Galbraith D M, Rawlinson J A, Munro P
Med Phys. 1984 Mar-Apr;11(2):197-203. doi: 10.1118/1.595601.
Commercial plastics used for radiation dosimetry are good electrical insulators . Used in electron beams, these insulators store charge and produce internal electric fields large enough to measurably alter the electron dose distribution in the plastic. The reading per monitor unit from a cylindrical ion chamber imbedded in a polymethylmethacrylate (PMMA) or polystyrene phantom will increase with accumulated electron dose, the increase being detectable after about 20 Gy of 6-MeV electrons. The magnitude of the effect also depends on the type of the plastic, the thickness of the plastic, the wall thickness of the detector, the diameter and depth of the hole in the plastic, the energy of the electron beam, and the dose rate used. Effects of charge buildup have been documented elsewhere for very low energy electrons at extremely high doses and dose rates. Here we draw attention to the charging effects in plastics at the dose levels encountered in therapy dosimetry where ion chamber or other dosimeter readings may easily increase by 5% to 10% and where a phantom, once charged, will also affect subsequent readings taken in 60Co beams and high-energy electron and x-ray beams for periods of several days to many months. It is recommended that conducting plastic phantoms replace PMMA and polystyrene phantoms in radiation dosimetry.
用于辐射剂量测定的商用塑料是良好的电绝缘体。在电子束中使用时，这些绝缘体会储存电荷并产生足够大的内部电场，从而可测量地改变塑料中的电子剂量分布。嵌入聚甲基丙烯酸甲酯（PMMA）或聚苯乙烯模体中的圆柱形电离室每监测单位的读数会随着累积电子剂量的增加而增加，在约20 Gy的6 MeV电子照射后这种增加就可被检测到。这种效应的大小还取决于塑料的类型、塑料的厚度、探测器的壁厚、塑料中孔的直径和深度、电子束的能量以及使用的剂量率。在其他地方已经记录了在极高剂量和剂量率下极低能量电子的电荷积累效应。在此，我们提请注意在治疗剂量测定中遇到的剂量水平下塑料中的充电效应，在这种情况下，电离室或其他剂量计的读数可能很容易增加5%至10%，并且模体一旦带电，在60Co射线束、高能电子束和X射线束中，会在数天至数月的时间内影响后续的读数。建议在辐射剂量测定中使用导电塑料模体取代PMMA和聚苯乙烯模体。
