Gamma Ray Spectrum from Thermal Neutron Capture on Gadolinium-157

2018
We have measured the $\ gamma$- rayenergy 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-PARCand 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 spectrometer's detection efficiency for $\ gamma$ raysat 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 $\ gamma$- rayspectrum 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 $\ gamma$- rayspectrum from the ${}^{157}$Gd($n$,$\gamma$) reaction. Our model combines these two components. The results of the comparison between the observed $\ gamma$- rayspectra from the reaction and the model are reported in detail.
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