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

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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.