Polarity and recombination in plane parallel ionisation chambers

An investigation has been made of polarity and recombination corrections for several ionisation chambers used in dosimetry. Markus, NACP, Calcam, Vinten-631 and Attix chambers were irradiated in cobalt-60 radiation and in electron beams ranging in energy from 4 to 18 MeV. Experimental and theoretical evaluations of recombination were made over a range of dose-rates of pulsed radiation in addition to continuous radiation. At standard clinical dose rates, recombination correction factors were generally found to be in the order of 0.1-0.2% for 60Co, and 0.2-0.4% for pulsed electron beams. The variation of polarity effect with field size and depth of measurement was investigated. Under standard 'calibration' conditions, polarity corrections were in the order of 0.3%. In this work, particular emphasis was placed on evaluating the importance of these corrections in typical calibration and other clinical settings