The level scheme of the compound ${}^{153}\mathrm{Sm}$ nucleus formed via the ${}^{152}\mathrm{Sm}(n_{\text{th}},\gamma )$ reaction is studied by using a $\gamma -\gamma$ coincidence spectrometer at Dalat Nuclear Research Institute, Vietnam. All the $\gamma$ cascades, which correspond to the decays from the compound state to 12 final levels of 0 (${\frac{3}{2}}^{+}$), 7.535 (${\frac{5}{2}}^{+}$), 35.844 (${\frac{3}{2}}^{-}$), 90.875 (${\frac{5}{2}}^{-}$), 126.412 (${\frac{1}{2}}^{-}$), 127.298 (${\frac{3}{2}}^{-}$), 182.902 (${\frac{5}{2}}^{-}$), 321.113 (${\frac{3}{2}}^{+}$), 404.129 (${\frac{1}{2}}^{-}$), 405.470 (${\frac{3}{2}}^{-}$), 414.924 (${\frac{1}{2}}^{+}$), and 481.088 (${\frac{3}{2}}^{+}$) keV, have been measured. A total number of 386 cascades corresponding to 576 $\gamma$ transitions has been detected. Among these cascades, 103 primary $\gamma$ transitions together with their corresponding intermediate levels and 299 secondary transitions have been determined. In addition, 29 primary $\gamma$ transitions, 42 intermediate levels, and 8 secondary transitions have been found to be the same as those extracted from the Evaluated Nuclear Structure Data File (ENSDF) data. The remaining 74 primary $\gamma$ transitions, 61 intermediate levels, and 291 secondary transitions are therefore considered as new data. In particular, based on an assumption that most of the transitions are dipole, we have tentatively assigned the unique spin value of $\frac{3}{2}\hslash$ for 53 observed intermediate levels corresponding to the cascades from the compound state to the final levels of 7.535 (${\frac{5}{2}}^{+}$), 90.875 (${\frac{5}{2}}^{-}$), and 182.902 (${\frac{5}{2}}^{-}$) keV, whereas the remaining levels are assigned with the spin values in the range of $[\frac{1}{2},\frac{3}{2}]\hslash$. Moreover, the total and partial (for the spin range of $[\frac{1}{2},\frac{3}{2}]\hslash$) cumulative numbers of levels have been constructed by combining the ENSDF data with the new data obtained in the present experiment. Comparison between these new cumulative curves and those extracted from the nuclear level density (NLD) data, which are obtained using the Oslo method, shows that the maximum excitation energy $E_{\max }$, defined as the energy threshold below which most of the excited levels are observed, is extended to about 1.2 and 1.8 MeV for the total and partial NLD data, respectively. These values of $E_{\max }$ are higher than those obtained by using the present ENSDF data, which are around 1 MeV. The new cumulative curves have also been compared with different phenomenological and microscopic NLD models, and the recent exact pairing plus independent-particle model at finite temperature ($\mathrm{EP}+\mathrm{IPM}$), in which no fitting parameter has been employed, is found to be the best-fit one. The present findings provide updated information on the nuclear level structure and make a step toward the completed level schemes of excited compound nuclei.

• Received 18 February 2019
• Revised 19 June 2019

DOI:https://doi.org/10.1103/PhysRevC.100.024324