Persistence of the weak-coupling limit around $^{132}Sn$: The case of the $1p\mbox{-}2h ^{131}Sb$ nucleus.

In this contribution, the lifetime of the first $11/2^{+}$ state in the 1p-2h, $^{131}Sb$ nucleus around doubly magic $^{132}Sn$ was measured by fast-timing techniques, yielding $\tau=5(3)$ ps. This finding, at the limit of sensitivity of the experimental method, enabled to asses the transition probability to the $7/2^{+}$ ground state, providing $B(E2)i =1.5(9)$ W.u., similar to the value reported for the first $2^{+}$ state of $^{130}Sn$. The experiment was carried out at Insititut Laue-Langevin (ILL) by using neutron-induced fissions of $^{235}U$ and the LOHENGRIN spectrometer, with a detection setup at the focal plane composed by an ionization chamber, two HPGe detectors and four $LaBr_{3}(Ce)$ fast scintillators. The experimental results were compared to realistic large-scale shell-model calculations, with a $^{100}Sn$ core and a two-body effective interaction derived from the CD-Bonn $NN$ potential. This suggests the validity of the weak-coupling limit description in this nucleus and implications on the emergence of collectivity around $^{132}Sn$ will be discussed.