THE EFFECT OF DELTA-RAY ON THE IONOMETRIC DOSIMETRY OF PROTON-BEAMS

The interface effects arising in the measurement of absorbed dose by i onization chambers, owing to the inhomogeneity between the walls and t he gas, have been evaluated by an analytical model. The geometrical si tuation considered here is appropriate for representing the behaviour of a plane-parallel ionization chamber exposed to a radiotherapeutic b eam of protons. Two gases, dry air and tissue equivalent gas (methane based), as well as six materials commonly used in ionization chamber w alls, i.e. graphite, A-150 tissue equivalent plastic, C-522 air equiva lent plastic, nylon type 6, polymethyl methacrylate and polystyrene, h ave been examined. The analysis of the results shows that, within the limits of the detector dimensions and proton energies commonly used in the dosimetry of radiotherapeutic beams, these effects, if not taken into account in the measurement interpretation, can entail deviations of up to about 2% with respect to the correct absorbed dose in gas.