A Monte Carlo simulation to study a design of a gamma-ray detector for neutron resonance densitometry

Neutron resonance densitometry (NRD) has been proposed to quantify nuclear materials in melted fuel (MF) that will be removed from the Fukushima Daiichi nuclear power plant. The problem is complex due to the expected presence of strong neutron absorbing impurities such as 10B and high radiation field that is mainly caused by 137Cs. To identify the impurities under the high radiation field, NRD is based on a combination of neutron resonance transmission analysis (NRTA) and neutron resonance capture analysis (NRCA). We investigated with Geant4 the performance of a gamma-ray detector for NRCA in NRD. The gamma-ray detector has a well shape, consisting of cylindrical and tube type LaBr3 scintillators. We show how it measures 478 keV gamma rays derived from 10B(n, αγ) reaction in MF under a high 137Cs-radiation environment. It was found that the gamma-ray detector was able to well suppress the Compton edge of 662-keV gamma rays of 137Cs and had a high peak-to-Compton continuum ratio, by using the tube type scintillator as a back-catcher detector. Then, we demonstrate that with this ability, detection of 478-keV gamma rays from 10B is accomplished in realistic measuring time