In-beam $\gamma$-ray spectroscopy of a neutron-rich nucleus of $^{240}$U

We have measured deexcitation $\gamma$ rays in a neutron-rich nucleus of $^{240}$U for the first time. The $^{240}$U nuclei were produced by the two-neutron-transfer reaction of a 200-MeV $^{18}$O beam with a $^{238}$U target at the JAERI tandem booster facility. Outgoing nuclei and $\gamma$ rays were measured using 4 Si $\Delta E$-$E$ detectors and 7 Ge detectors, respectively, and $\Delta E$-$E$-$\gamma$-($\gamma$)-$t$ coincidence data were recorded. The outgoing nuclei were clearly separated not only by the atomic number but by the mass number on the $E$-$\Delta E$ plots. The $\gamma$ rays in $^{240}$U were identified by taking coincidence with $^{16}$O; the excitation energies of $^{240}$U were selected below the neutron separation energy by the kinetic energies of $^{16}$O. The multipolarities of $\gamma$-rays in $^{240}$U were determined by the in-plane to out-of-plane anisotropies of $\gamma$ rays. The ground-state band and the $K^{\pi}=0^{-}$ octupole band of $^{240}$U were established up to 12$^{+}$ and 9$^{-}$, respectively. The moment of inertia for the ground-state band of $^{240}$U is consistent with the systematics of $\beta_2$ and $\beta_4$ deformations in actinide nuclei. The octupole-band head of $^{240}$U is higher than those of $^{236,238}$U by about a hundred keV, suggesting that a secondary maximum of octupole correlations exists at $N=144-146$ in U isotopes.