Neutron one-quasiparticle states in 251Fm151 populated via the α decay of 255No

Excited states in Fm-251 populated via the alpha decay of No-255 are studied in detail through alpha-gamma coincidence and alpha fine-structure measurements. Five excited states reported previously in Fm-251 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 Fm-251 as well as the ground state of No-255 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.