Level density andγ-decay properties of closed shell Pb nuclei

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The level densities and $\ensuremath{\gamma}$-ray strength functions of $^{205\ensuremath{-}208}\mathrm{Pb}$ have been measured with the Oslo method, using the $(^{3}\mathrm{He},^{3}\mathrm{He}{}^{'}\ensuremath{\gamma})$ and $(^{3}\mathrm{He},\ensuremath{\alpha}\ensuremath{\gamma})$ reactions on the target nuclei $^{206}\mathrm{Pb}$ and $^{208}\mathrm{Pb}$. 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 $\ensuremath{\Delta}S~0.9{k}_{B}$ has been observed between $^{205}\mathrm{Pb}$ and $^{206}\mathrm{Pb}$. The $\ensuremath{\gamma}$-ray strength functions in $^{205\ensuremath{-}208}\mathrm{Pb}$ have been extracted and compared with two models; however, none of them describe the data adequately. Intermediate structures have been observed in the $\ensuremath{\gamma}$-ray strength functions for $\ensuremath{\gamma}$-ray energies below neutron threshold in all the analyzed Pb nuclei. These structures become less pronounced while moving from the doubly magic nucleus $^{208}\mathrm{Pb}$ to $^{205}\mathrm{Pb}$.