MEASUREMENT OF SATURATION CORRECTION FACTORS OF THIMBLE-TYPE IONIZATION CHAMBERS IN PULSED PHOTON BEAMS

The charge collection efficiency of six models of thimble-type ionization chamber was measured in pulsed high-energy photon beams. For some chamber types the resulting saturation correction factors agree well with values calculated from Boag's formula for volume recombination, with a small constant term of 0.1% accounting for initial recombination. For other types, however, common correction schemes such as the 'two-voltage technique' are not applicable, since additional effects such as charge multiplication in regions of high electric field strength, which are not considered in the theory, may significantly influence the behaviour of the chambers. The magnitude of these effects not only depends on the chamber model but may also vary with individual chambers of the same model.