Experimental investigation of the level density in highly excited nuclei around ~(208)Pb

摘要

The reactions ~(11)B + ~(198)Pt → ~(209)Bi~* and ~(10)B + ~(198)Pt → ~(208)Bi~* have been investigated by measuring low- and high-energy γ-rays and proton-γ coincidences. The nuclear temperature of the ~(208)Pb populated by first-chance proton emission from ~(209)Bi 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 ~(208)Pb are substantially washed out. Proton and high-energy γ-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 γ-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 γ-ray spectra, suggesting that the actual knowledge of the level densities in the ~(208)Pb region is not sufficiently accurate.