Medipix

The Technology

The Medipix family consists of the Medipix 1 chip, the Medipix 2 chip, with an improved resolution, Timepix, a modified version of Medipix2 with the additional functionality of time measurements and Medipix3, which not only counts all of the photons but also determines the energy level of each individual photon detected.

A particle detector can observe each individual proton, electron, neutron or other exotic particle that fall within its measurement range. The intricacies of the LHC operation require the detectors to have certain qualities – they must be fast, noise-free and have high resolutions. CERN researchers saw the potential of transferring this technology outside the High Energy Physics domain, thus the Medipix project was born.

The Medipix chip is a single photon counting chip. In conventional methods several photons are needed to obtain a (black) image, but the Medipix chip requires just one and, therefore, requires less radiation. Another important advantage compared to conventional techniques is that no signal is measured if no photon enters. This means that there is no noise irrespective of the period of exposure.

Both the sensors and the microchips - which together form a hybrid detector - are divided into tiny sensitive elements (pixels), similar to those in a digital camera. These hybrid pixel detectors produce images with high resolution, high contrast and almost no noise. They are so sensitive that they can detect individual X-ray photons.

Application of Medipix chip

• Medical imaging
• Material analysis
• Nuclear power plant decommissioning
• Optics
• Education

Parameters of Medipix:

• Pixels: 256 x 256
• Pixel size: 55 x 55 mm2
• Area: 1.5 x 1.5 cm2

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.

Medipix: Knowledge Translfer[online]. 2014 [cit. 2014-01-23]. Source: http://knowledgetransfer.web.cern.ch/life-sciences/from-physics-to-medicine/medipix

Presentations about Medipix chip

Presentation 1: Medipix chip on the ISS

Presentation 2: Energy calibration of Medipix chip