Design and characterization of a tissue-equivalent CVD-diamond detector for clinical dosimetry in high-energy photon beams.

New solid-state detectors, based on chemical vapour deposited (CVD) polycrystalline diamonds produced by hot-filament (HF) or microwave plasma (MW) assisted deposition methods, were constructed for radiation therapy dosimetry. Properties of diamond crystals, such as high radiation sensitivity, resistance to radiation damage and tissue-equivalence giving a low-energy dependence are very advantageous for clinical dosimetry. Therefore the encapsulation was specially designed for these detectors to have as little influence as possible on the radiation response. The prototypes were irradiated with use of a wide range of photon beam qualities ((60)Co gamma-rays, 6 and 18 MV X-rays). The radiation sensitivity varied considerably between samples deposited with HF (9 nC Gy(-1)mm(-3)) and MW (66 and 144 nC Gy(-1)mm(-3)) methods. For all detectors the leakage current was of the order of 10% of the radiation-induced current (bias voltage 100 V, dose rate 0.3 Gy/min). When irradiated with (60)Co gamma-rays, the detectors showed a dose-rate linearity with an exponential Delta parameter close to unity. However, a difference of 8% was found between Delta values for the different beam qualities. A small energy dependence was observed, for which the most probable sources are interface effects due to the silver electrodes and partly the geometry of the encapsulation which needs to be further optimized. Despite some limitations in the performance of present prototype detectors, with an improved CVD technique producing crystals of better electrical and dosimetric properties, and with a well-designed tissue-equivalent encapsulation, CVD-diamonds could serve as very good dosimeters for radiotherapy.