Characterisation of a single photon counting pixel system for imaging of low-contrast objects

In the framework of the Medipix11The Medipix collaboration is formed by CERN, the University of Freiburg, University of Glasgow and the Universities and INFN of Pisa, Napoli and Sassari. boration the PCC, a single photon counting pixel chip, has been developed with the aim of improving the contrast resolution in medical imaging applications. The PCC consists of a matrix of 64×64 square pixels with 170 μm side length, each pixel comprising a 15-bit counter and a pulse-height discriminator. The chip has been bump bonded to equally segmented 200 μm thick SI-LEC GaAs detectors showing a very high absorption energy for X-rays used in diagnostics. An absolute calibration of the system with a radioactive source and a synchrotron beam are described resulting in the value of the test input capacitance of ∼24.7 fF. Using this value a full characterisation of the system from electrical measurements is presented. The entire system can reach a minimum threshold of ∼2100e− with ∼250e− rms noise. One of the characteristic features of the PCC is the possibility to adjust the thresholds of all pixels on a pixel-by-pixel basis with 3-bit precision. The threshold distribution after adjustment is ∼120e− rms. The spatial resolution of the system has been measured to be 3.6 lp/mm. A comparison of a tooth image taken with the PCC and with a screen-CCD-system demonstrates its imaging capabilities.