Thwaites D I
Phys Med Biol. 1985 Jan;30(1):41-53. doi: 10.1088/0031-9155/30/1/005.
Measurements of central axis depth ionisation curves and ionisation at ionisation maximum were carried out in water, clear polystyrene and a commercially available 'solid water' phantom material. Flat and cylindrical chambers were used for electron beams of 5-10 MeV. Displacements for the cylindrical chambers were determined, indicating a recommended value of 0.55 times cavity radius (without perturbation corrections). The use of a single scaling parameter is considered for converting depth ionisation curves obtained in plastic to those in water. This is shown to be valid within 2 mm at these energies and for these materials. Ionisation ratios between water and polystyrene are presented, i.e. correction factors for converting ionisation readings in the plastic to readings in water required for electron dosimetry determinations. These show 3% differences on average at these energies, ionisation in water being higher. Variations were observed with chamber wall material and chamber type. Measurements in the solid water material showed it to be a better water substitute than polystyrene for electron dosimetry in this energy range, although still significantly different from a true water phantom. Experiments at 10 MeV on the effect of charge storage on chamber reading in polystyrene, as a result of previous irradiation with electron beams, were carried out, indicating that such effects have negligible influence on the ionisation ratios reported.
在水、透明聚苯乙烯和一种市售的“固体水”体模材料中进行了中心轴深度电离曲线和电离最大值处电离的测量。使用扁平室和圆柱形电离室测量5 - 10 MeV的电子束。确定了圆柱形电离室的位移，表明推荐值为腔半径的0.55倍（无扰动校正）。考虑使用单个缩放参数将在塑料中获得的深度电离曲线转换为在水中的曲线。结果表明，在这些能量下以及对于这些材料，在2 mm范围内此方法有效。给出了水和聚苯乙烯之间的电离比，即电子剂量测定中用于将塑料中的电离读数转换为水中读数所需的校正因子。在这些能量下，这些电离比平均显示出3%的差异，水中的电离更高。观察到电离比随电离室壁材料和电离室类型而变化。在固体水体模材料中的测量表明，在该能量范围内，对于电子剂量测定而言，它是比聚苯乙烯更好的水替代物，尽管仍与真实水体模有显著差异。进行了10 MeV的实验，研究先前电子束照射导致的聚苯乙烯中电荷存储对电离室读数的影响，结果表明这种影响对所报告的电离比的影响可忽略不计。
