Measurements of ionisation in water, polystyrene and a 'solid water' phantom material for electron beams.

Measurements of central axis depth ionisation curves and ionisation at ionisation maximum were carried out in water, clear polystyrene and a commercially available 'solid water' phantom material. Flat and cylindrical chambers were used for electron beams of 5-10 MeV. Displacements for the cylindrical chambers were determined, indicating a recommended value of 0.55 times cavity radius (without perturbation corrections). The use of a single scaling parameter is considered for converting depth ionisation curves obtained in plastic to those in water. This is shown to be valid within 2 mm at these energies and for these materials. Ionisation ratios between water and polystyrene are presented, i.e. correction factors for converting ionisation readings in the plastic to readings in water required for electron dosimetry determinations. These show 3% differences on average at these energies, ionisation in water being higher. Variations were observed with chamber wall material and chamber type. Measurements in the solid water material showed it to be a better water substitute than polystyrene for electron dosimetry in this energy range, although still significantly different from a true water phantom. Experiments at 10 MeV on the effect of charge storage on chamber reading in polystyrene, as a result of previous irradiation with electron beams, were carried out, indicating that such effects have negligible influence on the ionisation ratios reported.