Lifetimes of Three Low-Lying Excited States inIn117

Abstract

The lifetimes of three low-lying excited states in $^{117}\mathrm{In}$ have been measured by delayed-coincidence techniques using a time-to-amplitude converter. These states at 748, 659, and 587 keV are fed by ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay from the ${s}_{\frac{1}{2}}$ ground state of $^{117}\mathrm{Cd}$ (2.4 h). The measured half-lives are 4.6\ifmmode\pm\else\textpm\fi{}0.3, 59.7\ifmmode\pm\else\textpm\fi{}2.0, and 0.17\ifmmode\pm\else\textpm\fi{}0.03 nsec, respectively. All three levels de-excite directly to the 314-keV ${p}_{\frac{1}{2}}$ isomeric state but not to the ${g}_{\frac{9}{2}}$ ground state of $^{117}\mathrm{In}$. A comparison of the partial half-lives with the corresponding single-particle estimates has been performed assuming $E1$, $M1$, or $E2$ character for the transitions. Higher multipolarities are ruled out by the short half-lives of the levels. $\mathrm{Log}\mathrm{ft}$ values of transitions from $^{117}\mathrm{Cd}$ indicate negative parities for the three levels, and systematics of known $\ensuremath{\gamma}$-ray transition probabilities suggest $M1\ensuremath{-}E2$ character for all the transitions involved in their de-excitation. However, when considered as pure $E2$, the transitions from the 748- and 659-keV levels are delayed by factors ranging from about 3 to 20 compared with the single-particle transition probabilities, whereas when they are considered as pure $M1$ transitions, delays of the order of ${10}^{4}$ result. On the other hand, the 274-keV transition from the 587-keV level is enhanced by a factor of about 60 when assumed as pure $E2$. The possibility of considering this level as a member of the doublet obtained by coupling the ${2}^{+}$ phonon excitation of the $^{116}\mathrm{Cd}$ or $^{118}\mathrm{Sn}$ even-even core to the ${p}_{\frac{1}{2}}$ single-particle state is discussed.