High-bandgap semiconductor dosimeters for radiotherapy applications

We present a comparison between the performance of on-line radiotherapy dosimeters made with different high-bandgap semiconductor materials. We analysed the performances of a Schottky diode made with an epitaxial n-type 4H–SiC and a Chemical Vapour Deposited (CVD) diamond film with ohmic contacts. The current response of the dosimeters has been tested under exposure to a Co60 γ-source and to 6 MV photons beam from a linear accelerator. The dose range covered is 0.1–10 Gy with dose rates 0.1–10 Gy/min. The two devices show a charge response linear with the dose when a constant dose rate is used. The SiC diode current response increase linearly with the dose rate; for diamond a quasi-linear behaviour is observed. The epitaxial SiC device shows no priming effects and a fast velocity of response, due to the low density of lattice defects in this material. The diamond performances are affected by trapping–detrapping mechanisms at defects with energy levels at ∼1 eV. To de-activate these levels the diamond sample has been pre-irradiated with fast neutrons up to a fluence of 5×1014 cm−2. The sensitivity of the two devices compare favourably to those of standard silicon dosimeters.