Statistical structure and $gamma$-decay properties of closed shell Pb nuclei

The level densities and gamma-ray strength functions of 205-208Pb have been measured with the Oslo method, utilizing the (3He, 3He gamma) and (3He,alpha gamma) 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 micro-canonical ensemble have been deduced from the experimental level density. An average entropy difference of Delta S ~ 1.8 k_B has been observed between 205Pb and 206Pb. The gamma-ray strength functions in 205-208Pb are extracted and compared with two models; however, none of them describe the data adequately. Intermediate structures have been observed at lower gamma-ray energies in all the analyzed Pb nuclei. These structures are less pronounced while moving from the doubly-magic nucleus 208Pb to 205Pb.