Rotational alignments in Np-235 and the possible role of j(15/2) neutrons

The role j(15/2) neutron orbitals play in the transuranic region of actinides has been studied by exploring gamma-ray transitions between yrast states in Np-235, populated utilizing the nucleon-transfer reaction Np-237(Sn-116,Sn-118). Two rotational sequences, presumably the two signatures of the ground-state band, have been delineated to high spin for the first time, with the alpha = +1/2 and alpha = -1/2 signature partners reaching 49/2(+(h) over bar) (tentatively 53/2(+)(h) over bar) and 47/2(+)(h) over bar (tentatively 51/2(+)(h) over bar), respectively. Definite isotopic assignments for these in-band transitions were established through gamma-ray cross correlations between Np-235 and Sn-118 and events where at least three gamma 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 nu j(15/2) configuration crossing the ground-state band in Np-235, which is based on a pi i(13/2) orbital. However, the lack of any signature splitting over the observed frequency range of the Np-235 rotational sequences cannot remove the possibility of a pi h(9/2) assignment for the observed band. The role of the nu j(15/2) and pi i(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.