Numerical simulation of Gamma-Induced background current in cobalt Self-Powered neutron detectors

Cobalt self-powered neutron detectors (SPNDs) are widely used in pressurized water reactor (PWR) protection systems because of their prompt response characteristics. However, in-core cobalt SPNDs are exposed to intense gamma radiation that generates interfering currents, compromising the accuracy of neutron flux measurements. This paper presents a high-fidelity Monte Carlo (MC) simulation method that uses a two-step reactor–detector coupling strategy to evaluate the gamma-induced background current in cobalt SPNDs. The strategy first obtains the neutron and gamma energy spectra at the detector location from a detailed MC model of a typical PWR core, and then uses these spectra as the source term for a fine-scale SPND model with the insulator divided into 50 radial layers. The contributions of positrons and electrons are explicitly distinguished, and the space charge effect is incorporated into the electron transport calculation. Applied to two prototype cobalt SPNDs (ZTCo121 and ZTCo122), the method yields gamma-induced background current fractions of 2.56% and 4.19% of the total signal, both of which are below 5%. The resulting impact on detector sensitivity remains within the acceptable engineering uncertainty. Results validated against irradiation experiments performed in a swimming-pool reactor confirm that neglecting gamma interference in initial sensitivity calibration is justifiable. The method is general and extendable to other prompt-response SPNDs.