正比计数器测量气体核素活度中小幅度脉冲漏计数修正的Geant4模拟研究

Simulation of Small-amplitude Pulse Counting Loss Correction in Gas Radioactivity Measurement Based on Geant4

采用内充气正比计数管进行放射性气体核素活度绝对测量的方法中,对小幅度脉冲计数损失进行合理修正是准确获得活度测量结果的关键。本工作以3种典型的放射性气体核素37 Ar、3H和85 Kr为例,利用Geant4蒙特卡罗模拟软件,对气体核素活度绝对测量过程中阈值以下漏计数损失修正进行了研究。结果表明:在核素37 Ar活度测量中,小脉冲漏计数主要为电子俘获过程释放的X射线所产生的“壁效应”计数损失,可通过计数率-气压倒数外推的方式进行修正;核素3H衰变释放的β射线能量较低,小脉冲漏计数主要来自于β射线的低能端,可在低计数阈值条件下采用能谱外推的方式予以修正;核素85 Kr测量过程中,计数损失主要来源于高能β射线产生的“壁效应”损失,需依次采用β能谱外推和气压外推的方式进行修正。采用以上方法,可消除小幅度脉冲损失产生的影响。

In the absolute measurement method of nuclide radioactivity by the internal gas proportional counter,the reasonable correction of the small pulse counting loss is the key to accurately obtain the measurement results.During the activity measurement of radionuclides,counting loss from low-energyβ-ray and wall effects can generally be corrected by extrapolating the energy deposition spectrum,changing the working gas pressure,and using Monte Carlo simulations or theoretical calculations.Considering the decay type and energy of radioactive gas nuclides,the influence of the low-energyβ-ray and the wall effect counting loss on the activity measurement results is also different.To this end,three typical radioactive gas nuclides 37 Ar,3H and 85 Kr were used to study the cause of counting loss based on the Geant4 simulation.According to the simulated particle decay characteristics and energy ranges,the physical process package FTFP_BERT built in Geant4 was selected.The results show that the counting loss of small pulse in the activity measurement of 37 Ar mainly comes from the wall effect generated by X-ray.Within the given gas pressure of 60-300 kPa,the simulated wall effect correction factors are 1.063-1.021,and the counting loss of the wall effect can be completely corrected by extrapolating the pressure and detection efficiency.The decay energy ofβ-ray generated by 3H is very low,and there is no obvious wall effect.The small pulse counting loss mainly comes from the low-energyβ-ray contribution with the energy below the counting threshold,which can be corrected by extrapolating theβenergy spectrum at a lower counting threshold(below 1 keV).For 85 Kr nuclide,the small pulse count loss is mainly due to the wall effect of high-energyβ-ray,and it can’t be completely corrected only by the extrapolation correction ofβenergy spectrum.In addition,the wall effect correction factors were also simulated in this paper.Below the threshold 1 keV at the pressure of 60-300 kPa,the results are 1.017-1.005,and the correction factors decrease as the counting threshold decrease.