Purpose: An ideal dosimeter for radiation therapy applications is typically described as water-equivalent to eliminate inhomogeneity-related perturbations and simplify interpretation of measurements. In practice corrections are required, especially in small-field dosimetry where both field-size and detector-specific corrections are employed. Water-equivalency becomes even more challenging for solid state detectors, restricting choices to Si or diamond. We explore an alternative approach of purposely modifying electron spectra reaching the sensitive volume, studying high-efficiency Cadmium Telluride (CdTe) detector as a practical example.
Methods: Since the stopping-power ratios are not affected by the field-size-dependency changes in photon fluence, modifying electron spectra within detector sensitive volume becomes feasible. MCNP5 package was used to model dosimeter designs combining a semiconductor layer with a metal back reflector. CdTe of 30 and 300μm thickness was simulated in combination with copper (Cu) and lead (Pb) of varying thickness, with surrounding PMMA layer. The absorbed dose was calculated based on electron fluences and stopping power.
Results: Based on the electron spectrum generated at the surface of CdTe under 6MV photon beam representative of Varian TrueBeam linac, we were able to acquire fluences due to electrons backscattered in CdTe from the interface with a metal layer, thus adjusting the final dose in CdTe. Varying thickness of Cu and Pb and thus changing electron backscattered fluence and spectra from the interface we can adjust dose ratio of CdTe to water in a controlled manner. Further design modifications can be done using electron backscatter coefficients for the corresponding interfaces.
Conclusion: Cadmium Telluride (CdTe) detectors offer high sensitivity, limited dark current, and mature manufacturing technology. High sensitivity of solid-state detectors is an attractive attribute for radiation dosimetry applications. Our approach of modifying electron spectra through addition of a back-reflector metal to CdTe detector opens a venue for design of a new class of correction-less dosimeters.