Geant4 simulation study of β-γ coincidence detector for 41Ar measurement

[Background] As one of the main radionuclides released into the environment by the operation of reactors and accelerators, the measurement of 41Ar activity concentration is of great significance for ensuring public health. In the field of radioactive gas measurement, the β-γ coincidence method is widely used because it can significantly reduce the background and improve the sensitivity of the detector. However, present, there is little study on β-γ coincidence detectors for 41Ar measurement. [Purpose]This study aims to optimize the β-γ coincidence detector composed of plastic scintillator and CsI(Tl) scintillator to realize high sensitivity measurement of 41Ar. [Methods] An optimization method of detector structure based on minimum detectable activity concentration (MDC) is proposed. The optimization process of the detector is realized based on Geant4 simulation. Firstly, the penetration percentage of β-rays depositing energy into CsI(Tl) through different thicknesses of BC404 and 0.5 mm aluminum film was simulated. Secondly, the full-energy peak efficiency of 1 293.6 keV γ-ray in CsI(Tl) scintillator with different thicknesses was simulated. Thirdly, assuming that the sampling time of argon is proportional to the volume of the detector’s gas chamber, by simulating with different gas chamber volume, the comprehensive effects of β-rays detection efficiency, γ-ray full-energy peak efficiency, gas chamber volume and sampling time on MDC were analyzed. Finally, the influence of measurement time on MDC under different background counting rates was analyzed. [Results] It is concluded that the penetration percentage of β-rays emitted from the decay of 41Ar is about 0.78% when the thickness of BC404 is 3 mm. The γ-ray full-energy peak efficiency increases with the increase of CsI(Tl) scintillator thickness. With the argon sampling rate of 600 mL∙h−1, the optimal detector size parameters that minimize MDC are completely determined. When the background count rate is 1×10-3~1 s-1, the recommended measurement time for 41Ar is about 200 min. [Conclusions] When the measurement time is 200 min, the sample cooling time is 30 min and the background count rate is 5×10-3s-1, the MDC of the optimized β-γ coincidence detector for 41Ar measurement is about 1.7 Bq∙m−3.