Tessier Frédéric, Cojocaru Claudiu D, Ross Carl K
Ionizing Radiation Standards, Measurement Science and Standards, National Research Council Canada, Ottawa, Ontario, K1A 0R6, Canada.
Med Phys. 2018 Jan;45(1):370-381. doi: 10.1002/mp.12660. Epub 2017 Dec 12.
The average energy expended by an energetic electron to create an ion pair in dry air, W , is a key quantity in radiation dosimetry. Although W is well established for electron energies up to about 3 MeV, there is limited data for higher energies. The measurements by Domen and Lamperti [Med. Phys. 3, 294-301 (1976)] using electron beams in the energy range from 15 to 50 MeV can, in principle, be used to deduce values for W , if the electron stopping power of graphite and air are known. A previous analysis of these data revealed an anomalous variation of 2% in W as a function of the electron energy. We use Monte Carlo simulation techniques to reanalyze the original data and obtain new estimates for W , and to investigate the source of the reported anomaly.
Domen and Lamperti (DL) reported the ratio of the response of a graphite calorimeter to that of a graphite ionization chamber for broad beams of electrons with energies between 15 and 50 MeV and at different depths in graphite (including depths well beyond the range of the primary electrons, i.e., in the bremsstrahlung photon regime). Using a detailed EGSnrc model of the DL apparatus, as well as up-to-date stopping powers, we compute the dose ratio between the ionization chamber cavity and the calorimeter core, for plane-parallel electron beams. This dose ratio, multiplied by the DL measured ratio, provides a direct estimate for W .
Despite an improved analysis of the original work, the extracted values of W still exhibit an increase as the mean electron energy at the point of measurement decreases below about 15 MeV. This anomalous trend is dubious physically, and inconsistent with extensive data for W obtained at lower energies. A thorough sensitivity analysis indicates that this trend is unlikely to stem from errors in extrapolation and correction procedures, uncertainties in electron stopping powers, or bias in calorimetry or ionization chamber measurements. However, we find that results are quite sensitive to the intrinsic graphite mass thickness of the detectors and to the incident beam energy.
The DL experiment provides data in an energy regime where the electron stopping power is insensitive to the mean excitation energy of graphite - an issue plaguing W experiments at lower energies. Unfortunately, state-of-the-art scrutiny of the original data cannot explain the anomalous trend in terms of perturbation effects or extrapolation bias. It can only be understood in terms of speculative offsets in graphite mass thickness or beam energy. Therefore higher accuracy measurements for electron energies above 15 MeV are recommended to further resolve the value of W .
在干燥空气中，一个高能电子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关键量。虽然对于能量高达约3兆电子伏的电子，(W)已经确定，但更高能量的数据有限。Domen和Lamperti [《医学物理》3, 294 - 301 (1976)] 使用能量范围为15至50兆电子伏的电子束进行的测量，原则上，如果已知石墨和空气的电子阻止本领，就可用于推导(W)的值。对这些数据的先前分析揭示了(W)随电子能量的变化存在2%的异常。我们使用蒙特卡罗模拟技术重新分析原始数据，获得(W)的新估计值，并研究报告异常的来源。
Domen和Lamperti（DL）报告了石墨量热计与石墨电离室对能量在15至50兆电子伏之间的宽电子束以及在石墨中不同深度（包括远超出初级电子射程的深度，即在轫致辐射光子区域）的响应之比。使用DL装置的详细EGSnrc模型以及最新的阻止本领，我们计算平面平行电子束在电离室腔和量热计芯之间的剂量比。这个剂量比乘以DL测量的比值，可直接得到(W)的估计值。
尽管对原始工作进行了改进分析，但提取的(W)值在测量点的平均电子能量降至约15兆电子伏以下时仍呈现增加趋势。这种异常趋势在物理上令人怀疑，并且与在较低能量下获得的大量(W)数据不一致。全面的灵敏度分析表明，这种趋势不太可能源于外推和校正程序中的误差、电子阻止本领的不确定性或量热法或电离室测量中的偏差。然而，我们发现结果对探测器的固有石墨质量厚度和入射束能量相当敏感。
DL实验提供了在电子阻止本领对石墨平均激发能不敏感的能量区域的数据——这是困扰较低能量下(W)实验的一个问题。不幸的是，对原始数据的最新审查无法根据微扰效应或外推偏差来解释这种异常趋势。只能从石墨质量厚度或束能量的推测性偏移来理解。因此，建议对15兆电子伏以上的电子能量进行更高精度的测量，以进一步确定(W)的值。
