A chemical vapour deposition diamond detector fabricated at Rome Tor Vergata University was investigated for its applicability as transfer dosimeter in radiotherapy photon beams with small field sizes. The detector consists of a single crystal diamond with a very small sensitive volume (0.004mm 3). The detector showed a measurement repeatability of 0.1% and a long term reproducibility of 0.4%. Monte Carlo simulations revealed a response dependence on the photon beam energy of about 2% from the 60Co quality to 10MV photon beam. The calculated detector response was found to be independent of field size within 0.5% from 10cm×10cm to 2cm×2cm beam size for both 6MV and 10MV photon beams, increasing in smaller field sizes. D w values obtained by the diamond detector were found to be in agreement with D w values obtained by a small volume ionization chamber in photon beams with field size down to 2cm×2cm.
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