Study of a CVD diamond detector for absorbed dose measurement in photon beams with small field sizes

A new diamond detector to perform absorbed-dose- to-water measurement in radiotherapy photon beams with small field sizes is being developed in the framework of the EURAMET/EC FP7 project “External Beam Cancer Therapy”. An objective of the project is to obtain detectors capable of ensuring traceability of absorbed dose measurements in radiotherapy photon beams with field size down to 1 cm x 1 cm. To this end the CVD diamond detectors up to now developed were found not adequate. The present project requires detectors with high spatial resolution, good stability and low energy dependence. Diamond detectors were chosen for their high spatial resolution and good tissue equivalence (Z = 6). The detector studied in this work is based on a single crystal CVD diamond with dimensions 3 mm x 3 mm x 0.5 mm on which 0.2 μm electrodes were thermally evaporated. Particular care was addressed to ensure the tissue-equivalence of the detector by using appropriate materials. A thorough analysis of the priming procedure, stability and signal reproducibility was carried out in a Co-60 gamma beam at dose rates in the range from 0.3 Gy min− 1 to 1.38 Gy min− 1. Moreover the detector response was studied by Monte Carlo calculations as a function both of the beam quality, from Co-60 to 10 MV photon beams, and field size, from 10 cm x 10 cm to 1 cm x 1 cm. The perturbation due to the non-water equivalence of electrodes was also determined by Monte Carlo simulations.