Level density and γ-decay properties of closed shell Pb nuclei

The level densities and γ-ray strength functions of 205-208Pb have been measured with the Oslo method, using the (3He,3He'γ) and (3He,αγ) reactions on the target nuclei 206Pb and 208Pb. The extracted level densities are consistent with known discrete levels at low excitation energies. The entropies and temperatures in the microcanonical ensemble have been deduced from the experimental level density. An average entropy difference of ΔS∼0.9kB has been observed between 205Pb and 206Pb. The γ-ray strength functions in 205-208Pb have been extracted and compared with two models; however, none of them describe the data adequately. Intermediate structures have been observed in the γ-ray strength functions for γ-ray energies below neutron threshold in all the analyzed Pb nuclei. These structures become less pronounced while moving from the doubly magic nucleus 208Pb to 205Pb