Ionization chamber shift correction and surface dose measurements in electron beams.
Ionization chamber shift correction and surface dose measurements in electron beams.
复制标题
电子束中的电离室位移校正和表面剂量测量。
DOI：
--
复制
发表时间：
1998
影响因子：
3.5
通讯作者：
Chee
中科院分区：
文献类型：
--
作者：
I. Das;S. McNeeley;Chee
圆柱形电离室在介质中产生微扰(梯度和通量)，因此在电子束剂量学中不能准确地定义测量点。根据电离室的类型，梯度微扰通常通过移位方法进行校正。根据剂量学方案，从电离室中心上游的电离室的内径(R)的0.33-0.85倍的范围内。这种漂移的变化导致表面剂量由于高剂量梯度而不确定。对电子束中圆柱形电离室的有效测量点进行了研究。电离测量采用空气中的电离室和源到电离室距离分别为<；100 cm和>；100 cm的体模。空气中的数据和体模中的数据分别用反平方剂量函数和电子深度剂量函数进行了拟合。这两个函数的交集提供了对电离室位移和表面剂量的准确估计。我们的结果表明，电离室的位移修正与能量有关。在6 MeV和20 MeV束流之间，测量到的漂移分别为0.9r到0.5r。用电离室测得的表面剂量与数学测定值符合在3%以内。本文提出的方法对于估算电子束剂量学所需的表面剂量和漂移是明确、快速和可靠的。
Cylindrical ionization chambers produce perturbations (gradient and fluence) in the medium, and hence the point of measurement is not accurately defined in electron beam dosimetry. The gradient perturbation is often corrected by a shift method depending on the type of ion chamber. The shift is in the range of 0.33-0.85 times the inner radius (r) of the ion chamber, upstream from the centre of the chamber, depending upon the dosimetry protocol. This variation in shift causes the surface dose to be uncertain due to the high dose gradient. An investigation was conducted to estimate the effective point of measurement of cylindrical ion chambers in electron beams. Ionization measurements were taken with the ion chamber in air and in a phantom at source to chamber distances of <100 cm and >100 cm respectively. The data in air and in the phantom were fitted with the inverse square and electron depth dose functions, respectively. The intersection of the two functions provides an accurate estimate of the ion chamber shift and the surface dose. Our results show that the shift correction for an ion chamber is energy dependent. The measured shifts vary from 0.9r to 0.5r between 6 MeV and 20 MeV beams respectively. The surface dose measured with the ion chambers and mathematically determined values are in agreement to within 3%. The method presented in this report is unambiguous, fast and reliable for the estimation of surface dose and the shift needed in electron beam dosimetry.