Using 0.25μm standard CMOS we have developed 2-D semiconductor matrix detectors with sophisticated functionality integrated inside each pixel of a hybrid sensor module. One of these sensor modules is a matrix of 256x256 square 55μm pixels intended for X-ray imaging. This device is called 'Medipix2' and features a fast amplifier and two-level discrimination for signals between 1000 and 100000 equivalent electrons, with overall signal noise ~150 e- rms. Signal polarity and comparator thresholds are programmable. A maximum count rate of nearly 1 MHz per pixel can be achieved, which corresponds to an average flux of 3x10exp10 photons per cm2. The selected signals can be accumulated in each pixel in a 13-bit register. The serial readout takes 5-10 ms. A parallel readout of ~300 μs could also be used. Housekeeping functions such as local dark current compensation, test pulse generation, silencing of noisy pixels and threshold tuning in each pixel contribute to the homogeneous response over a large sensor area. The sensor material can be adapted to the energy of the X-rays. Best results have been obtained with high-resistivity silicon detectors, but also CdTe and GaAs detectors have been used. The lowest detectable X-ray energy was about 4 keV. Background measurements have been made, as well as measurements of the uniformity of imaging by photon counting. Very low photon count rates are feasible and noise-free at room temperature. The readout matrix can be used also with visible photons if an energy or charge intensifier structure is interposed such as a gaseous amplification layer or a microchannel plate or acceleration field in vacuum.
A 4096 pixel Photon Counting Chip (PCC) has been developed and tested. It is aimed primarily at medical imaging although it can be used for other applications involving particle counting. The readout chip consists of a matrix of 64 by 64 identical square pixels, whose side measures 170 micrometers and is bump-bonded to a similar matrix of GaAs or Si pixel diodes covering a sensitive area of 1.18 cm<SUP>2</SUP>. The electronics in each cell comprises a preamplifier, a discriminator with variable threshold and a 3-bit threshold tune as well as a 15-bit counter. Each pixel can be individually addressed for electrical test or masked during acquisition. A shutter allows for switching between the counting and readout modes and the use of static logic in the counter enables long data taking periods. Electrical test of the chip have shown a maximum counting and readout modes and the use of static logic in the counter enables long data taking periods. Electrical test of the chip have shown a maximum counting rate of up to 2 MHz in each pixel. The minimum reachable threshold is 1400 e with a variation of 350 e rms that can be reduced to 80 e rms after tuning with the 3-bit adjustment. Electrical noise at the input is 170 e rms. Several read-out chips have been bump bonded to 200 micrometers thick GaAs pixel detectors. Test with (gamma) -ray and (beta) sources have been carried out. A number of objects have been imaged and a 260 micrometers thick aluminum foil which represents a contrast to the surrounding air of only 1.9 percent has been correctly imaged.