McEwen M R, Niven D
Ionizing Radiation Standards, Institute for National Measurement Standards, National Research Council of Canada, Ottawa, Canada.
Med Phys. 2006 Apr;33(4):876-87. doi: 10.1118/1.2174186.
The material Virtual Water has been characterized in photon and electron beams. Range-scaling factors and fluence correction factors were obtained, the latter with an uncertainty of around 0.2%. This level of uncertainty means that it may be possible to perform dosimetry in a solid phantom with an accuracy approaching that of measurements in water. Two formulations of Virtual Water were investigated with nominally the same elemental composition but differing densities. For photon beams neither formulation showed exact water equivalence-the water/Virtual Water dose ratio varied with the depth of measurement with a difference of over 1% at 10 cm depth. However, by using a density (range) scaling factor very good agreement (<0.2%) between water and Virtual Water at all depths was obtained. In the case of electron beams a range-scaling factor was also required to match the shapes of the depth dose curves in water and Virtual Water. However, there remained a difference in the measured fluence in the two phantoms after this scaling factor had been applied. For measurements around the peak of the depth-dose curve and the reference depth this difference showed some small energy dependence but was in the range 0.1%-0.4%. Perturbation measurements have indicated that small slabs of material upstream of a detector have a small (<0.1% effect) on the chamber reading but material behind the detector can have a larger effect. This has consequences for the design of experiments and in the comparison of measurements and Monte Carlo-derived values.
物质“虚拟水”已在光子束和电子束中进行了特性表征。获得了射程缩放因子和注量修正因子，后者的不确定度约为0.2%。这种不确定度水平意味着有可能在固体模体中进行剂量测定，其精度接近在水中的测量精度。研究了两种标称元素组成相同但密度不同的“虚拟水”配方。对于光子束，两种配方均未显示出与水完全等效——水/“虚拟水”剂量比随测量深度而变化，在10 cm深度处差异超过1%。然而，通过使用密度（射程）缩放因子，在所有深度处水和“虚拟水”之间都获得了非常好的一致性（<0.2%）。对于电子束，也需要一个射程缩放因子来匹配水和“虚拟水”中深度剂量曲线的形状。然而，在应用此缩放因子后，两个模体中测量的注量仍存在差异。对于深度剂量曲线峰值和参考深度附近的测量，这种差异显示出一些小的能量依赖性，但在0.1% - 0.4%的范围内。微扰测量表明，探测器上游的小材料板对电离室读数有小的影响（<0.1%），但探测器后面的材料可能有更大的影响。这对实验设计以及测量值与蒙特卡罗推导值的比较有影响。
