Alpha-gamma decay studies of $^{261}$Rf and $^{257}$No

Alpha-gamma and alpha-electron coincidence measurements were performed for the $\alpha$ decay of $^{261}$Rf and $^{257}$No to establish Nilsson single-particle states in odd-mass superheavy nuclei. The neutron single-particle configuration of 3/2[622] has been assigned to the ground state of $^{257}$No as well as to the 124.1 keV level in $^{253}$Fm. It was found that the ground state configuration of $^{257}$No is different from that of the lighter $N=155$ isotones $^{255}$Fm and $^{253}$Cf. Measured excitation energy in $^{257}$No populated by the $\alpha$ decay of $^{261}$Rf ($T_{1/2}=67$~s) revealed that another $\alpha$-decaying state in $^{261}$Rf with $T_{1/2}=4.2$~s reported in the $\alpha$-decay chain of $^{277}$112 is not the ground state but an isomeric state. The ground state configuration of $^{261}$Rf is discussed on the basis of the $\alpha$-$\gamma$ coincidence results.