A Monte Carlo comparison of the response of the PTW-diamond and the TL-diamond detectors in megavoltage photon beams

A detailed Monte Carlo study of the PTW-diamond solid state detector respon se in megavoltage photon beams (Co-60 gamma rays to 25 MV x rays) has been performed with the EGS4 Monte Carlo Code. The sensitive volume of the diamo nd detector is a disk of diameter 4.4 mm and thickness 0.40 mm. The phantom material was water and the irradiation depth was usually 3 cm but addition al simulations were performed at six other depths for the 10 and 25 MV x ra ys. Results show that the PTW-diamond detector response per unit of absorbe d dose is constant within 1% for photon beam energies ranging from Co-60 ga mma rays to 25 MV x rays. Accurate depth dose curves for 10 and 25 MV x-ray beams may be measured with the diamond detector since the response per uni t of absorbed dose at different depths in a water phantom is also constant to within 1% for depths ranging from 3 to 25 cm and field sizes ranging fro m 2.5 cm by 2.5 cm to 10 cm by 10 cm. An examination of the difference betw een the PTW-diamond detector and the wall-less form of the detector (e.g., TLDs) revealed that there is no significant difference in their response in megavoltage photon beams. This implies that the encapsulation of the diamo nd dosimeter causes less than a 1.3% change in its response for these megav oltage photon beams. Analysis of the total dose deposited in the sensitive volume of the detector shows that the PTW-diamond detector behaves as an in termediate-sized cavity, not a simple Bragg-Gray cavity, since the dose con tribution from photon interactions within the cavity (cr,) to the total cav ity dose is 8% for 25 MV x rays and increases to 42% for Co-60 gamma rays. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)015 11-4].