Abstract
This thesis describes the development of a robust and portable calorimeter for use in radiotherapy photon and electron beams. The system consists of the calorimeter itself, means for thermal isolation and temperature control, and a temperature measurement system. The requirement for portability placed restrictions on the design which led to higher heat transfer between components than was desirable and much effort was put into thermodynamic modelling of the system. Effort was also focussed on the development of a temperature control system sensitive enough to allow measurements of temperature rises of the order of 1 mK. The control system maintains the temperature of the calorimeter to within +/- 0.2 mK over several hours in an uncontrolled environment. A full characterisation of the calorimeter was earned out at NPL in 60Co radiation and X-ray and electron beams from the NPL linear accelerator. This showed the system is capable of measuring a dose of 1 Gy at doserates as low as 1.5 Gy min-1 with a measurement uncertainty of +/- 0.3% (1 standard deviation). In comparisons with the present NPL primary standards the calorimeter gave agreement within the overall uncertainties. The robustness and portability of the calorimeter were proven in a number of external tests, firstly in an intercomparison between NPL and the Bureau International des Poids et Mesures in the BIPM's 60Co beam in Paris. In the final test an ion chamber was calibrated in a 6 MV X-ray beam from a clinical linac, yielding a calibration factor 0.68% different from that obtained in the NPL linac beam. This difference is within the measurement uncertainties and indicated that there is no significant error in the transfer of ion chamber calibrations from NPL to the radiotherapy clinic.