Arib Mehenna, Medjadj Toufik, Boudouma Youcef
Secondary Standard Dosimetry Laboratory, Nuclear Research Centre of Algiers, Algeria.
J Appl Clin Med Phys. 2006 Aug 24;7(3):55-64. doi: 10.1120/jacmp.v7i3.2264.
In the International Atomic Energy Agency's (IAEA) code of practice (TRS 398) and the American Association of Physicists in Medicine's dosimetry protocol (TG-51), full-scatter water phantoms are recommended for the determination of the absorbed dose for both photon and electron beams and, consequently, for the calibration of the user's ionization chambers. This procedure is applied in the Secondary Standard Dosimetry Laboratory, where the calibration is performed on a 60Co gamma beam, in comparison with reference chambers whose absorbed dose-to-water calibration coefficients, ND,w, are known. In this work, we present the results of the calibration of 10 Farmer-like ionization chambers calibrated in three water phantoms (sizes 20 x 20 x 15 cm3, 30 x 30 x 30 cm3, and 35 x 35 x 37 cm3) and two plastic phantoms (size 20 x 20 x 20 cm3) polymethyl methacrlyate (PMMA) and polystyrene). Calibrations are performed by the substitution method using an ionization chamber whose ND,w has been supplied by the IAEA's reference laboratory. It is shown that the results, expressed as the percentage ratio of the calibration coefficient in a given phantom to that of the standard IAEA phantom, is less than 0.35% for all investigated chambers, and that the standard deviation of the mean of the ND,w calibration coefficients determined in all five phantoms is less than 0.06%, except for one nylon-walled ionization chamber, where the observed 0.34% value could be explained by the hygroscopic properties of nylon. Furthermore, a chamber-to-chamber dependence of the calibration coefficient has been shown to vary by up to 2.8%. These results emphasize that the phantom dimensions and its material are not sensitive criteria for the calibration of cylindrical ionization chambers in terms of absorbed dose to water. The results also show that generic calibration coefficients could not be considered for a given type of chamber.
在国际原子能机构（IAEA）的实践准则（TRS 398）和美国医学物理学家协会的剂量测定协议（TG - 51）中，推荐使用全散射水模体来确定光子束和电子束的吸收剂量，进而用于用户电离室的校准。此程序应用于二级标准剂量测定实验室，该实验室在60Co伽马射线下进行校准，并与已知吸收剂量 - 水校准系数ND,w的参考电离室进行比较。在这项工作中，我们展示了在三个水模体（尺寸分别为20×20×15 cm3、30×30×30 cm3和35×35×37 cm3）以及两个塑料模体（尺寸为20×20×20 cm3，材料分别为聚甲基丙烯酸甲酯（PMMA）和聚苯乙烯）中校准10个类 Farmer 电离室的结果。校准采用替代法，使用一个由IAEA参考实验室提供ND,w的电离室。结果表明，以给定模体中的校准系数与标准IAEA模体中的校准系数的百分比比值表示，所有研究的电离室该比值均小于0.35%，并且在所有五个模体中确定的ND,w校准系数平均值的标准偏差小于0.06%，但有一个尼龙壁电离室除外，其观测到的0.34%的值可由尼龙的吸湿特性来解释。此外，已表明校准系数的室间依赖性变化高达2.8%。这些结果强调，就水吸收剂量而言，模体尺寸及其材料对于圆柱形电离室的校准并非敏感标准。结果还表明，对于给定类型的电离室不能考虑通用校准系数。
