Holt J G, Fleischman R C, Perry D J, Buffa A
Med Phys. 1979 Jul-Aug;6(4):280-4. doi: 10.1118/1.594582.
The calibration of a cobalt-60 beam in a phantom with an ion chamber, which has been calibrated with respect to exposure, requires the use of a displacement correction factor which essentially corrects the photon fluence for the attenuation and scatter when the chamber with buildup cap is removed and replaced by phantom material. To determine the displacement factor, Ac, a special set of cylindrical ionization chambers with various volumes were constructed out of polystyrene. Tissue-air ratios were measured with these chambers for cobalt-60 gamma rays in a polystyrene phantom, and the ratio Ac/Aeq was experimentally determined. In order to calculate Ac from this ratio, Aeq was determined also. It was found that Ac depended on chamber diameter only, and not on field size or depth. A value of 0.990 for Aeq is recommended and a table of Ac for chambers of different outer diameters is included.
使用已针对照射量进行校准的电离室在模体中对钴 - 60射线束进行校准，需要使用位移校正因子，该因子主要用于校正当带有建成帽的电离室被模体材料移除并替换时，光子注量因衰减和散射而产生的变化。为了确定位移因子Ac，用聚苯乙烯制作了一组特殊的、具有不同体积的圆柱形电离室。用这些电离室在聚苯乙烯模体中测量了钴 - 60伽马射线的组织空气比，并通过实验确定了Ac/Aeq的比值。为了从该比值计算Ac，还确定了Aeq。结果发现Ac仅取决于电离室直径，而不取决于射野大小或深度。建议Aeq的值为0.990，并包含了不同外径电离室的Ac表格。
