100 Years of Nuclear Isomers

The long chain of Sn isotopes is a formidable testing ground for nuclear models studying the evolution of shell structure and interplay between pairing and quadrupole correlations. A transition from superfluid nuclei at midshell to spherical nuclei is also expected approaching the neutron shell closures at N = 50, where the seniority scheme can be adopted to describe the energy spectra. However, the corresponding $B(E2:0^{+}\to 2^{+})$ values have shown a presumed deviation from the expected parabolic behavior. From a theoretical point of view, various attempts have been done to explain the experimental results, in particular by including core-breaking excitations in the shell-model calculations by activating protons and neutrons from the $g_{\frac{9}{2}}$ orbital to the higher ones. From experimental side, limited data are available beyond ${}^{104}$Sn on this very neutron-deficient region, leading to a difficulty in a firmly establishment of core-braking effect.

In this presentation, we will report on the first lifetime measurement for the 4${}_1^{+}$ state in ${}^{102}$Sn which is sensitive to the balance between the pairing and quadrupole terms in the nuclear interaction. The experiment is performed at GSI based on the use of hybrid AIDA+HPGe+LaBr${}_3$(Ce) array, made available by the HISPEC/DESPEC collaboration. The nuclei of interest were separated and identified through the FRS separator, following the production via fragmentation reaction of ${}^{124}$Xe beam incident on a ${}^9$Be target. The ${}^{102}$Sn ions are stopped by AIDA array and $\gamma$ rays emitted from the 6${}^{+}$ seniority isomer are collected by FATIMA array which allows a direct lifetime measurement with a precision up to few tens of ps. The obtained experimental data would be compared with theoretical predictions, shedding light on the detailed wave function and the core breaking contribution.