Excited states in ${}^{251}\mathrm{Fm}$ populated via the $\alpha$ decay of ${}^{255}\mathrm{No}$ are studied in detail through $\alpha$-$\gamma$ coincidence and $\alpha$ fine-structure measurements. Five excited states reported previously in ${}^{251}\mathrm{Fm}$ are firmly established through the $\alpha$-$\gamma$ coincidence measurement, and rotational bands built on one-quasiparticle states are newly established through the $\alpha$ 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 $\gamma$ transitions, rotational-band energies built on one-quasiparticle states, and hindrance factors of $\alpha$ 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 $Z⩾100$, 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 (${\beta }_4$) and hexacontatetrapole (${\beta }_6$) deformations.

• Received 7 November 2010

DOI:https://doi.org/10.1103/PhysRevC.83.014315