The Medipix1 chip or PCC (photon counting chip) was the first large area single photon counting CMOS imaging chip. It comprises a matrix of 64 x 64 identical pixels each measuring 170 μm x 170 μm. Each pixel contains a preamplifier, comparator and a 15-bit pseudorandom counter. It operates like an electronic camera counting hits above threshold while the shutter is open and permitting readout of the acquired image when the shutter is closed. It was bump bonded to identically segmented sensors of both silicon and GaAs. Although the number of pixels was rather low, the pixel pitch was rather high and it could not be abutted to neighbouring chips it demonstrated the potential of single photon counting X-ray imaging.
The Medipix1 chip