Ground-state configuration of theN=157nucleus259No

The ground-state configuration of the $N=157$ nucleus ${}^{259}$No has been identified through $\ensuremath{\alpha}$-$\ensuremath{\gamma}$ coincidence and $\ensuremath{\alpha}$-singles measurements. Three $\ensuremath{\gamma}$ transitions were observed for the first time in the $\ensuremath{\alpha}$ decay of ${}^{259}$No, and its decay scheme was established. The neutron 9/2${}^{+}$[615] configuration was assigned to the ground state of ${}^{259}$No as well as to the 231.4 keV level in ${}^{255}$Fm. Ground-state deformations and neutron single-particle energies in $Z=102$ isotopes were calculated with the macroscopic-microscopic model. The 9/2${}^{+}$[615] orbital was calculated to be the highest among the five orbitals between the $N=152$ and 162 deformed shell gaps, which is consistent with the experimental one-quasiparticle energies in $N=153$ and 155 isotones, but is inconsistent with the present experimental result of the 9/2${}^{+}$[615] ground state at $N=157$. To reproduce the 9/2${}^{+}$[615] ground state at $N=157$, the order of the neutron orbitals should be different between the $N=153$ and 157 isotones.