S Rossomme, J Horn, S Brons, O Jäkel, A Mairani, M Ciocca, V Floquet, F Romano, D Rodriguez Garcia, S Vynckier, H Palmans

Based on international reference dosimetry protocols for light-ion beams, a correction factor (<i>k_s</i>) has to be applied to the response of a plane-parallel ionisation chamber, to account for recombination of negative and positive charges in its air cavity before these charges can be collected on the electrodes. In this work, <i>k_s</i> for IBA PPC40 Roos-type chambers is investigated in four scanned light-ion beams (proton, helium, carbon and oxygen). To take into account the high dose-rates used with scanned beams and LET-values, experimental results are compared to a model combining two theories. One theory, developed by Jaffé, describes the variation of <i>k_s</i> with the ionization density within the ion track (initial recombination) and the other theory, developed by Boag, describes the variation of <i>k_s</i> with the dose rate (volume recombination). Excellent agreement is found between experimental and theoretical <i>k_s</i>-values. All results confirm that <i>k_s</i> cannot be neglected. The solution to minimise <i>k_s</i> is to use the ionisation chamber at high voltage. However, one must be aware that charge multiplication may complicate the interpretation of the measurement. For the chamber tested, it was found that a voltage of 300 V can be used without further complication. As the initial recombination has a logarithmic variation as a function of 1/V, the two-voltage method is not applicable to these scanned beams.