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

Abstract

We have measured the -ray energy spectrum from the thermal neutron capture, 157 Gd(n, ), 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, ) reaction were used to determine the spectrometer's detection efficiency for 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 -ray spectrum from the thermal 157 Gd(n, ) 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 -ray spectrum from the 157 Gd(n, ) reaction. Our model combines these two components. The results of the comparison between the observed -ray spectra from the reaction and the model are reported in detail. .........