Gamma-ray spectrum from thermal neutron capture on gadolinium-157

Abstract

We have measured the $\gamma$-ray energy spectrum from the thermal neutron capture, ${}{157}$Gd$(n,\gamma)$, on an enriched ${157}$Gd target (Gd$ {2}$O$ {3}$) in the energy range from 0.11 MeV up to about 8 MeV. The target was placed inside the germanium spectrometer of the ANNRI detector at J-PARC and exposed to a neutron beam from the Japan Spallation Neutron Source (JSNS). Radioactive sources (${60}$Co, ${137}$Cs, and ${152}$Eu) and the ${35}$Cl($n$,$\gamma$) reaction were used to determine the spectrometer's detection efficiency for $\gamma$ rays at energies from 0.3 to 8.5 MeV. Using a Geant4-based Monte Carlo simulation of the detector and based on our data, we have developed a model to describe the $\gamma$-ray spectrum from the thermal ${}{157}$Gd($n$,$\gamma$) reaction. While we include the strength information of 15 prominent peaks above 5 MeV and associated peaks below 1.6 MeV from our data directly into the model, we rely on the theoretical inputs of nuclear level density and the photon strength function of ${}{158}$Gd to describe the continuum $\gamma$-ray spectrum from the ${}{157}$Gd($n$,$\gamma$) reaction. Our model combines these two components. The results of the comparison between the observed $\gamma$-ray spectra from the reaction and the model are reported in detail.