Dose perturbations produced by thermoluminescent dosimeters and ionization chambers in high energy photon beams (in French)

Abstract

The thesis studied perturbation by thermoluminescent dosimeters (TLDs) and ionization chambers of the absorbed dose measured in a medium irradiated with photon beams from γ‐rays to 25‐MV x‐rays. The perturbations were evaluated using experimental and theoretical techniques involving analytical calculations and the Monte Carlo (MC) method (EGS4‐PRESTA I). The MC results agree with experimental data better than within 0.4%, with specific uncertainties (1 s.d.) less than 0.1% for MC and 0.3% for experimental values. We used Burlin's general cavity theory and MC to determine the influence of TLD detector material, its form, size, density, and dopant concentration in the energy correction factors. The size and density influence the energy correction factors about 2%. The dopant concentration does not have a significant effect. For the specific use of TLDs in radiotherapy postal quality checks by the Gustave–Roussy Institute, we investigated the influence on absorbed dose of the TLD holder (0.5% to 2%), the thermal conditioning of the TLD (1%), and heterogeneities around the dosimeters (2%). Uncertainties in the absorbed dose determination in water with TLD were evaluated to be about 1.7% for the and 2% for the x‐rays. In ionization chambers, correction factors were determined for the walls, waterproofing caps, and the central electrode from Exradin, Nuclear Enterprise, and Wellhöfer chambers. Global perturbation correction factors were evaluated for primary (BIPM) and secondary (IRD) standard ionization chambers. This determination validated the MC method for the calculation of global perturbation correction factors for the determination of the absorbed dose to water.