Krithivas G, Rao S N
Med Phys. 1986 Sep-Oct;13(5):674-7. doi: 10.1118/1.595872.
A parallel-plate ion chamber has been investigated with a view to determining Ngas, the cavity-gas calibration factor, introduced by the Task Group 21 protocol for dose determination. In-air and in-phantom irradiation of a Memorial Hospital parallel-plate ion chamber in a 60Co beam led to the determination of ion collecting volume and a calibration factor for the chamber. When these quantities were used to deduce Ngas factors for the chamber, an agreement within 1% was found. An average of the cavity-gas factors obtained from 60Co data agreed to within 0.5% with the factor determined through force fitting the doses received by the parallel-plate chamber and a cylindrical chamber in a 22-MeV (nominal) electron beam. Additionally, the parallel-plate chamber was utilized to study the effectiveness, in electron beam dosimetry, of the electron fluence perturbation correction factors introduced when the phantom material is replaced by a cylindrical ion chamber.
为了确定任务组21剂量测定协议引入的腔体气体校准因子Ngas，对平行板电离室进行了研究。在60Co射束中对纪念医院平行板电离室进行空气和体模内照射，从而确定了该电离室的离子收集体积和校准因子。当使用这些量来推导该电离室的Ngas因子时，发现两者的一致性在1%以内。从60Co数据获得的腔体气体因子的平均值与通过对平行板电离室和圆柱形电离室在22 MeV（标称）电子束中所接收剂量进行强制拟合确定的因子之间的一致性在0.5%以内。此外，平行板电离室还用于研究在电子束剂量学中，当用圆柱形电离室替代体模材料时引入的电子注量扰动校正因子的有效性。
