The influence of defect levels on the dose rate dependence of synthetic diamond detectors of various types on exposures to high-energy radiotherapy beams

The linear response of a radiation dosimeter with absorbed dose rate is a principal requirement in radiotherapy. Fowler's model for electrical conductivity, s of a solid-state detector and absorbed dose rate, Dr is of the form σ ∝ Δ Dr where Δ is the linearity index that can take on a range of values around unity. Utilising synthetic diamond detectors of various types as sensors, this study investigates the influence of defect levels on the Δ values of the sensors and the dependence ofΔ on bias voltage, beam energy and type in the dosimetry of highenergy photon and electron therapy beams. One main objective of the study was to establish whether for a given diamond detector, Δ could be determined only once for any given beam energy and then used for other beam energies of clinical interest. In order to attain the ICRU overall 75% uncertainty of absorbed dose delivery in radiotherapy, 72% accuracy was considered. The study was conducted on one HPHT and eight CVD synthesised diamonds of optical grade (OG) and detector grade (DG) qualities using 6 and 15MV photon, and 7 and 12 MeV electron energies. Values of Δ ranging from 0.79–1.03 to 0.85–0.96 were obtained for the electron and photon beams, respectively for all the diamond sensors at 1 kV/cm. The Δ values were found to change with various defect levels present within the crystals as characterised by Raman spectroscopy, ESR, FTIR spectroscopy and TL emission, and it was observed that the Δ values of crystals with high defect levels varied strongly with bias voltage. Whereas the Δ values of the HPHT diamond were found not vary with the electron and photon energies, only those of three CVD samples of a given class showed a variation within 2% between the two energies of each beam type. However, for all the crystals testedΔ showed amaximumvariation of 3.4% between the photon energies unlike the electron energies where a very strong variation (45%) was observed for three OG CVD crystals. The results of this study have suggested that differences in crystal quality due to the presence of defects could cause Δ to vary with bias voltage, beam energy and type. & 2014 Elsevier Ltd. All rights reserved.