Standardisation of absorbed dose by means of an aluminium calorimeter

Abstract

An aluminium calorimeter was used to determine an absorbed 'dose in water' calibration factor for a thimble ionisation chamber for Co gamma radiation. Several distinct steps were involved in the calibration. An absorbed 'dose in aluminium' calibration factor for the chamber was measured to determine the effect of depth on this factor; this calibration factor was used to transfer experimentally the measured dose in aluminium to dose in a water phantom at depths of 5 cm and 7 cm for a range of field sizes. A build-up factor (to describe the radiation quality) in the water phantom was calculated as a function of depth and field size. Using these steps, an absorbed dose in water calibration factor for the thimble chamber was measured over a limited range of radiation quality. Two further experiments were made to test the overall calibration procedure. The dose in water measurements were used to determine the G-value for ferric ion production by irradiating ferrous sulphate dosimeters for two or three field sizes at depths of 5 and 7 cm respectively. The measured G-values were compared with those obtained by other workers using calorimetry. The long-term stability of the calorimeter was observed over a period of about 14 months, during which time no change was detected in the calorimeter.