Neutron one-quasiparticle states in 251 Fm 151 populated via the α decay of No 255

Excited states in $^{251}\mathrm{Fm}$ populated via the $\ensuremath{\alpha}$ decay of $^{255}\mathrm{No}$ are studied in detail through $\ensuremath{\alpha}$-$\ensuremath{\gamma}$ coincidence and $\ensuremath{\alpha}$ fine-structure measurements. Five excited states reported previously in $^{251}\mathrm{Fm}$ are firmly established through the $\ensuremath{\alpha}$-$\ensuremath{\gamma}$ coincidence measurement, and rotational bands built on one-quasiparticle states are newly established through the $\ensuremath{\alpha}$ fine-structure measurement. Spin-parities and neutron configurations of the excited states in $^{251}\mathrm{Fm}$ as well as the ground state of $^{255}\mathrm{No}$ are definitely identified on the basis of deduced internal conversion coefficients, lifetimes of $\ensuremath{\gamma}$ transitions, rotational-band energies built on one-quasiparticle states, and hindrance factors of $\ensuremath{\alpha}$ transitions. It is found that the excitation energy of the 1/2${}^{+}$[620] state in $N=151$ isotones increases with the atomic number, especially at $Z\ensuremath{\geqslant}100$, while that of the 1/2${}^{+}$[631] state decreases at $Z=100$. Ground-state deformations and energies of neutron one-quasiparticle states in the $N=151$ isotones are calculated using a macroscopic-microscopic model, and the energy systematics of the one-quasiparticle states in the isotones are discussed in terms of the evolution of nuclear deformation involving the hexadecapole (${\ensuremath{\beta}}_{4}$) and hexacontatetrapole (${\ensuremath{\beta}}_{6}$) deformations.