Single Photon 2 { D Imaging System Prototype forBiopsy Digital Mammography

We have developed a digital radiography system based on double sided mi-crostrip silicon crystals. The detector is 300 m thick, equipped with 320 channels for each side. The strip pitch is 100 m. The read{out system consists of a VLSI integrated analog circuit with low noise preampliication, pulse shaping, threshold discrimination. The two main issues of this Front End chip are low noise and good timing information. It has 64 channels which provide a digital pulse whenever one strip of the detector is hit. The channel number is then encoded by the digital chip which is directly bonded to the analog Front{End. This is accomplished on both sides independently. To reconstruct the spatial position an X-Y coincidence unit is used. The two signals produced along the two coordinates are time related by sampling the Front{ End outputs with a common clock signal. The two correlated signals are then presented to the coincidence unit at the same clock cycle. To accomplish the on{line image reconstruction the time unit recognizes these correlated signals as one real event and updates a related histogram. This system is suitable for X{Ray imaging, optimized for stereotaxic mam-mography in the 18 ? 22 keV photon energy range.