SU-E-T-153: Detector-Grade CVD Diamond for Radiotherapy Dosimetry

To evaluate the use of commercially available detector-grade synthetic diamond films made via chemical vapor deposition (CVD) as x- ray detectors for radiotherapy dosimetry.

A detector was fabricated using high-quality single crystal CVD diamond films (0.5 × 3 × 3 mm̂3) with 0.4 mm̂3 sensitive volumes, which were encapsulated with PMMA. The detector was placed in a (30 × 30 × 30 cm̂3) PTW water phantom. Six- and ten-MV photons from an Elekta Synergy linac were measured using an SSD of 90 cm and typically a 10-cm phantom depth with a 10 × 10 cm̂2 field size in the central axis of the beam. Data acquisition was performed using a PTW UNIDOS E electrometer with a 100-V bias. The detector was evaluated by measuring leakage current, priming dose, response dynamics, dose linearity, dependence on dose rate, percent depth dose (6 and 10 MV photons) and output factors. Some measurements were compared with a Si diode detector, 0.04 and/or 0.13-cc ion chamber(s).

Leakage currents were negligible (∼1 pA) given the overall average sensitivity of the material (680 nC/Gy at 100 V). Detector current rise and fall times were < 2 s. Dose measurements up to 5 Gy were linear. A slight dependence on nominal dose rates up to 500 MU/min was seen with Fowler fitting parameter delta = 0.981. Percent depth dose profiles compared well to a 0.13-cc ion chamber up to 25 cm depths for 10-MV photons. Output factors measured well for field widths 5-15 cm; below 5 cm, differences are seen between all four different detectors as expected.

The type of diamond tested has potential to be used for small field dosimetry due to its small sensitive volume and high sensitivity. Further experiments are ongoing and detector packaging is yet to be optimized.