Correction of the measured current of a small-gap plane-parallel ionization chamber in proton beams in the presence of charge multiplication

Purpose: The aims of this work are to study the response of a small-gap plane-parallel ionization chamber in the presence of charge multiplication and suggest an experimental method to determine the product of the recombination correction factor (k_s) and the charge multiplication correction factor (k_CM) in order to investigate the latter.

Methods: Experimental data were acquired in scanned proton beams and in a Cobalt-60 beam. Measurements were carried out using an IBA PPC05 chambers of which the electrode gap is 0.6mm. The study is based on the determination of Jaffé plots by operating the chambers at different voltages. Experimental results are compared to theoretical equations describing initial and volume recombination as well as charge multiplication for continuous and pulsed beams.

Results: Results obtained in protons and Cobalt-60 with the same PPC05 chamber indicate that the charge multiplication effect is independent of the beam quality, while results obtained in different proton beams with two different PPC05 chambers show that the charge multiplication effect is chamber dependent.

Conclusions: The approach to be taken when using a small-gap plane-parallel ionization chamber with a high voltage (e.g. 300V or 500V) for reference dosimetry in scanned proton beams depends on which correction factors were applied to the chamber response during its calibration in terms of absorbed dose to water: In both cases, it is recommended to use the ionization chamber at the same operating voltage used during its N_D,w-calibration. Another solution consists of operating the PPC05 chamber at a lower voltage (e.g. 50V) with larger k_s and smaller k_CM and determining the product of both factors with higher accuracy using a linear extrapolation method.