衰减法则光子跟它环境中的原子互相起作用的概率,可用组成环境的材料的微观截面,也可用它的宏观截面来表达。微观截面(σ_c)的定义是一个原子向一个γ光子摆出使它能起相互作用的有效截面面积。在推导γ辐射衰减法则方面,宏观截面(μ)比微观截面有用。宏观截面的定义是光子轨迹每单位长度的起作用概率,而跟微观截面有下列的关系(A1) μ=Nσ_c,(见参考文献1,1—61页) Attenuation Law The probability that a photon interacts with atoms in its environment can be expressed in terms of the microscopic section of the material that makes up the environment or its macro section. The microscopic cross-section (σ_c) is defined as the effective cross-sectional area for an atom to swing into a γ-photon such that it interacts. The macroscopic section (μ) is more useful than the microscopic section in deriving the gamma radiation attenuation law. The definition of macroscopic section is the working probability per unit length of the photon trajectory, and has the following relationship with the microscopic section (A1) μ = Nσ_c (see references 1, 1-61)