich Sn and Sb isotopes. The isomeric configurations in Sn and Sb were found to be relatively pure. Further, the calculations revealed that the presence of a single valence proton, mainly in the $g_{7/2}$ orbital in Sb isotopes, leads to significant mixing (due to the $\hat{I}½\ddot{I}€$ interaction) of: (i) the neutron seniorities (${\upsilon }_{\hat{I}}½$) and (ii) the neutron angular momentum ($I_{\hat{I}}½$). The above features have a weak impact on the excitation energies, but have an important impact on the $B(E2)$ transition probabilities. In addition, a constancy of the relative excitation energies irrespective of neutron seniority and neutron number in Sn and Sb was observed.

23 pages, 18 figures, 10 tables