Several experimental investigations have reported evidence of collective enhancement of the nuclear level density and its fadeout. However, a suitable method is needed for experimental determination of the enhancement factor as a function of excitation energy. In this study, neutron spectra were measured in coincidence with evaporated $\alpha$ particles produced in the reactions ${}^{11}\mathrm{B}+{}^{181}\mathrm{Ta},{}^{197}\mathrm{Au}$. The nuclear level density parameter has been extracted for the $\mathrm{Os}(A\approx 188)$ and $\mathrm{Pb}(A\approx 204)$ isotopes by comparing neutron spectra with statistical model prediction. Evidence for collective enhancement has been found for Os nuclei whereas no such enhancement has been seen for Pb nuclei. The energy-dependent enhancement factor has been extracted by simultaneous fitting of the neutron spectra at various excitation energies. Near a temperature of 0.8 MeV, the enhancement starts to fadeout which is lower than the theoretically predicted temperature of 1.4 MeV for ${}^{187}\mathrm{Os}$. Also, free energy surface calculation shows that the ${}^{187}\mathrm{Os}$ nucleus undergoes a transition from collective prolate to noncollective oblate shape close to the temperature of 0.8 MeV, corroborating the early fadeout. No such shape transition is seen for ${}^{203}\mathrm{Pb}$.

• Received 14 March 2019
• Revised 18 June 2019

DOI:https://doi.org/10.1103/PhysRevC.100.011602