MEDIPIX: a VLSI chip for a GaAs pixel detector for digital radiology

A GaAs pixel detector designed for digital mammography, equipped with a 36- channel single photon counting discrete read-out electronics, was tested us ing a test object developed for quality control purposes in mammography. Ea ch pixel was 200 x 200 mu m(2) large, and 200 mu m deep. The choice of GaAs with respect to silicon (largely used in other applicati ons and with a more established technique) has been made because of the muc h better detection efficiency at mammographic energies, combined with a ver y good charge collection efficiency achieved thanks to new ohmic contacts. This GaAs detector is able to perform a measurement of low-contrast details , with minimum contrast lower (nearly a factor two) than that typically ach ievable with standard mammographic film + screen systems in the same condit ions of clinical routine. This should allow for an earlier diagnosis of bre ast tumour masses. Due to these encouraging results, the next step in the evolution of our ima ging system based on GaAs detectors has been the development of a VLSI fron t-end prototype chip (MEDIPIX) in order to cover a much larger diagnostic a rea. The chip reads 64 x 64 channels in single photon counting mode, each o ne 170 mu m wide. Each channel contains also a test input where a signal ca n be simulated, injecting a known charge through a 16 f F capacitor. Fake s ignals have been injected via the test input measuring and equalizing minim um thresholds for all the channels. On an average, in most of the performing chips available up to now, we have found that it is possible to set a threshold as low as 1800 electrons with an RMS of 150 electrons (10 standard deviations lower than the 20 keV phot on signal roughly equivalent to 4500 electrons). The detector, bump-bonded to the chip, will be tested and a ladder of detectors will be prepared to b e able to scan large surface objects. (C) 1999 Elsevier Science B.V. All ri ghts reserved.