The level scheme of the ${}_{58}{}^{}{}_{}{}^{}\mathrm{Ce}_{83}^{141}{}_{}{}^{}$ nucleus was studied by observing the $\gamma$ rays from the ${\mathrm{Ce}}^{140}(n_{\mathrm{th}}, \gamma )$ reaction at the Brookhaven high flux beam reactor. $\gamma$-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 $\gamma$ rays were assigned to ${\mathrm{Ce}}^{141}$ and all but three of these $\gamma$ 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±0.5(48.0), 4291.4±0.6(22.1), 3619.7±0.6(4.0), 3435.0±0.6(2.0), 3239.0±1.5(2.7), 3092.5±0.5(2.8), 3017.1±0.7(4.7), 3003.3±0.6(3.9), and 2905.9±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±0.1, 1137.0±0.3, 1497.3±0.4, 1808.7±0.5, 1994.0±0.6, 2189.6±0.5, 2336.3±1.0, 2410.8±0.6, 2425.6±0.8, and 2522.9±0.6 keV. The measured neutron separation energy is 5428.6±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 $N=83\mathrm{}$ 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 ($n, \gamma$) and ($d, p$) reactions. The 1808.7-keV state, shown here to have $\frac{3}{2}$— spin and parity and not $\frac{5}{2}$— 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 $E2\mathrm{}$ transition proceeds from this state to the $\frac{7}{2}$— ground state in competition with $M1\mathrm{}$ transitions to the $\frac{3}{2}$— and ½— 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-$Z$, $N=83\mathrm{}$ nuclei.

• Received 22 October 1969

DOI:https://doi.org/10.1103/PhysRevC.1.1052