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

TL;DR

This study measures the photoneutron cross sections of $^{208}$Pb in the Giant Dipole Resonance region using laser Compton backscattering gamma-ray beams, revealing fine structure and providing detailed reaction data.

Contribution

First measurements of absolute photoneutron cross sections on $^{208}$Pb in the GDR region using quasi-monochromatic gamma-ray beams and neutron multiplicity sorting.

Findings

Observed fine structure in the $( ext{γ},n)$ cross sections below 13 MeV.

Obtained photoabsorption cross sections from reaction data.

Estimated neutron energy spectra and partial cross sections for residual states.

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.