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

Collective enhancement in nuclear level density (CELD) and its fadeout has been studied experimentally using neutron and high energy gamma-ray spectra earlier. Attempts to probe CELD with alpha-particle spectra were made for compound nucleus (CN) Hf-178 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 alpha particles, if any, would be found at excitation lower than 25 MeV. With the aim to observe CELD and its fadeout with alpha-particle spectra, two reactions C-12 + Sn-116, Tb-159, forming CN in different mass region (A approximate to 128 and A approximate to 171) were studied. Evaporation 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 alpha-particle spectra. As a function of CN excitation energy, the k value showed peak like structure for the reaction C-12 + 159Tb which indicates fadeout of CELD. No such evidence was found for the reaction C-12 + Sn-116. Collective enhancement factor for daughter nuclei populated in C-12 + Tb-159 reaction was extracted. Critical energy of the fadeout was found to be similar to that of mass A approximate to 188 region.