B(E2) anomaly in $6^+$ isomers of $^{134-138}$Sn isotopes and neutron single-particle energies beyond N=82

TL;DR

This study investigates the anomalous B(E2) behavior of $6^+$ isomers in neutron-rich $^{134-138}$Sn isotopes using shell model and generalized seniority schemes, revealing a new sub-shell closure at N=112 and refining neutron orbital energies.

Contribution

It introduces a combined approach of shell model and generalized seniority calculations to explain B(E2) anomalies and refines neutron orbital energies beyond N=82.

Findings

Confirmed the generalized seniority nature of $6^+$ isomers.

Identified a new sub-shell closure at N=112.

Reproduced experimental data with minor adjustments to two-body matrix elements.

Abstract

Isomeric studies in neutron-rich nuclei present a powerful tool to explore the structure at the nuclear extremes. We recently used the shell model calculations with Renormalized Charge Depen- dent Bonn (RCDB) effective interaction to calculate the properties of the $6^+$ seniority isomers in $^{134-138}$Sn in an attempt to resolve the anomalous B(E2) behavior of the $6^+$ isomer in $^{136}$Sn [Phys. Rev. C 91, 024321 (2015)]. We further explore these isomers by using the generalized seniority scheme for multi-j orbitals recently presented by us [Phys. Lett. B 753, 122 (2016)]; the B(E2) values so calculated reproduce the experimental data quite well, including the anomaly at $^{136}$Sn confirming the generalized seniority nature of the $6^+$ isomers. We then use the generalized seniority guided Large Scale Shell Model (LSSM) calculations, along with the latest single particle energies from Jones et al : [Nature (London) 465, 454 (2010)] and Allmond et al : [Phys. Rev. Lett. 112, 172701 (2014)] to estimate more accurate location of i$_{13/2}$ neutron orbital in the extreme neutron rich N = 82-126 region. This entails a new sub-shell closure at N = 112 due to the higher location of i$_{13/2}$ neutron orbital, also consistent with the choice of orbitals in the generalized seniority scheme. However, a small reduction in the f$_{7/2}$ two-body matrix elements is still required in the LSSM calculations to reproduce the experimental level energies as well as the transition probabilities in $^{134-138}$Sn isotopes in a consistent way.