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Ion Recombination in a Liquid-Filled Ionization Chamber in High-Energy Photon Beams

E Chung

E Chung1*, S Davis2, J Seuntjens1, (1) McGill University, Montreal, QC, CA (2) McGill Univ Health Center, Montreal, QC, CA

TU-A-BRB-9 Tuesday 8:00:00 AM - 9:55:00 AM Room: Ballroom B

Purpose: To characterize the ion recombination effect of a liquid-filled ionization chamber (LIC) in high-energy photon beams.

Methods: The ion recombination effect on the response of a PTW micro liquid ion chamber model 31018 (microLion) was investigated with a 6 MV photon beam in normal and SRS modes produced from a Varian Novalis Tx linear accelerator. Repetition rates were set to 100, 400 and 1000 MU/min, which correspond to pulse repetition frequencies of 60, 240 and 600 Hz, respectively. Polarization voltages applied to the microLion were +800 V and +400 V. The collection efficiency of the microLion as a function of dose per pulse was experimentally measured and theoretically calculated with changing polarization voltage and repetition rate.

Results: The relative collection efficiency of the microLion decreased with increasing dose per pulse or pulse repetition frequency from the linear accelerator, or with decreasing polarization voltage to the microLion because of more ion recombination in the liquid. For repetition rates of 100, 400 and 1000 MU/min, the collection efficiency decreased by up to 0.62%, 0.71% and 1.9%, respectively, in a range of dose per pulse from 0.062 to 0.429 mGy/pulse with the polarization voltage of +800 V. For +400 V, the collection efficiency decrease was by up to 0.92%, 2.0% and 5.8% at 100, 400 and 1000 MU/min, respectively. For pulsed radiation with high repetition rate, the experimental relative collection efficiency for all polarization voltages tends to the theoretically calculated efficiency for continuous beams because of low ion mobility in the liquid. We proposed a correction technique for differences in recombination between different radiation fields.

Conclusions: Ion recombination effects in a LIC were quantified and a practical correction method was proposed thereby enabling the use of the LIC for accurate measurements of output factors and relative doses in small and nonstandard fields.

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