Evaluation of different methods for determining the magnitude of initial recombination in ionization chambers

The charge loss due to initial recombination was measured for various therapy level ionization chamber types. Different methods were applied. Measurements were performed in Co beams, pulsed high-energy photon beams and pulsed high-energy electron beams. The results are mutually consistent within less than 0.05 %. Within this uncertainty initial recombination and diffusion do not depend on the radiation quality. For dose measurements in high energy photon or electrton beams it is shown that the incomplete charge collection in an ionization chamber cavity can be corrected for within less than 0.05 % if appropriate procedures are applied.