New isomer found in ${}_{51}{}^{140}\mathrm{Sb}_{89}$: Sphericity and shell evolution between $N=82$ and $N=90$

In this article we report on the first spectroscopic information on ${}_{51}{}^{140}\mathrm{Sb}_{89}$ and the observation of an isomer in this nucleus with $T_{1/2}=41\left(8\right)$ $\mu \mathrm{s}$. It is located in close proximity to ${}^{140}\mathrm{Sn}$ for $N=90$, where the $\nu f_{7/2}$ orbit is completely filled. The possible origin of the isomeric state is a $\pi g_{7/2}^1\times \nu f_{7/2}^{-1}$ coupling configuration, resulting in a $\left(6^{-}\right)$ or $\left(7^{-}\right)$ 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.

Figure 1

The delayed and background-subtracted $\gamma$-ray spectrum for ${}^{140}\mathrm{Sb}$ (a). Lifetime fit is shown in the inset and coincidence-gate spectra on each of the transitions are plotted in panels (b) and (c).

Figure 2

Experimental level scheme of the observed isomer in ${}^{140}\mathrm{Sb}$ compared to the results of SM calculations. Yrast and yrare states are represented by solid and dotted lines, respectively.

Figure 3

Theoretical $B(E2;j\to j-2)$ and $B(M1;j\to j-1)$ transition rates according to the SM (a), most relevant $\nu$ orbital occupations (b), and most relevant $\pi$ orbital occupations (c) for the first (I) excited and second (II) excited states in ${}^{140}\mathrm{Sb}$.

Figure 4

Possible spin traps according to the MF, representing the spin and excitation energy of states expected to be occupied in ${}^{140}\mathrm{Sb}$.