The levels in ${}^{129}\mathrm{Sn}$ populated from the ${\beta }^{-}$ decay of ${}^{129}\mathrm{In}$ isomers were investigated at the ISOLDE facility of CERN using the newly commissioned ISOLDE Decay Station (IDS). The lowest $1/2^{+}$ state and the $3/2^{+}$ ground state in ${}^{129}\mathrm{Sn}$ are expected to have configurations dominated by the neutron $s_{1/2}$ ($l=0$) and $d_{3/2}$ ($l=2$) single-particle states, respectively. Consequently, these states should be connected by a somewhat slow $l$-forbidden $M1$ transition. Using fast-timing spectroscopy we have measured the half-life of the $1/2^{+}$ 315.3-keV state, $T_{1/2}=$ 19(10) ps, which corresponds to a moderately fast $M1$ transition. Shell-model calculations using the CD-Bonn effective interaction, with standard effective charges and $g$ factors, predict a 4-ns half-life for this level. We can reconcile the shell-model calculations to the measured $T_{1/2}$ value by the renormalization of the $M1$ effective operator for neutron holes.

• Received 12 January 2016

DOI:https://doi.org/10.1103/PhysRevC.93.044303

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society