Gamma Ray Spectrum from Thermal Neutron Capture on Gadolinium-157

We have measured the $gamma$-ray energy spectrum from the thermal neutron capture, ${}^{157}$Gd$(n,gamma){}^{158}$Gd, 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 reaction $^{35}$Cl($n$,$gamma$) were used to determine the spectrometers 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.