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

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Excited states in ^<251>Fm populated via the α decay of ^<255>No are studied in detail through α-γ coincidence and α fine-structure measurements. Five excited states reported previously in ^<251>Fm 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 ^<251>Fm as well as the ground state of ^<255>No 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 (β_4) and hexacontatetrapole (β_6) deformations.
2011-01-27