Unified framework for precise background modeling to enhance rare event detection at the Kuo-Sheng nuclear reactor laboratory

Abstract

A comprehensive GEANT4 simulation framework was developed to model the background of the TEXONO experiment, including contributions from radioactive isotopes in detector components and the surrounding environment. The HPGe detector front-end electronics (pre-amplifier) were modeled with trace amounts of naturally occurring radionuclides 238U, 232Th, and 235U from manufacturing materials. Results show that the 238U and 232Th decay chains dominate the background below 400 keV, each contributing O(1) counts kg^-1 keV^-1 day^-1. Trace impurities were also introduced into the anti-Compton veto (ACV) detectors to represent realistic materials: 40K in the NaI(Tl) crystal and 137Cs in the CsI(Tl) detector. Simulations identified measurable background contributions from both isotopes, with the residual spectrum dominated by 40K gamma-rays and smaller contributions from 137Cs. The 40K background rate is about 0.1 counts kg^-1 keV^-1 day^-1, nearly 10 times larger than that from 137Cs below 400 keV. Environmental radioactivity was modeled using 60Co, 54Mn, and 135Xe distributed in the air gap between the copper end-cap and the NaI(Tl) ACV detector, representing airborne and surface contamination. These sources contribute minor background components below 100 keV, at levels of about 10^-2, 10^-2, and 0.1 counts kg^-1 keV^-1 day^-1 for 135Xe, 54Mn, and 60Co, respectively. Comparison of simulated and measured spectra shows good overall agreement, with only minor deviations at specific gamma-lines, validating the background model and demonstrating the robustness of the simulation framework for detector and shielding design.