The principle of calorimetry in ionization radiation dosimetry is to measure the temperature rise in a well-defined material sample as a result of the deposited energy from ionizing radiation. This is an absolute measurement methodology, that can form the basis for primary standards, and other less ideal (but more practical) dosimeters can then be calibrated or characterized against such standards.
Calorimeters are technically very challenging to design and construct. One has to block competing processes to the heat production (e.g. radiation induced chemical reactions) and to correct for heat losses due to imperfections in adiabatic conditions. Furthermore, calorimeters needed for the clinical dose range require highly sensitive temperature measurements as the temperature rises are in the mK range.
A simple small-core graphite calorimeter has been designed by DTU for detector characterization studies in small MV photon beams and protons as part of a research project (Danish Council for Independent Research, grant FTP, DFF 4185-00151). The core has a diameter of 5 mm and a height of 7 mm. The calorimeter has been used for characterizing solid state detectors such as alanine and organic plastic scintillators with a size similar to the core.
How much does the temperature increase for a 2 Gy dose?
Radiotherapy is typically delivered in fractions of 2 Gy to the tumor region per day. With a specific heat capacity of graphite of 700 J/(K kg), 2 Gy of absorbed dose will result in a temperature rise of about 2.8 mK. So, if the temperature of the graphite was 20.0000 degC before the irradiation, then it will be 20.0028 degC after the dose of 2 Gy. Such small changes in temperature are measured with high precision using a 10 kohm thermistor coupled to a stable, low-level current source (20 uA), a nano voltmeter and a Wheatstone bridge.
