Effect of collectivity on the nuclear level density

Neutron evaporation spectra at backward angles from ${}^{201}$Tl${}^{*}$, ${}^{185}$Re${}^{*}$, and ${}^{169}$Tm${}^{*}$ compound nuclei, having different ground-state deformations, have been measured at two excitation energies ($E^{*}$ $\sim$ 37 and 26 MeV). The values of the inverse level density parameter ($k$), extracted at these excitations using statistical model calculations, are observed to decrease substantially at the lower excitation energy ($\sim$26 MeV) for nuclei having large ground-state deformation (residues of ${}^{185}$Re${}^{*}$ and ${}^{169}$Tm${}^{*}$), whereas for near-spherical nuclei (residues of ${}^{201}$Tl${}^{*}$), the $k$ value remains unchanged at the two energies. The decrease in $k$ at the lower excitation energy for the deformed systems amounts to a relative increase in nuclear level density, indicating a collective enhancement. The present observation clearly establishes the existence of a strong correlation between collectivity and ground-state deformation.

Figure 1

Measured neutron energy spectra (symbols) for the ${}^4$He+${}^{197}$Au system at $E_{\mathrm{lab}}=40$ and 28 MeV, along with statistical model calculation performed with gemini$++$ (continuous lines).

Figure 2

Same as in Fig. 1 for the ${}^4$He+${}^{181}$Ta system at $E_{\mathrm{lab}}$ $=$ 40 and 30 MeV.

Figure 3

Same as in Fig. 1 for the ${}^4$He+${}^{165}$Ho system at $E_{\mathrm{lab}}=40$ and 28 MeV. Predictions of gemini$++$ with default level density parameter are shown by the dashed curves.