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

Abstract A comprehensive GEANT4 simulation framework is developed to model the background of the TEXONO experiment, considering contributions from radioactive isotopes in both the detector components and the ambient environment. In this framework, the HPGe detector's front-end electronics (pre-amplifier) are modeled to contain trace amounts of naturally occurring radionuclides 238 U, 232 Th, and 235 U from manufacturing materials. The results confirm that the decay chains of 238 U and 232 Th dominate the background in this region. The observed background contributions from both isotopes are at the 𝒪(1) counts kg -1 keV -1 day -1 for energies below 400 keV. Trace radioisotopic impurities were introduced into the anti-Compton veto (ACV) detectors to reflect realistic material compositions: 40 K was incorporated in the NaI(Tl) crystal consistent with its natural isotopic abundance, and 137 Cs was included in the CsI(Tl) detector, representing a common anthropogenic contaminant. The analysis identifies minor yet measurable background components originating from 40 K in the NaI(Tl) ACV detector and 137 Cs in the CsI(Tl) scintillator. The residual spectrum is dominated by 40 K γ -rays, with smaller components from 137 Cs, in agreement with simulation results. The background rate from 40 K is uniform at ∼ 0.1 counts kg -1 keV -1 day -1 , approximately 10 times higher than that from 137 Cs below 400 keV. To incorporate environmental sources of radioactivity, simulations included isotopes 60 Co, 54 Mn, and 135 Xe distributed within the air gap between the copper end-cap and the NaI(Tl) ACV detector, representing airborne and surface-induced contamination. Although these sources contribute to the overall background (10 -2 , 10 -2 , and 0.1 counts kg -1 keV -1 day -1 below 100 keV for 135 Xe, 54 Mn, and 60 Co, respectively), their effects are well controlled and constitute only minor components of the total background. The comparison between simulated and measured spectra, while showing minor deviations at specific γ -lines, demonstrates the validity of the background model and the robustness of the simulation framework for guiding detector and shielding design.