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One of the goals of the INFN HASPIDE project (Hydrogenated Amorphous Silicon DEtectors) is to explore the possibility to use a-Si:H detectors to be employed in medical physics application. The choice of this material as sensitive layer is driven by its resistance to radiation damage which allows the sensors to operate in very demanding environments like skin dosimeter and/or FLASH therapy. Furthermore, a sensitive layer very thin and deposited on a thin flexible plastic layer of Polyimmide permits to develop matrix of sensors with a great variety of shapes. First tests on clinical photon beams are reported.

Four a-Si:H n-i-p diode structure of 2.5µm thickness are fabricated on Kapton substrate. The upper contact is an ITO/aSi stack while the bottom contact is a stack of Cr/Al/Cr/aSi deposited directly on the kapton substrate. The pad diode area is 2x2 mm2. Samples are mechanically fixed to a kapton pigtail 35cm long via double sided sticky tape. Electrical contacts are made with MG Chemicals 838AR Carbon Conductive Paint and insulated copper wire with 50 µm diameter. 70 µm thick Kapton tape placed over the sample for protection of the electrical contact as well as to create a consistent and reproducible build-up layer for dosimetry.
Tests of the HASPIDE dosimeters were performed by means of an Elekta VERSAHD LINAC, with conventional 6MV photon beams. The dosimeter was placed at the isocenter and sandwiched inside a phantom of water equivalent material at 10cm depth, SSD=90cm. Signal repeatability, linearity with dose and sensitivity was studied for each dosimeter.
Tests were performed with a radiation field of 10 × 10 cm2 and nominal dose rate of 500cGy/min. Linearity with dose was evaluated by fitting the charge signal of each pixel against the dose for a fixed dose-rate, in the dose range 2-1000 cGy. To evaluate the sensitivity, we performed a least-square fit by using the relation Q = αD + β, with Q the signal in charge and D the dose. The sensitivity was supplied by the angular coefficient α.

Fig.1  shows charge signal against the dose of four Haspide pixels, both data and best fit are shown. The sensitivity of the four pixels is in the range of 140 and 160 fC/cGy with a linearity 0,999 and the signal to noise ratio is about 70.

The good performances of the detector  and its physical dimensions show that the Haspide device is suitable for application in radiotherapy. Due its small thickness future application in skin dosimeter and flash therapy should be investigated.