Ground-state bands of neutron-richTh236andU242nuclei and implication of spherical shell closure atN=164

Abstract

The ground-state bands of the neutron-rich $^{236}\mathrm{Th}$ and $^{242}\mathrm{U}$ nuclei were established up to spin 10 and 8, respectively, by in-beam \ensuremath{\gamma}-ray spectroscopy using the ($^{18}\mathrm{O}$, $^{20}\mathrm{Ne}$) two-proton pickup reaction with a $^{238}\mathrm{U}$ and a $^{244}\mathrm{Pu}$ target. Deexcitation \ensuremath{\gamma} rays in $^{236}\mathrm{Th}$ and $^{242}\mathrm{U}$ were identified by selecting the kinetic energies of $^{20}\mathrm{Ne}$ using Si $\ensuremath{\Delta}E\text{\ensuremath{-}}E$ detectors. The excitation energies of the first ${2}^{+}$ states in U and Pu isotopes have local minima at $N\ensuremath{\simeq}146$, suggesting the possibility that nuclei with $Z\ensuremath{\simeq}92$ have a spherical shell closure of $N=164$. Calculation using the Koura-Yamada single-particle potential gives an energy gap of 1.8 MeV at $N=164$ for $^{256}\mathrm{U}$.