The influence of unattached electrons on the collection efficiency of ionisation chambers for the measurement of radiation pulses of high dose rate

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Abstract
Due to their high mobility, electrons represent the major component in the initial phase of the current in an ionisation chamber irradiated with short pulses of ionising radiation. The authors have developed 'fast' ionisation chambers and, by means of these, have directly measured the electron component of the chamber current. The actual number of electrons collected appeared to be significantly greater than that calculated when using attachment coefficient and drift velocity values found in the literature. For example, approximately four times the expected value is collected in room air. The deficit of negative ions resulting from the collection of electrons is incorporated within a modified version of Boag's recombination equation. The recombination loss up to an exposure per pulse of, at least, 2.58*10-2C kg-1(100R) can be calculated in this manner.