Cojocaru C D, Ross C K
Ionizing Radiation Standards, NRC, Ottawa.
Med Phys. 2012 Jul;39(7Part4):4645. doi: 10.1118/1.4740209.
The average energy expended by an energetic electron to create an ion pair in air, W is an important quantity in radiation dosimetry. The data obtained by Domen and Lamperti using electron beams in the energy range from 15 to 50 MeV can be used to extract a value for W if the electron stopping powers of graphite and air are assumed to be known. We use Monte Carlo techniques to reanalyze these data and obtain a new estimate for the value of W Using the EGSnrc Monte Carlo and its associated user codes, as well as the best availabl-e stopping power data for graphite, we calculate the perturbation effects due to the calorimeter and ionization chamber and the effect of extrapolating from scattered to plane-parallel beams. Without further adjustments, the extracted values of W show a significant trend as the mean electron energy decreases. We show that part of this trend can be attributed to an incorrect value of the density assigned to the graphite absorbers and part to the likelihood that the nominal energy assigned to the low-energy electron beams is not correct. Using all the data, we obtain a value for W of 33.84 eV per ion pair with a relative standard uncertainty of 0.4 %. This result serves to complement values obtained using Co γ-rays, for which the value of the mean excitation of graphite contributes significantly to the uncertainty.
高能电子在空气中产生一对离子所消耗的平均能量W是辐射剂量学中的一个重要量。如果假设已知石墨和空气的电子阻止本领，那么多梅恩和兰佩尔蒂利用能量范围在15至50兆电子伏的电子束所获得的数据可用于提取W的值。我们使用蒙特卡罗技术重新分析这些数据，并获得W值的新估计。利用EGSnrc蒙特卡罗及其相关用户代码，以及石墨的最佳可用阻止本领数据，我们计算了量热计和电离室引起的微扰效应以及从散射束外推到平行平面束的效应。在没有进一步调整的情况下，随着平均电子能量降低，提取的W值呈现出显著趋势。我们表明，这种趋势部分可归因于赋予石墨吸收体的密度值不正确，部分可归因于赋予低能电子束的标称能量可能不正确。利用所有数据，我们得到每离子对W的值为33.84电子伏，相对标准不确定度为0.4%。该结果有助于补充使用钴γ射线获得的值，对于钴γ射线，石墨平均激发能的值对不确定度有显著贡献。
