Gamma Ray Spectrum from Thermal Neutron Capture on Gadolinium-157

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We have measured the γ-ray energy spectrum from the thermal neutron capture, ¹⁵⁷Gd(n,γ)¹⁵⁸Gd, on an enriched ¹⁵⁷Gd target (Gd₂O₃) 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 (⁶⁰Co, ¹³⁷Cs, and ¹⁵²Eu) and the reaction ³⁵Cl(n,γ) were used to determine the spectrometer's detection efficiency for γ 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 γ-ray spectrum from the thermal ¹⁵⁷Gd(n,γ) 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 ¹⁵⁸Gd to describe the continuum γ-ray spectrum from the ¹⁵⁷Gd(n,γ) reaction. Our model combines these two components. The results of the comparison between the observed γ-ray spectra from the reaction and the model are reported in detail.