Test of a GaAs-based pixel device for digital mammography

We are characterising a prototype GaAs pixel detector able to perform digit al mammography which contains two features which give enhanced performance compared with standard mammographic systems: it can detect low contrast tes t objects, inaccessible for traditional systems and, using its higher intri nsic detection efficiency, can reduce the radiation dose to the patient. Th e prototype is a hybrid assembly of a GaAs detector divided into 64 x 64 pi xels, 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 cel ls of the readout chip are identical in size to the detector pixels and wor k 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 conditi ons used in a standard mammographic system we have tested the prototype wit h 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. (C) 2 001 Elsevier Science B.V. All rights reserved.