Level Structure ofCe141from theCe140(n, γ)Ce141Reaction

Abstract

The level scheme of the $_{58}\mathrm{Ce}_{83}^{141}$ nucleus was studied by observing the $\ensuremath{\gamma}$ rays from the ${\mathrm{Ce}}^{140}({n}_{\mathrm{th}}, \ensuremath{\gamma})$ reaction at the Brookhaven high flux beam reactor. $\ensuremath{\gamma}$-ray singles and coincidence spectra were obtained with a 20-${\mathrm{cm}}^{3}$ Ge(Li) detector and a Ge(Li)-Ge(Li) detector combination. Thirty-two $\ensuremath{\gamma}$ rays were assigned to ${\mathrm{Ce}}^{141}$ and all but three of these $\ensuremath{\gamma}$ rays were included in the ${\mathrm{Ce}}^{141}$ level scheme. We have identified nine of these transitions as primary transitions from the neutron capture state to low-lying levels in ${\mathrm{Ce}}^{141}$. These primary transitions have the following energies (in keV) and relative intensities [I(662) = 100]; 4766.6\ifmmode\pm\else\textpm\fi{}0.5(48.0), 4291.4\ifmmode\pm\else\textpm\fi{}0.6(22.1), 3619.7\ifmmode\pm\else\textpm\fi{}0.6(4.0), 3435.0\ifmmode\pm\else\textpm\fi{}0.6(2.0), 3239.0\ifmmode\pm\else\textpm\fi{}1.5(2.7), 3092.5\ifmmode\pm\else\textpm\fi{}0.5(2.8), 3017.1\ifmmode\pm\else\textpm\fi{}0.7(4.7), 3003.3\ifmmode\pm\else\textpm\fi{}0.6(3.9), and 2905.9\ifmmode\pm\else\textpm\fi{}0.5(3.0). Our data, together with the information available from earlier charged particle reaction and radioactive decay studies, establish levels in ${\mathrm{Ce}}^{141}$ with energies 662.0\ifmmode\pm\else\textpm\fi{}0.1, 1137.0\ifmmode\pm\else\textpm\fi{}0.3, 1497.3\ifmmode\pm\else\textpm\fi{}0.4, 1808.7\ifmmode\pm\else\textpm\fi{}0.5, 1994.0\ifmmode\pm\else\textpm\fi{}0.6, 2189.6\ifmmode\pm\else\textpm\fi{}0.5, 2336.3\ifmmode\pm\else\textpm\fi{}1.0, 2410.8\ifmmode\pm\else\textpm\fi{}0.6, 2425.6\ifmmode\pm\else\textpm\fi{}0.8, and 2522.9\ifmmode\pm\else\textpm\fi{}0.6 keV. The measured neutron separation energy is 5428.6\ifmmode\pm\else\textpm\fi{}0.6 keV. All of these levels may be identified with levels observed earlier with lower accuracy in the ${\mathrm{Ce}}^{140}(d, p){\mathrm{Ce}}^{141}$ reaction. As in the neighboring $N=83$ nuclei ${\mathrm{Ba}}^{139}$ and ${\mathrm{Nd}}^{143}$, there appears to be a correlation between the strengths of excitation of the ${p}_{\frac{3}{2}}$ and ${p}_{\frac{1}{2}}$ levels in the ($n, \ensuremath{\gamma}$) and ($d, p$) reactions. The 1808.7-keV state, shown here to have $\frac{3}{2}$\char22{} spin and parity and not $\frac{5}{2}$\char22{} as previously believed, exhibits properties typical of a core excitation state formed from the coupling of an ${f}_{\frac{7}{2}}$ neutron to the first excited 2+ state of the semimagic ${\mathrm{Ce}}^{140}$ core. In particular, an $E2$ transition proceeds from this state to the $\frac{7}{2}$\char22{} ground state in competition with $M1$ transitions to the $\frac{3}{2}$\char22{} and \textonehalf{}\char22{} first and second excited states. Accordingly, a calculation of the properties of the low-lying excited states in ${\mathrm{Ce}}^{141}$ was carried out on the basis of the weak-coupling model. The results obtained are in reasonable agreement with the measured properties of the levels of ${\mathrm{Ce}}^{141}$. In particular, it is shown that the measured branching ratios of the transitions from the 1497.3- and 1808.7-keV levels are correctly predicted by this model. The level scheme of ${\mathrm{Ce}}^{141}$ is compared with the level schemes of the other even-$Z$, $N=83$ nuclei.