A multi-layer silicon microstrip detector for single photon counting digital mammography

The SYRMEP/FRONTRAD (SYnchrotron Radiation for MEdical Physics/FRONTier RADiology) collaboration has developed a digital X-Ray imager for the mammography beamline at ELETTRA, the synchrotron radiation facility in operation in Trieste, Italy. This imaging device is composed of three stacked layers of FOXFET-biased silicon microstrip detectors, positioned with the strips along the beam direction. In this way a very high absorption efficiency is guaranteed, since the strip length (1 cm) absorbs nearly all the incoming photons in the energy range of interest (15 - 30 keV). Furthermore, the active surface is by construction subdivided into pixels, the dimensions of which are determined by the strip pitch (200 mu m) in one direction and by the thickness of the single layer (300 mu m) in the other. Each layer has 256 implanted strips, and therefore the whole device has an active surface of -50x1 mm(2) subdivided into 764 pixels (the inter-layer distance is nearly equal to 100 mu m) The electronic chain reading out the detector signal operates in "single photon counting" mode: this gives the possibility of extracting the maximum information from the radiation beam, since the only limitation to the signal-to-noise ratio in the images is the quantum noise. Two-dimensional images are obtained by scanning the samples through the synchrotron laminar beam, while the detector is kept stationary with respect to the beam itself. The desired statistics is obtained by combining the information from the three layers, thus reducing the overall acquisition time by a factor of three. Results from extensive tests carried out on this imaging device by means of a synchrotron radiation beam are reported on this paper.