Experimental investigation of the level density in highly excited nuclei around Pb-208

The reactions B-11 + Pt-198 --> Bi-209* and B-10 + Pt-198 --> Bi-208* have been investigated by measuring low- and high-energy gamma-rays and proton-gamma coincidences. The nuclear temperature of the Pb-208 populated by first-chance proton emission from Bi-209 has been determined by using an unfolding technique. The extracted value, T = 1.6 +/- 0.2 MeV, is compared with predictions from various level density parametrisations in which the shell effects play different roles. The comparison shows that, as expected in this excitation energy range, the shell effects in Pb-208 are substantially washed out. Proton and high-energy gamma-ray spectra as well as neutron average multiplicities have been also compared with predictions from Statistical Model calculations making use of a constant (a = A/10 MeV-1) or an excitation-energy-dependent level density parameter. The obtained results demonstrate that the high-energy gamma-rays are more sensitive than protons to the level density variation with the excitation energy. However, Statistical Model calculations fail in reproducing satisfactorily the line-shape of the experimental gamma-ray spectra, suggesting that the actual knowledge of the level densities in the Pb-208 region is not sufficiently accurate.