Development of hydrogenated amorphous silicon sensors for high energy photon radiotherapy imaging

Measurements with a-Si:H n-i-p photodiodes with sensitive areas of ~0.6 mm² exposed to a 6 MV radiation therapy treatment beam have been performed. Such photodiodes can be configured into large two-dimensional arrays of addressable sensors suitable for real-time imaging of megavoltage treatment beams as well as other applications such as diagnostic X-ray imaging. Signal sizes per radiation pulse up to 3.9×10⁶ and 69×10⁶ electrons are observed when the sensors are used with cronex (CaWO₄) and lanex (gadolinium-oxysulfide) intensifying screens, respectively. For the cronex and lanex screens the size of the measured signals can be enhanced by reducing the thickness of the top p-layer, thereby reducing the attenuation of the incident light. However, this is a smaller effect for the lanex screen, which provides substantially more light signal. The variation in cronex signal size with p-layer thickness is consistent with calculations based on known attenuation coefficients