Detailed structure of Sn131 populated in the β decay of isomerically purified In131 states

Abstract

The excited structure of the single-hole nucleus ${}^{131}\mathrm{Sn}$ populated by the ${\beta }^{-}$ decay of ${}^{131}\mathrm{In}$ was investigated in detail at the ISOLDE facility at CERN. This new experiment took advantage of isomeric purification capabilities provided by resonant ionization, making it possible to independently study the decay of each isomer for the first time. The position of the first-excited $\nu h_{11/2}$ neutron-hole state was confirmed via an independent mass spectroscopy experiment performed at the Ion Guide Isotope Separator On-Line facility at the University of Jyväskylä. The level scheme of ${}^{131}\mathrm{Sn}$ was notably expanded with the addition of 31 new $\gamma$-ray transitions and 22 new excited levels. The $\gamma$-emitting excited levels above the neutron separation energy in ${}^{131}\mathrm{Sn}$ were investigated, revealing a large number of states, which in some cases decay by transitions to other neutron-unbound states. Our analysis showed the dependence between the population of these states in ${}^{131}\mathrm{Sn}$ and the $\beta$-decaying ${}^{131}\mathrm{In}$ state feeding them. Profiting from the isomer selectivity, it was possible to estimate the direct $\beta$ feeding to the $3/2^{+}$ ground and $11/2^{-}$ isomeric states, disentangling the contributions from the three indium parent states. This made possible to resolve the discrepancies in $\log ft$ for first-forbidden transitions observed in previous studies, and to determine the $\beta$-delayed neutron decay probability $\left(P_n\right)$ values of each indium isomers independently. The first measurement of subnanosecond lifetimes in ${}^{131}\mathrm{Sn}$ was performed in this work. A short $T_{1/2}=18\left(4\right)\text{−}\mathrm{ps}$ value was measured for the $1/2^{+}$ neutron single-hole 332-keV state, which indicates an enhanced $l$-forbidden $M1$ behavior for the $\nu 3s_{1/2}^{-1}\to \nu 3d_{3/2}^{-1}$ transition. The measured half-lives of high-energy states populated in the $\beta$ decay of the $(21/2^{+})$ second isomeric state $\left({}^{131m2}\mathrm{In}\right)$ provided valuable information on transition rates, supporting the interpretation of these levels as core-excited states analogous to those observed in the doubly-magic ${}^{132}\mathrm{Sn}$.