Ferrous Sulphate Dosimetry for Electrons: A re-evaluation

The ferrous sulphate dosimeter is often used as a reference dosimeter when careful absorbed dose measurements are to be carried out especially in electron beams.It is therefore very unsatisfactory that the uncertainty in the G-value recommended by the ICRU report No. 21 for electron radiation, is as large as 4 per cent.Recent investigations (EGGERMONT et coll.1978) also indicate that the majority of the molar extinction coefficients used in experiments for G-value determination probably are 1 to 2 per cent too high.Furthermore, most G-values are determined without considering the perturbation effect introduced when the absorber of a calorimeter is replaced by the ferrous sulphate dosimeter.Finally, the reliability of the G-values determined by ionization chamber dosimetry must be doubted, as they in the past were based on insufficient knowledge of the cavity theory for electrons due to large uncertainties in stopping power ratios and perturbation corrections.Therefore, a reevaluation of the ferrous sulphate dosimetry seems necessary.The present report is mainly concerned with electron dosimetry, but most of the discussions apply equally well to photons. Absorbed dose to the ferrous sulphate dosimeterThe mean absorbed dose, a,, to the ferrous sulphate dosimeter is obtained from the change in absorbance in optical density units by the relation