Test of a GaAs-based pixel device for digital mammography

We are characterising a prototype GaAs pixel detector able to perform digital mammography which contains two features which give enhanced performance compared with standard mammographic systems: it can detect low contrast test objects, inaccessible for traditional systems and, using its higher intrinsic detection efficiency, can reduce the radiation dose to the patient. The prototype is a hybrid assembly of a GaAs detector divided into 64 x 64 pixels, each of which measures 170 mum on the side, which is bump-bonded to a photon counting chip, PCC, developed by the Medipix collaboration. The cells of the readout chip are identical in size to the detector pixels and work in single photon counting mode. We have studied the detection efficiency of some assemblies for different bias voltages, reaching in some cases the detector breakdown point. To test the assembly in the same clinical conditions used in a standard mammographic system we have tested the prototype with radiation produced by a standard mammographic X-ray tube operating at 28 kVp, In particular, working not too far from the detector breakdown point, we are checking its imaging capabilities using a mammographic test phantom with known composition of the materials and therefore known contrast.