Improved measurement of the half-life of theJπ=8−nuclear isomerEu152m2

Abstract

The standard $\ensuremath{\gamma}$-ray energy calibration source $^{152}\mathrm{Eu}$ is well known based on the 13.5 y decay of its ground state. However, in addition to this decay $^{152}\mathrm{Eu}$ also has two relatively long-lived isomeric states: a 9 h ${J}^{\ensuremath{\pi}}={0}^{\ensuremath{-}}$ state at ${E}^{*}=46$ keV and a 96 min ${J}^{\ensuremath{\pi}}={8}^{\ensuremath{-}}$ state at ${E}^{*}=148$ keV. Here we report a new measurement of the half-lives of both of these isomeric states. Excited states in $^{152}\mathrm{Eu}$ were populated following the $^{154}\mathrm{Sm}(p,3n)$ reaction using a 25 MeV proton beam from the K-150 cyclotron at the Cyclotron Institute of Texas A University. Post irradiation, $\ensuremath{\gamma}$ rays from the de-excitation of the long lived isomeric states were measured using the six BGO shielded high-purity germanium (HPGe) clover detectors that are part of the STARLiTeR array. The half-life of the ${J}^{\ensuremath{\pi}}={8}^{\ensuremath{-}}$ isomer $^{152{m}_{2}}\mathrm{Eu}$ was obtained by measuring the decrease in intensity of the 90 keV $\ensuremath{\gamma}$ ray from the cascade to the ground state. The half-life of this state was measured to be 95.8(4) min which is in agreement with and significantly more precise than the previously measured value of 96(1) min. In a manner similar to the ground state the second long-lived isomer $^{151{m}_{1}}\mathrm{Eu}$, the ${J}^{\ensuremath{\pi}}={0}^{\ensuremath{-}}$ state at 46 keV, $\ensuremath{\beta}$ decays to excited states in $^{152}\mathrm{Gd}$ and $^{152}\mathrm{Sm}$. The half-life of this state was measured to be 9.39(7) h using five $\ensuremath{\gamma}$-ray transitions.