edipix

The Medipix2 Chip

The Medipix2 CMOS ASIC is the successor of the Medipix1 (or PCC) photon counting chip. It benefits from the quick progress of CMOS technology which allows enhanced functionality of the pixel cell at the same time as providing a significant reduction in pixel size.

• The square pixel size of 55 µm side length overcomes one of the limitations of the Medipix1 chip and makes the Medipix2 chip competitive with most of the existing imaging devices in terms of spatial resolution.
• Direct X-ray conversion in a semiconductor sensor minimises image blurring and avoids an extra conversion stage from X-rays into visible light.
• The chip is designed to accept either positive or negative charge input in order not to restrict the choice of the sensor material (Si, GaAs, CdZnTe,...). Detector leakage current gets compensated pixelwise at the input.
• With the Medipix2 chip it is possible to select a window in energy. Upper and lower threshold can be adjusted pixelwise with 3 bits for uniform performance of the whole pixel matrix and will open new measurement perspectives. The photon counting principle in contrast to systems based on charge integration suppresses noise and leads to superior SNR properties.
• Exposure times can be chosen arbitrarily. Data is accumulated in a 13-bit counter per pixel. Each pixel can handle count rates of about 100 kHz of randomly arriving particles. Read-out is performed after exposure to avoid dead time.
• Parallel and serial read-out will be realised.
• The Medipix2 chip has an active area of about 2 cm². 256 x 256 pixels form the pixel matrix.
• Larger area coverage is a big concern of the collaboration. Medipix2 will therefore be 3-side buttable.