Effect of collectivity on the nuclear level density

Abstract

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 (${E}^{*}$ $\ensuremath{\sim}$ 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 ($\ensuremath{\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.