129. Recombination effects in parallel-plates ionization chambers under clinical continuous active scanning proton beams

Abstract

PurposeSeveral factors can influence the response of an ionization chamber (IC) when exposed to radiation. One of these is the ion recombination. It depends mainly on dose rate (volumetric recombination, Vrec) and ionization density (initial recombination, Irec). This work aimed to determine the recombination correction factor (Ks) for parallel-plates (PP) ICs irradiated under continuous pencil beam scanning proton beams. MethodsPPICs were exposed to clinical proton beams produced by an isochronous cyclotron (Proteus®PLUS-Iba). To compensate for fluctuations of the beam two PPICs were set face-to-face, one used as monitor and the second was the PPIC under study, as explained in [1]. To separate Vrec and Irec, measurements were performed at different dose-rates (minimum cyclotron current 75 nA, maximum 300 nA) and different voltage (20 V–500 V). Ks was obtained as the ratio between the saturation current (Isat) and the ionization current (I) measured at the operating voltage (Ks=Isat/I) [1]. This was carried-out for three scenarios: (a) single 10 × 10 cm2 unmodulated 100 MeV layer; (b) single 10 × 10 cm2 unmodulated 226 MeV layer; (c) modulated box-like field (range 15 cm-modulation 10 cm). A theoretical model is under investigation to be compared to Ks-values determined with the two-voltage method [2] for continuous beam. Only results obtained using an PPC05 IC (Iba-Dosimetry) are reported. ResultsAll scenarios show similar results (Figure). At high voltages, data has an exponential trend, highlighting the presence of charge multiplication effects (CME). This should be due to the small gap (0.6 mm) between electrodes. At low voltages, there is a rather linear relation between 1/I and 1/V2. Using the two-voltage method, at +300 V, a Ks-value of 1.001 is found in all scenarios. Due to CME this value could be overestimated. ConclusionsAlthough the proton beam used can be considered as continuous, the variation of Ks against 1/V2 is not linear due to CME, so the two-voltage method use might be unsuitable. Further investigations are currently made to decouple recombination effect from CME and derive a model for accurately determine Ks.▪