不同海拔高度下γ射线空气衰减系数的计算

在航空γ剂量率测量过程中由于海拔高度跨度大，γ射线空气衰减系数会受到海拔高度、温度、气压与空气密度的影响，故需要进行相应修正。利用蒙特卡罗（MCNP）法模拟γ射线在不同空气密度下的线衰减系数。利用经验公式计算不同海拔高度下的空气密度，利用MCNP法建立γ谱仪模型，计算不同海拔高度、不同密度条件下的γ光子注量，根据指数衰减原理计算不同空气密度下的线衰减系数。结果表明，由经验公式求出的空气密度模拟得出的空气质量衰减系数与美国NIST推荐值最大相对误差为-17.3%；在γ射线参考辐射场中用衰减板进行实验验证，⁶⁰Co源的最大相对误差为6.0%，¹³⁷Cs源的最大相对误差为5.3%。本工作为后续低空近地辐射剂量的研究工作奠定了基础。

Calculation and Research of γ-ray Air Attenuation Coefficient at Different Altitudes

In the process of aviation gamma dose rate measurement, due to the large altitude span, the gamma-ray air attenuation coefficient will be affected by altitude, temperature, air pressure and air density, so corresponding corrections are needed. The Monte Carlo method is used to simulate the linear attenuation coefficient of γ-ray in the air at different densities. First, use empirical formulas to calculate the air density at different altitudes; then use MCNP software to build a gamma spectrometer model to calculate the gamma photon fluence at different altitudes and different densities; finally, calculate the air density at different air densities based on the principle of exponential decay line attenuation coefficient. The results show that the maximum relative error between the air quality attenuation coefficient obtained by the air density simulation calculated by the empirical formula and the recommended value of NIST in the United States is -17.3%; the attenuation plate is used for experimental verification in the γ-ray reference radiation field, and the results show the theoretical calculation and the actual measurement relative error of value: the maximum relative error of ⁶⁰Co source is 6.0%, and the maximum relative error of ¹³⁷Cs is 5.3%. This work lays the foundation for the follow-up study of low-altitude near-earth radiation dose.