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

In the framework of the Medipix1 collaboration 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 170lm side length, each pixel comprising a 15-bit counter and a pulse-height discriminator. The chip has been bump bonded to equally segmented 200lm 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. ( 2001 Elsevier Science B.V. All rights reserved.