Photoneutron cross section measurements on ${}^{208}$Pb in the Giant Dipole Resonance region

Abstract

Photoneutron reactions on ${}^{208}$Pb in the Giant Dipole Resonance energy region have been investigated at the $\gamma$-ray beam line of the NewSUBARU facility in Japan. The measurements made use of quasi-monochromatic laser Compton backscattering $\gamma$-ray beams in a broad energy range, from the neutron threshold up to 38 MeV, and of a flat-efficiency moderated ${}^3$He neutron detection system along with associated neutron-multiplicity sorting methods. We report absolute cross sections and mean photoneutron energies for the ${}^{208}$Pb$(\gamma ,inX)$ reactions with $i$~=~1 to 4. The fine structure present in the ${}^{208}$Pb$(\gamma ,n)$ cross sections at incident energies lower than 13~MeV has been observed. The photoabsorption cross section has been obtained as the sum of the $(\gamma ,inX)$ reaction cross sections. By reproducing the measured ring-ratio values at excitation energies below the two neutron separation energy, we were able to extract estimations on the ${}^{208}$Pb$(\gamma ,n)$ photoneutron energy spectra and on the partial photoneutron cross sections for leaving the residual ${}^{207}$Pb in its ground and first two excited states. The present results are compared with data from the literature and statistical model calculations.

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