High-bandgap semiconductor dosimeters for radiotherapy applications

We present a comparison between the performance of on-line radiotherapy dosimeters made with different highbandgap 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 gamma-source and to 6MV 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 primingeffects 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 B1 eV. To de-activate these levels the diamond sample has been preirradiated with fast neutrons up to a fluence of 5x10^14 cm^(-2). The sensitivity of the two devices compare favourably to those of standard silicon dosimeters.