The dose rate dependence of synthetic diamond detectors in the relative dosimetry of high-energy electron therapy beams

Evaluation of the linear response of a radiation detector with absorbed dose rate should be of paramount importance in clinical dosimetry. As modelled by Fowler, electrical conductivity, sigma, of a solid-state detector and absorbed dose rate, D-r, are related by sigma similar to D-r(Delta) where Delta is the linearity index. The detector is thus independent of dose rate if Delta is unity. This contribution investigates and evaluates the dependence of Delta of synthetic diamond detectors of various types on therapy electron energy and its influence in relative electron dosimetry with the aim of selecting a suitable crystal. The study was conducted initially on one HPHT and eight CVD synthesised diamonds of optical grade (OG) and detector grade (DG) qualities using 6-14 MeV electron therapy beams. For quality control, the diamond specimens were characterised by Raman spectroscopy and electron spin resonance (ESR). Values of Delta ranging between 0.79 and 1.03 were obtained for all the nine diamond detectors at 1000 V/cm for 7 and 12 MeV electron beams. Whereas the Delta values of the HPHT diamond were found not to vary with the electron energies, those of three CVD samples of a given class varied with the electron energies within 2%. In addition, a very strong variation of about 9% was observed for two OG crystals of another class. The Delta values were found to decrease with increasing dose rate and there was a tendency for the Delta values to change with defect levels present within the crystals. Due to the independence of the HPHT diamond's Delta values on electron energy and its better stability of response to radiation, a small-size HPHT crystal was then evaluated of its potential applications in small radiation fields. Relative dose distributions measured with the diamond probe on exposure to 6, 12 and 14 MeV electron beams between 1 x 1 cm(2) and 10 x 10 cm(2) fields were compared with those obtained with reference ion chambers and a Dosimetry Diode E. The results showed that with careful selection of a suitable diamond crystal relative dose distributions taken with the diamond probe would compare favourably with those obtained with the reference detectors within or of the order of 1% with or without dose rate dependence corrections. In addition, the presented results have demonstrated for the CVD diamonds that Delta may vary with electron energy and it could be influenced by defect levels. (C) 2014 Elsevier Ltd. All rights reserved.