Presentation Study of recombination characteristics of therapeutic ion beams on ion-chamber dosimetry

松藤, 成弘

In heavy ion radiotherapy, ionization chambers have been regarded as a standard for determining the absorbed dose given to patient. In the ion dosimetry, it is necessary to correct for initial recombination effect dependent on the radiation quality. The aim of this study is to reveal the radiation-quality dependence of the initial recombination in air in ion dosimetry.Ionization charge was measured for the beams of proton 40~160 MeV, carbon 21~400 MeV/n and iron 23.5~500 MeV/n by identical two parallel plate ionization chambers placed along beam axis in series. Downstream one was used as a monitor operated with a constant applied voltage while the other was used for recombination measurement by changing the voltage. The ratio of the ionization charge by the two ionization chambers showed a linear relationship with the inverse of the voltage at high-voltage region. The initial recombination factor was estimated by extrapolating the linear relationship to zero bias. The extent of initial recombination at zero bias was compared with track structure model.The initial recombination factor was found increased with decreasing the incident energy or increasing the atomic number of the beam. This behavior can be explained as the increase of ionization density in the core region of the track structure model with decreasing energy or increasing atomic number which lead to denser initial ion production and resultant higher recombination probability. The measured and calculated initial recombination factors agrees satisfactorily: 1.010 and 1.009 for proton, 1.021 and 1.020 for carbon and 1.134 and 1.126 for iron beams, respectively.
The 2015 International Conference on Applications of Nuclear Techniques (CRETE15)

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