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

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Excited states in 251Fm populated via the α decay of 255No are studied in detail through α-γ coincidence and α fine-structure measurements. Five excited states reported previously in 251Fm are firmly established through the α-γ coincidence measurement, and rotational bands built on one-quasiparticle states are newly established through the α fine-structure measurement. Spin-parities and neutron configurations of the excited states in 251Fm as well as the ground state of 255No are definitely identified on the basis of deduced internal conversion coefficients, lifetimes of γ transitions, rotational-band energies built on one-quasiparticle states, and hindrance factors of α 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 (β4) and hexacontatetrapole (β6) deformations.