Rotational alignments in Np235 and the possible role of j15/2 neutrons

Abstract

The role j 15/2 neutron orbitals play in the transuranic region of actinides has been studied by exploring γ-ray transitions between yrast states in 235 Np, populated utilizing the nucleon-transfer reaction 137 Np( 116 Sn, 118 Sn). Two rotational sequences, presumably the two signatures of the ground-state band, have been delineated to high spin for the first time, with the α = +1/2 and α=-1/2 signature partners reaching 49/2+ h (tentatively 53/ 2 + h) and 47/2 + ħ (tentatively 51/2 + ħ), respectively. Definite isotopic assignments for these in-band transitions were established through γ-ray cross correlations between 235 Np and 118 Sn and events where at least three γ rays corresponding to neptunium-like particles were detected. These transitions reveal clear upbends in the aligned angular momentum and kinematic moment of inertia plots; such a phenomenon could indicate a strong interaction between an aligned νj 15/2 configuration crossing the ground-state band in 135 Np, which is based on a πi 13/2 orbital. However, the lack of any signature splitting over the observed frequency range of the 135 Np rotational sequences cannot remove the possibility of a πh 9/2 assignment for the observed band. The role of the νj 15/2 and πi 13/2 alignment mechanisms in the deformed U-Pu region is discussed in light of the current spectroscopic data and in the context of the cranked-shell model.