A portable calorimeter for measuring absorbed dose in the radiotherapy clinic

This paper describes the development of a robust and portable calorimeter for use in clinical electron and photon beams. Although intended for therapy-level dosimetry, the new calorimeter can also be used for high-dose measurements at industrial facilities. The system consists of a front end (the calorimeter itself), means for thermal isolation and temperature control, and a measurement system based on thermistors in a dc Wheatstone bridge. It was found from investigation that the heat transfer between components was significant. The restrictions on the design placed by the requirement for portability led to higher heat transfer than was desirable. Much effort was put into thermodynamic modelling of the system and determining the heat transfer coefficients. Effort was also focused 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 responds to the calorimeter, phantom and air temperatures and maintains the temperature of the calorimeter to within +/-0.2 mK over several hours. Initial operation at the National Physical Laboratory (NPL) in x-ray and electron beams from the NPL linear accelerator shows that the system is capable of measurements of 1 Gy at 2 Gy min(-1) with a random uncertainty of +/-0.3% (1 standard deviation). Operation in 60Co at a doserate of 1 Gy min(-1) has also been achieved with a similar uncertainty. It is intended to test the calorimeter 'in the field' during 2000.