Rotational alignments inNp235and the possible role ofj15/2neutrons

Abstract

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