An Improved Numerical Model for Quantitatively Computing the Space Charge Effect of Self-Powered Neutron Detector

Self-powered neutron detector (SPND) has been widely applied in reactors for monitoring neutron flux. However, the electrons emitted from SPNDs' emitter may deposit in the insulator and accumulate an obvious space charge distribution because of insufficient kinetic energy and high resistivity. The distribution of space charge in the insulator creates an electric field distribution, which influences the transportation of electrons and subsequently affects the output current and neutron sensitivity. This phenomenon is known as the space charge effect. Moreover, if the maximum electric field exceeds the breakdown field strength, the insulator will lose its insulating ability and the detector will not work normally. Therefore, quantitative calculation of the electric field is required. In the previous work, tracing whole tracks of electrons and recording positive charges are used to acquire the accurate charge deposition in the insulator, and fitting the charge distribution is needed for solving Poisson's equation to obtain the electric field distribution, which results in the final results' instability and unreliability. In order to overcome these existing problems, an efficient way for recording charge disposition based on the law of charge conservation is proposed and the image method is introduced to calculate the electric field. This method has been integrated into our simulation toolkit SPNDSignal, and the results have been compared with the old method, indicating its stability and reliability. In addition, it has been used to study the space charge effect's dependences on the emitter's radius, insulator thickness, insulator bulk resistivity, and neutron flux density. Finally, a formula is proposed to estimate the maximum electric field for a given SPND under a typical thermal neutron spectrum, regarding the neutron flux and the insulator bulk resistivity.