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1 基本原理 戈瑞（Gray）研究了γ射线在介质中能量吸收的问题。在被照射的介质中有一个小的充气空腔,围绕这个空腔附近介质的吸收能量_mE_w值,按他的理论推导和实验得出所熟悉的布拉格-戈瑞（Bragg-Gray）公式:其中_mE_w为单位质量介质所吸收的能量; J_g为介质中空腔内单位质量气体所形成的离子对数; W_g为空腔内气体中形成每个离子对所消耗的平均能量; _mS_w/_mS_g为介质对气体的电子的质量碰撞阻止本领比值。 上式成立的条件是:在空腔气体中产生电离的全部电子都是在包围空腔的介质内产生的,因而空腔的存在不会扭歪电子的注量。 为了测量照射量,上式中的介质应是空腔电离室的壁。壁应有足够的厚度（必须大于电子在介质中的最大射程）,以排除由其它介质所产生的电子。可以使用任何壁材料和任何气体,只需要知道它们的阻止本领比值和在该气体中形成每个离子对所需要的平均能量。如果电离室的壁材料是碳,那么单位质量空气所吸收的能量为
1 Fundamentals Gray studied the problem of energy absorption of gamma rays in a medium. There is a small inflated cavity in the medium to be irradiated, around which the value of the absorbed energy _mE_w of the medium near this cavity derives from the theoretical Bragg-Gray formula: _mE_w is the energy absorbed by the unit mass medium; J_g is the number of ion pairs formed by the unit mass of gas in the cavity of the medium; W_g is the average energy consumed by each ion pair formed in the gas in the cavity; _mS_w / _mS_g is the medium The mass collisions with the electrons in the gas prevent this ratio. The condition for the above equation is that all the electrons that generate ionization in the cavity gas are generated in the medium that surrounds the cavity, so that the presence of the cavity does not distort the electron fluence. In order to measure the exposure, the medium in the above equation should be the wall of the chamber ionization chamber. The walls should have a sufficient thickness (must be greater than the maximum range of electrons in the medium) to exclude the electrons generated by other media. Any wall material and any gas can be used, only the stopping power ratio and the average energy required to form each ion pair in the gas need to be known. If the ionization chamber wall material is carbon, then the energy absorbed by the unit mass of air is