Design of a radiation shield for an HPGe detector for a reactor-based PGNAA facility

The purpose of this study was to design an appropriate neutron and gamma radiation shield for the HPGe detector to be used in the PGNAA facility in the Isfahan MNSR reactor. In order to improve the parameters related to shield design, including the thickness of the material used in the design, the different type of materials considered and the relative reduction of neutron and gamma dose, shields were simulated by MCNPX code and the results of calculations were compared. It was concluded that the optimal thickness of lead as gamma absorber is 4 cm and the optimal thickness of borated polyethylene as neutron absorber is 10 cm. In this case, the neutron and gamma dose rate at the location of the detector inside the shield was equal to 5.11 μSv.h−1 and 7.5 μSv.h−1, respectively. Also, according to the investigations conducted for different materials as neutron absorbers, it was found that the best absorber for neutrons is borated polyethylene, which absorbs more neutrons and produces less secondary gamma than other materials and the best absorber for gamma is bismuth. The shielding has brought the relative percentage of neutron and gamma dose reduction about 90%.