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

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 $<sup>{134-138}$Sn in an attempt to resolve the anomalous B(E2) behavior of the $6^+$ isomer in $<sup>{136}$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 $<sup>{136}$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$<em>{13/2}$ 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$</em>{13/2}$ 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 $<sup>{134-138}$Sn isotopes in a consistent way.