van Dijk Eduard, Kolkman-Deurloo Inger-Karine K, Damen Patricia M G
Nederlands Meetinstituut bv, 2600 AR Delft, The Netherlands.
Med Phys. 2004 Oct;31(10):2826-33. doi: 10.1118/1.1791352.
Different methods exist to determine the air kerma calibration factor of an ionization chamber for the spectrum of a 192Ir high-dose-rate (HDR) or pulsed-dose-rate (PDR) source. An analysis of two methods to obtain such a calibration factor was performed: (i) the method recommended by [Goetsch et al., Med. Phys. 18, 462-467 (1991)] and (ii) the method employed by the Dutch national standards institute NMi [Petersen et al., Report S-EI-94.01 (NMi, Delft, The Netherlands, 1994)]. This analysis showed a systematic difference on the order of 1% in the determination of the strength of 192Ir HDR and PDR sources depending on the method used for determining the air kerma calibration factor. The definitive significance of the difference between these methods can only be addressed after performing an accurate analysis of the associated uncertainties. For an NE 2561 (or equivalent) ionization chamber and an in-air jig, a typical uncertainty budget of 0.94% was found with the NMi method. The largest contribution in the type-B uncertainty is the uncertainty in the air kerma calibration factor for isotope i, N(i)k, as determined by the primary or secondary standards laboratories. This uncertainty is dominated by the uncertainties in the physical constants for the average mass-energy absorption coefficient ratio and the stopping power ratios. This means that it is not foreseeable that the standards laboratories can decrease the uncertainty in the air kerma calibration factors for ionization chambers in the short term. When the results of the determination of the 192Ir reference air kerma rates in, e.g., different institutes are compared, the uncertainties in the physical constants are the same. To compare the applied techniques, the ratio of the results can be judged by leaving out the uncertainties due to these physical constants. In that case an uncertainty budget of 0.40% (coverage factor=2) should be taken into account. Due to the differences in approach between the method used by NMi and the method recommended by Goetsch et al., an extra type-B uncertainty of 0.9% (k= 1) has to be taken into account when the method of Goetsch et al. is applied. Compared to the uncertainty of 1% (k= 2) found for the air calibration of 192Ir, the difference of 0.9% found is significant.
存在多种方法来确定用于192Ir高剂量率（HDR）或脉冲剂量率（PDR）源能谱的电离室的空气比释动能校准因子。对获取此类校准因子的两种方法进行了分析：（i）[Goetsch等人，《医学物理》18, 462 - 467 (1991)]推荐的方法，以及（ii）荷兰国家标准机构NMi采用的方法[Petersen等人，报告S - EI - 94.01（荷兰代尔夫特NMi，1994）]。该分析表明，根据用于确定空气比释动能校准因子的方法不同，在确定192Ir HDR和PDR源强度时存在约1%的系统差异。只有在对相关不确定度进行准确分析之后，才能探讨这些方法之间差异的最终意义。对于NE 2561（或等效）电离室和空气夹具，采用NMi方法时典型的不确定度预算为0.94%。B类不确定度中最大的贡献是由一级或二级标准实验室确定的同位素i的空气比释动能校准因子N(i)k的不确定度。这种不确定度主要由平均质能吸收系数比和阻止本领比的物理常数的不确定度主导。这意味着在短期内标准实验室不太可能降低电离室空气比释动能校准因子的不确定度。例如，当比较不同机构中192Ir参考空气比释动能率的测定结果时，物理常数的不确定度是相同的。为了比较所应用的技术，可以通过忽略这些物理常数引起的不确定度来判断结果的比值。在这种情况下，应考虑0.40%（覆盖因子 = 2）的不确定度预算。由于NMi使用的方法与Goetsch等人推荐的方法在方法上存在差异，当应用Goetsch等人的方法时，必须考虑额外的0.9%（k = 1）的B类不确定度。与192Ir空气校准的1%（k = 2）的不确定度相比，发现的0.9%的差异是显著的。
