多晶体航空γ能谱仪的地面辐射监测模拟仿真

Monte Carlo simulation of polycrystalline airborne γ-ray spectrometer for ground radiation monitoring

采用基于MCNP5的Monte Carlo模拟方法对多晶体航空γ能谱仪用于地面辐射监测进行模拟仿真。鉴于目前航空γ能谱仪主要用于资源勘探,对137Cs以及131I低能射线探测并不关注。本文以地面辐射为监测目标,通过建立多晶体航空γ能谱仪探测装置的Monte Carlo模拟模型,获得137Cs以及131I的模拟电子脉冲能量幅度分布谱,然后用计算得到的全能峰高斯展宽系数对模拟电子脉冲能量幅度分布谱进行高斯展宽,并对不同探测条件下最小可探测活度(Minimum detectable activity,MDA)进行计算。结果表明,Monte Carlo模拟方法能较好地对地面辐射监测进行仿真,在相同测量条件下,探测器外置计数比探测器内置计数大,且探测器外置时MDA优于探测器内置时MDA。所以实际航测过程中,建议采用探测器外置的方式,以减小MDA值。

Background： Airborne y-ray spectrometer has been used extensively over several decades for mineral exploration and geological mapping purposes to look for the peaks of potassium, uranium and thorium. And the low-energy ray is ignored. Purpose： In order to provide a basis for obtaining effective environmental radioactivity measurement results, Minimum detectable activity （MDA） values in monitoring ground radiation of the polycrystalline airborne y-ray spectrometer need to be calculated. Methods： MDA is related with the detection efficiency. A Monte Carlo simulation was performed using the MCNP5 code for different radionuclides in the ground environment. Equivalent mass thickness was proposed to reduce variance, and the secondary source was used in the MCNP5 input. Results： The pulse height distributions of external detectors and internal detectors for 137Cs and 131I at different heights were obtained, which represent the counting rate decreased as the altitude increases. And the MDA of external detectors is better than that of internal detectors. Conclusion： The external detector is suggested to adopt in flight measurement for enhancing MDA.