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

In the framework of the Medipix(1) collaboration the PCC, a single photon c ounting pixel chip, has been developed with the aim of improving the contra st resolution in medical imaging applications. The PCC consists of a matrix of 64 x 64 square pixels with 170 mum side length, each pixel comprising a 15-bit counter and a pulse-height discriminator. The chip has been bump bo nded to equally segmented 200 mum thick SI-LEC GaAs detectors showing a ver y high absorption energy for X-rays used in diagnostics. An absolute calibr ation of the system with a radioactive source and a synchrotron beam are de scribed resulting in the value of the test input capacitance of similar to 24.7 fF. Using this value a full characterisation of the system from electr ical measurements is presented. The entire system can reach a minimum thres hold of similar to 2100e(-) with similar to 250e(-) rms noise. One of the c haracteristic features of the PCC is the possibility to adjust the threshol ds of all pixels on a pixel-by-pixel basis with 3-bit precision. The thresh old distribution after adjustment is similar to 120e(-) rms. The spatial re solution of the system has been measured to be 3.61p/mm. A comparison of a tooth image taken with the PCC and with a screen-CCD-system demonstrates it s imaging capabilities. (C) 2001 Elsevier Science B.V. All rights reserved.