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1 基本原理 为绝对测量照射量而设计的电离室被称为自由空气电离室。由于Χ射线束通过电离室时不会撞击到任何室壁,所以其有效收集体积仅由空气电离柱的直径和收集电场的电力线所限定。在大气压力下,自由空气电离室只限于测量能量低于500kV的光子。这种限制主要是由于随着光子能量的增加,次级电子的最大射程也增加了。如对于~(60)Co的光子,次级电子的最大射程大约增加到5米,因此用1个大气压下的自由空气电离室测量这种能量的光子时,它的尺寸就要很大。 按照照射量的定义,对自由空气电离室的基本要求是:收集沿着所有的次级电子的径迹所形成的离子,这些次级电子是由光子束在围绕被研究点的小块空气体积中释放出来的,并需要测量它们的总电荷。但是直接测量这些电荷是不行的,实际的做法是:让一完全确定的待测量的X射线窄束,从电离室的两极板之间的中央通过,并收集和测量与X射线束的轴线相垂直的两个极板之间所产生的总电离。如果满足电子平衡条件,那么在这两个极板之间所产生的电离量,就几乎等于在所有的次级电子（它们是初级Χ射线在通过两极板之间的空气中释放出来的）径迹上所产生的电离量。所需要作出的小校正,可从理论和实验上加以研究。
1 Basic Principles Ionization chambers designed for absolute measurement of exposure are referred to as free air ionization chambers. Since the X-ray beam does not strike any wall of the chamber as it passes through the ionization chamber, its effective collection volume is limited only by the diameter of the air ionization column and the line of electric force collecting the electric field. At atmospheric pressure, the free air ionization chamber is limited to measuring photons with energies below 500 kV. This limitation is mainly due to the photon energy increases, the maximum range of secondary electrons also increased. For a photon of ~ (60) Co, the maximum range of a secondary electron increases to about 5 meters, so the size of a photon of this energy is very large when measured at 1 atmosphere of free-air ionization chamber. The basic requirement for a free air ionization chamber, as defined by the amount of radiation, is to collect ions formed along the tracks of all the secondary electrons generated by the photon beam in a small volume of air around the point of interest Released, and need to measure their total charge. However, it is not possible to measure these charges directly. The practical approach is to let a completely determined narrow beam of X-rays to be measured pass through the center between the plates of the ionization chamber and collect and measure the X-ray beam The total vertical distance between the two plates. If the electron balance conditions are satisfied then the amount of ionization produced between the two plates is almost equal to the difference between the diameters of all the secondary electrons (which are the primary X-rays released in the air passing between the plates) The amount of ionization generated on the trace. The small corrections needed to be made can be studied theoretically and experimentally.