Determination of Cλ and CE conversion factors and stopping power ratios using calibrated ferrous sulphate dosemeters

Abstract

Absorbed dose conversion factors C_lambda and C_E for a number of commercial ionisation chambers have been measured by means of ferrous sulphate (Fricke) dosemeters the response of which, in terms of absorbed dose, had been determined by an absolute method involving the total absorption of 5.6 MeV electrons and was assumed to be the same for other radiation qualities used. The measurements refer to photon radiation with beam qualities between that of cobalt-60 and 18 MV generating voltage and to electron radiation with energies between 6.5 and 17 MeV. The measured values of the conversion factors deviate by a few per cent from certain values given in the recent literature but are in good agreement with others. In addition, some values of the water to air mass stopping power ratio for photon and electron radiation have been experimentally determined and compared with theoretical values. The results obtained with the Fricke method as developed in this laboratory proved to be reproducible within less than 0.2% (95% confidence level), thereby demonstrating the suitability of the method for the calibration of therapy level dosemeters in this energy range.