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Effective measuring position for cylindrical ionization chambers when used for electron beam dosimetry

Department of Radiation Physics, Churchill Hospital, Headington, Oxford 0X3 7LJ

This excerpt was created in the absence of an abstract.

When depth dose measurements are made for electron beams it is recommended that a flat chamber be used (ICRU 1972). Suitable chambers are now coming onto the market, e.g. that designed at the National Physical Laboratory (HPA 1975), but they are not yet universally available and are somewhat bulky for some applications. Therefore the cylindrical type of thimble chamber, hitherto the only type available, continues to be used.

When an ionization chamber is used to make central axis depth dose measurements in tissue-equivalent material, the exposure registered on the dosemeter is converted to electron dose in rads by applying a factor C_E which varies with energy and thus depth in the medium. Additionally a number of precautions and corrections must also be taken into account: namely saturation, polarity effects, stem and cable effects and perturbation and displacement corrections.

Received for publication April 1, 1977. Revision received June 1, 1977.