Abdel-Rahman Wamied, Seuntjens Jan P, Verhaegen Frank, Podgorsak Ervin B
Department of Medical Physics, McGill University Health Centre, 1650 Avenue Cedar Montreal, Quebec H3G 1A4, Canada.
Med Phys. 2006 Sep;33(9):3094-104. doi: 10.1118/1.2208917.
Polarity effects in ionization chambers are caused by a radiation induced current, also known as Compton current, which arises as a charge imbalance due to charge deposition in electrodes of ionization chambers. We used a phantom-embedded extrapolation chamber (PEEC) for measurements of Compton current in megavoltage photon and electron beams. Electron contamination of photon beams and photon contamination of electron beams have a negligible effect on the measured Compton current. To allow for a theoretical understanding of the Compton current produced in the PEEC effect we carried out Monte Carlo calculations with a modified user code, the COMPTON/ EGSnrc. The Monte Carlo calculated COMPTON currents agree well with measured data for both photon and electron beams; the calculated polarity correction factors, on the other hand, do not agree with measurement results. The conclusions reached for the PEEC can be extended to parallel-plate ionization chambers in general.
电离室中的极性效应是由辐射感应电流引起的，也称为康普顿电流，它是由于电离室电极中的电荷沉积导致电荷不平衡而产生的。我们使用了一个嵌入体模的外推电离室（PEEC）来测量兆伏级光子束和电子束中的康普顿电流。光子束中的电子污染和电子束中的光子污染对测量的康普顿电流影响可忽略不计。为了从理论上理解PEEC效应中产生的康普顿电流，我们使用修改后的用户代码COMPTON/EGSnrc进行了蒙特卡罗计算。蒙特卡罗计算得到的康普顿电流与光子束和电子束的测量数据吻合良好；另一方面，计算得到的极性校正因子与测量结果不一致。一般来说，对于PEEC得出的结论可以推广到平行板电离室。
