Experimental Identification of Spin-Parities and Single-Particle Configurations inNo257and Itsα-Decay DaughterFm253

Abstract

$\alpha \mathrm{\text{−}}\gamma$ and $\alpha$-electron coincidence spectroscopy for a short-lived heavy actinide nucleus ${}^{257}\mathrm{No}$ ($T_{1/2}=24.5\mathrm{s}$) has been performed using a gas-jet transport system and an on-line isotope separator. Spin-parities of excited states in ${}^{253}\mathrm{Fm}$ fed by the $\alpha$ decay of ${}^{257}\mathrm{No}$ have been identified on the basis of the measured internal conversion coefficients. The $\nu 3/2^{+}[622]$ configuration has been assigned to the ground state of ${}^{257}\mathrm{No}$ as well as to the 124.1 keV level in ${}^{253}\mathrm{Fm}$. It was found that the ground-state configuration of ${}^{257}\mathrm{No}$ is different from that of lighter $N=155$ isotones.