Numerical simulation calculation of rhodium self-powered neutron detectors by considering the space charge effect and electrostatic field distribution within the insulant

Rhodium Self-Powered Neutron Detector (Rh-SPND) has become an indispensable in-core neutron detector for in-core neutron flux monitoring because of its simple structure, self-powered feature, and high radiation resistance. In the theoretical design of Rh-SPND, the presence of an electric field within the insulant can affect the electron transport process, thereby affecting the generation of detector's output current signal. Therefore, the influence of space charge effect and electrostatic field distribution must be considered when establishing a detector's response calculation model. At present, researches on the space charge effect mainly rely on the assumption of charge distribution, which cannot truly simulate the electron transportation within the insulant. In addition, these available methods cannot quantitatively analyze the key impact factors that can mostly affect the space charge effect. This paper proposes a method based on GEANT4 that can simulate and quantitatively analyze the space charge effect in the insulant, and then develops a corresponding precise numerical simulation calculation program. The charge distribution within the insulant and neutron sensitivity were calculated, the effects of emitter radius, insulant thickness and neutron flux density on space charge effect and neutron sensitivity were quantitatively analyzed, and the calculated results were compared with the experimental results. The results indicate that: (1) the space charge effect caused by different emitter radii has a relatively small impact on neutron sensitivity when the emitter radius is less than 0.02 cm; (2) when the insulant thickness is larger than 0.015 cm, it is necessary to consider the influence of space charge effect; (3) as the neutron flux density gradually increase, the deviation of neutron sensitivity caused by space charge effect increases significantly; (4) The deviation between the calculated neutron sensitivity and its experimental results is within 4% which indicates the high accuracy of the simulation results. The research results of this paper lay the foundation for the theoretical research and future design of Rh-SPND.