New isomer found in Sb8951140 : Sphericity and shell evolution between N=82 and N=90

In this article we report on the first spectroscopic information on Sb-140(51)89 and the observation of an isomer in this nucleus with T-1/2 = 41(8) mu s. It is located in close proximity to Sn-140 for N = 90, where the nu f(7/2) orbit is completely filled. The possible origin of the isomeric state is a pi g(7/2)(1) x nu f(7/2)(-1) coupling configuration, resulting in a (6(-)) or (7(-)) spin-parity. This is likely caused by an inversion of the pi g(7/2) and pi d(5/2) orbitals at N = 89. The existence of such an isomer far from stability is discussed extensively in the context of shell-model calculations and compared to results from mean-field calculations performed using a deformed Woods-Saxon potential. Both approaches suggest the observation of a single-particle excitation mode at an extreme neutron number.