Measurement of the stopping power of water in condensed phase for carbon ions in the energy range between 1 MeV and 5 MeV

准确了解中能区液态水对碳离子的阻挡能力对重离子剂量测量至关重要。在利用大多数粒子治疗中心使用的充气电离室测定吸收剂量时，不仅相对生物效应，而且靶体积内的吸收剂量直接取决于阻止功率，而且水与空气的阻止功率之比是关键参数。目前，电离室测量转换为吸收剂量的方法是基于蒙特卡罗模拟的结果。这些模拟使用了半经验和理论的阻止功率数据，这些数据只是布拉格峰区普遍存在的中间离子能量的近似值，因此对计算的吸收剂量的不确定度的最大贡献是由于阻止功率比水与空气的不准确[1]。研究发现，凝聚态物质和气态物质对重离子的阻挡能力相差高达48%[2]。对于中能区的碳离子，液体中物质的阻挡能力是比气相中的更小还是更大，甚至是不清楚的。不能排除使用蒙特卡罗模拟计算的水与空气的停止功率比受到高于规定剂量与提供剂量与目标体积之间的偏差的最大容差[1]的不确定度的影响。因此，必须用实验数据来检验停车功率比。尽管这一点很重要，但在中能区，还没有液态水对碳离子的阻挡能力的实验数据。缺乏实验数据的原因是，液体物质对中间离子能量的阻止能力的准确直接测量是不可行的，因为它们的范围很短。本项目的目的是利用一种复杂的实验技术，即所谓的倒置多普勒频移衰减(IDSA)方法，首次在实验上确定液态水对能量在1-5 MeV范围内的碳离子的阻挡能力。该项目将为临床剂量测定和更准确地评估碳离子癌症治疗的相对生物学效果提供重要数据。此外，这将有助于澄清与碳离子的凝聚相效应的符号和大小有关的不确定性。

The accurate knowledge of the stopping power of liquid water for carbon ions in the intermediate energy region is of paramount importance for heavy ion dosimetry. Not only the relative biological effectiveness but also the absorbed dose in the target volume directly depends on the stopping power but also the ratio of the stopping power of water to that of air is the crucial parameter in determining the absorbed dose by means of air-filled ionization chambers that are used in most particle therapy centers. At present, the conversion of the ionization-chamber measurements to absorbed dose is based on the results of Monte-Carlo simulations. These simulations employ semiempirical and theoretical stopping power data which are only approximate for intermediate ion energies prevailing in the Bragg peak area, with the consequence that the greatest contribution to the uncertainty of the calculated absorbed dose arises due to the inaccuracy of the stopping power ratio water-to-air [1]. It has been found that the stopping power of condensed and gaseous substances can differ up to 48% for heavy ions [2]. For carbon ions in the intermediate energy region, it is even not clear whether the stopping power of a substance in liquid phase is smaller or greater than that in gaseous phase. It cannot be excluded that stopping power ratios water-to-air calculated using Monte-Carlo simulations are subject to uncertainties higher than the maximal tolerance [1] for the deviation between the prescribed and delivered dose to the target volume. Therefore, the examination of the stopping power ratios by means of experimental data is mandatory. Despite this importance, there exist no experimental data for the stopping power of liquid water for carbon ions in the intermediate energy region. The reason for this lack of experimental data is that accurate direct measurement of the stopping power of liquid substances for intermediate ion energies is not feasible because of their very short ranges. The present project is aimed to experimentally determine the stopping power of liquid water for carbon ions in the energy range between 1 and 5 MeV for the first time by means of a sophisticated experimental technique, the so-called Inverted Doppler Shift Attenuation (IDSA) Method. The project will provide important data for clinical dosimetry and for a more accurate assessment of the relative biological effectiveness in carbon ion cancer therapy. Furthermore, it will help to clarify the uncertainties with respect to the sign and the magnitude of the condensed phase effect for carbon ions.