We are a member of the Medipix2 Consortium, based at CERN. Involved in these Medipix activities at NIKHEF are:
The goal of the collaboration is to transfer the technology, developed for semiconductor pixel detectors within High-Energy Physics, towards other applications such as Medical and Analytical X-ray imaging, see also the ITLO page at CERN, and the Technology Database.
A pixel detector for high-energy charged particle tracking consists of a sandwich of two Silicon chips, one chip containing a matrix of reverse-biased diodes implanted in a high-resistivity silicon material, the other containing a matching matrix of CMOS readout electronic circuits. Due to the small pixel size and correspondingly low input capacitance, an extremely low noise figure, in the order of 100 electrons rms on the input, can be reached at room temperature.
To use this technology for X-ray imaging, the sensor material is replaced with higher-Z materials like GaAs or Cd(Zn)Te, having better conversion efficiencies than Silicon for X-rays in the energy-range used for medical imaging (20 to 100 KeV). Also the complicated event-by-event readout circuitry is replaced by a simple 15-bits counter in each pixel-readout circuit.
The result is a unique device, performing single-photon counting without any noise or dark-current, at room temperature. First results with the Medipix-1 chip have shown that a reduction in X-ray dose of 30 times is attainable, with respect to current methods using fluorescence and integrating CCD-readout. The remaining disadvantage with respect to CCD's is the limited resolution (pixel size 170 x 170 micron-squared). But the Medipix-1 chip was designed several years ago in a SACMOS process with 1 micron minimum gatelength, while 0.25 micron is now available and 0.18 micron is expected this year. So NIKHEF is contributing to the design of a new chip in -.25 micron technology, which will have a pixel size of 55 x 55 micron-squared. This effort is done in close interdisciplinary collaboration with the IC-design group of Twente University and the Micro-electronics group at CERN. The submit of this Medipix2 chip is planned around September 2000.
A second NIKHEF effort is the development of a PCIbus interface for the readout of Medipix Chips (MUROS). This small multilayer printed-circuit board replaces a cumbersome, costly and complicated VME-based system. The MUROS-1 interface for Medipix-1 is already in use by several participants in the collaboration for imaging as well as for wafer-testing the new chips. The MUROS-2 upgrade for Medipix-2 will be based on high-speed serial signalling (LVDS).