DOSE-RATE DEPENDENCE OF A PTW DIAMOND DETECTOR IN THE DOSIMETRY OF A 6 MV PHOTON-BEAM

The dose rate dependence and current/voltage characteristics of a PTW Riga diamond detector in the dosimetry of a 6 MV photon beam have been investigated. Diamond detectors are radiosensitive resistors whose co nductivity (i) varies almost in proportion to dose rate and (ii) is al most independent of bias voltage for a constant dose rate. At the reco mmended bias of +100 V, and also at +200 V, the detector is operating with incomplete charge collection due to the electron-hole recombinati on time being shorter that the maximum time for an electron to be coll ected by the anode. As dose rate is varied by changing FSD or depth (c hanging dose per pulse), detector current and dose rate are related by the expression i is-proportional-to D(DELTA) where DELTA is approxima tely 0.98. This manifests itself in an overestimate in percentage dept h-dose at a depth of 30 cm of approximately 1% when compared to ioniza tion chamber results. A similar sublinearity is seen when pulse repeti tion frequency is varied, indicating that the dependence is an on aver age rather than an instantaneous dose rate. The dose rate dependence i s attributed to the reduction in recombination time as dose rate incre ases.