Dose build-up curves for cobalt-60 irradiation: a systematic error occurring with pancake chamber measurements

The authors present results to support their view that build-up curves measured with parallel plate ionisation chambers (pancake chambers) depend strongly on the geometry of the gas volume in which ions are collected and the geometry of its surroundings. Ionisation by the electrons scattered from the side walls into the gas volume must be expected to contribute a considerable fraction of the observed ionisation at the surface. Therefore the observed surface dose measured will be larger than that which would be obtained in the irradiation of the skin. Incorrect conclusions might thus be drawn if a 'measured dose' were to be used in the interpretation of erythema. (U.K.)