Calibration of ionization chamber with calorimetry

Details

Supervisors

Sterpin, Edmond ; Souris Kevin

Faculty

Faculté des sciences

Degree label

Master [120] en sciences physiques, à finalité spécialisée: physique médicale

Abstract

Calorimetry is a method of dosimetry. It is especially used to calibrate ionization chambers. This absolute dosimetry method relies on the temperature rise induced by radiation to measure the absorbed dose. The aim of this work is to evaluate the feasibility of measuring the absorbed dose for an 18 MV photon beam by water calorimetry. A McGill-type water calorimeter was used. The experiments were performed at the Saint-Luc University Clinics (UCLouvain) with the Elekta Infinity linear accelerator. The result was then compared to a reference. This reference was measured with a calibrated Farmer ionization chamber. The measurements were carried out 5 times with the calorimeter and 11 times with the Farmer chamber. Four correction factors have been taken into account in the water calorimetry determination: the radiation field perturbation kp, the water density kρ, the heat transfer kht and finally the heat defect khd. The heat defect is related to exothermic and endothermic chemical reactions. These effects can be avoided by operating with H2 saturated water and pre-irradiating the latter with 300 Gy. The heat transfer results from the phenomena of conduction and convection. The water density factor allows the conversion of the measurements carried out with the calorimeter at 4°C to those done with the ionization chamber at 21°C. These last two factors are negligible. Finally, the factor kp is linked to the perturbation associated with the glass vessel and the thermistors. It is calculated by means of Monte Carlo simulations. The result is 1.11. The absorbed dose determined with calorimetry is 1.963 Gy and the one determined with the ionization chamber is 1.542 Gy. This corresponds to a significant difference of 21.5%. The possible causes of this difference are diverse. The main one is probably due to the positioning of the chamber inside the calorimeter. The results of this work show that water calorimetry is a complex method. There are many sources of uncertainty, both in the dose measurements and in the calculation of the correction factors. This method must be used with great precision.