Alpha-gamma coincidence spectroscopy of $^{259}$Rf using a mixed Cf target

Gamma rays following the $\alpha$ decay of $^{259}$Rf have been observed for the first time by means of $\alpha$-$\gamma$ coincidence spectroscopy. $^{259}$Rf was produced via the $^ {251}$Cf($^{12}$C,4n)$^{259}$Rf reaction at the JAEA tandem accelerator. The target consists of 63\% $^{249}$Cf, 12\% $^ {250}$Cf, and 25\% $^{251}$Cf with a thickness of 420~$\mu$g/cm$^{2}$. Reaction products were transported with a He/KCl aerosol jet into a rotating wheel $\alpha$-$\gamma$ detection system equipped with two sets of two Si detectors and two Ge detectors. Two $\gamma$ lines were observed at 97.3 and 146.7~keV in coincidence with the 8770~keV $\alpha$ transition of $^{259}$Rf. In addition, a few $\gamma$ events appeared at $\sim$125~keV. The energy differences and intensities of these $\gamma$ transitions, which are very similar to those in the $\alpha$ decay of $^{257}$No, allow us to assign the 3/2$^{+} $[622] configuration to the 146.7~keV level in $^{255}$No as well as to the ground state of $^{259}$Rf. This result indicates that the order of neutron orbitals should be inverted between $^{255}$Fm and $^{257}$No in $N=155$ isotones.