New phenomenon in exotic neutron-rich Sn isotopes: role of 3-body force

Abstract

The excitation energies of the first 2+ states (E(2+1)) of even-even 134–140Sn have been calculated using shell model with empirical SMPN and realistic CWG interactions in the π(gdsh) × ν(hfpi) valence space above 132Sn core. The CWG predicts nearly constant E(2+1) for 134–138Sn isotopes, normally expected for singly-magic nuclei, and a weak shell closure at 140Sn. The prediction of SMPN differs dramatically, decreasing E(2+1) with increasing valence neutron number for 134–138Sn and a strong shell closure at 140Sn. This variation predicted by SMPN has striking similarity with the variation of experimental E(2+1) of even – even 18–22O and 42–48Ca, clearly showing that the N =84-88 spectra exhibit the effect of gradually filling up the ν(2f7/2) orbital which finally culminates in a new shell closure at 140Sn (N = 90). Spin-tensor decomposition of SMPN and CWG and the variation of their components with valence neutron number reveals that the origin of the decreasing E(2+) and shell closure at 140Sn might lie in the three-body effects. Calculations with CWG3M interaction, which is obtained by including a simple 3-body monopole term in the CWG predict decreasing E(2+1) for 134–138Sn and a shell closure at 140Sn.