Probing collective enhancement in nuclear level density with evaporation α particle spectra

Abstract

Collective enhancement in nuclear level density (CELD) and its fadeout has been studied experimentally using neutron and high energy $\ensuremath{\gamma}$-ray spectra earlier. Attempts to probe CELD with $\ensuremath{\alpha}$-particle spectra were made for compound nucleus (CN) $^{178}\mathrm{Hf}$ in the excitation energy $54--124$ MeV, but no signature of CELD was found. The possible reason behind this nonobservance has been discussed in a few subsequent reports. In our previous study, evidence of CELD was found using neutron spectra and enhancement faded away near 25 MeV of excitation energy. This implies that the effect of CELD on $\ensuremath{\alpha}$ particles, if any, would be found at excitation lower than 25 MeV. With the aim to observe CELD and its fadeout with $\ensuremath{\alpha}$-particle spectra, two reactions $^{12}\mathrm{C}+^{116}\mathrm{Sn},^{159}\mathrm{Tb}$, forming CN in different mass region ($A\ensuremath{\approx}128$ and $A\ensuremath{\approx}171$) were studied. Evaporation $\ensuremath{\alpha}$-particle spectra were measured for these reactions in singles as well as in coincidence with neutrons. Experimental data were compared with statistical model calculations and inverse level density parameter ($k$) were obtained from $\ensuremath{\alpha}$-particle spectra. As a function of CN excitation energy, the $k$ value showed peak like structure for the reaction $^{12}\mathrm{C}+^{159}\mathrm{Tb}$ which indicates fadeout of CELD. No such evidence was found for the reaction $^{12}\mathrm{C}+^{116}\mathrm{Sn}$. Collective enhancement factor for daughter nuclei populated in $^{12}\mathrm{C}+^{159}\mathrm{Tb}$ reaction was extracted. Critical energy of the fadeout was found to be similar to that of mass $A\ensuremath{\approx}188$ region.