Medipix

Principle of photon counting in a single pixel. The radiation generates electron-hole pairs (charge) in the sensor. The charge is collected to the appropriate pixel, amplified and compared with a pre-set comparator level (energy threshold). The counter is increased if the detected pulse is above the energy level.

These are hybrid detectors as a semiconductor sensor layer is bonded to a processing electronics layer.

The sensor layer is a semiconductor, such as silicon, GaAs, or CdTe in which the incident radiation makes an electron hole/cloud. The charge is then collected to pixel electrodes and, via bump bonds, conducted to the CMOS electronics layer.

The pixel electronics first amplifies the signal and then compares the signal amplitude with a pre-set discrimination level (an energy threshold). The subsequent signal processing depends on the type of device. A standard Medipix detector increases the counter in the appropriate pixel if the signal is above the discrimination level. The Medipix device also contains an upper discrimination level and hence only signals within a range of amplitude could be accepted (within an energy window).

Timepix devices offer two more modes of operation in addition to the counting. The first one is so called “Time-over-Threshold” mode (Wilkinson type analog-to-digital converter). It is a mode where the counter in each pixel records the number of clocks for which the pulse remains above the discrimination level. This number is proportional to the energy of detected radiation. This mode is useful for particle tracking applications or for direct spectral imaging.

The second mode of the Timepix chip is “Time-of-arrival”, in which pixel counters record time between a trigger and detection of radiation quanta with energy above the discrimination level. This mode of operation finds use in Time of flight (TOF) applications, for instance in neutron imaging.

Every individual hit of radiation is processed by the electronics integrated in each pixel this way, therefore the device could be considered as 65 536 individual counting detectors or even spectrometers. The energy discriminators are adjustable. Therefore, scanning with their level is possible to measure the full spectrum of the incoming radiation and thus enabling full spectroscopic x-ray imaging.

Medipix-2, Timepix, and Medipix-3 are all 256×256 pixels, each 0.055mm (55μm) square, forming a total area 14.08mm × 14.08mm. Larger area detectors can be created by bump-bonding multiple chips to larger monolithic sensors. Detectors of sizes from 2x2 to 2x4 chips are commonly used. Even larger, gapless areas could be created using the edgeless sensor technology. Medipix/Timepix chips each have its own sensor. These assemblies are tiled next to each other to create nearly arbitrarily sized detector arrays (the largest build using this technology has 10x10 chips, hence 14x14 cm and 2560x2560 pixels[1]).

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  Standard, single chip Medipix carrier board with USB readout.
• 
  The quad board: four Medipix2 chips with one common sensor chip to have a larger area with limited dead space.
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  Excalibur front end. Each of the three sensors has 16 chips flip chip bonded.
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  WidePix 10x10 with resolution of 2560x2560 pixels (6.5 Mpixels) and continuously sensitive area.

Photon counting pixel detectors represent the next generation of radiation imaging detectors. The photon counting technology overcomes limitations of current imaging devices. Comparison of photon counting with existing technologies is in the following table:

Medipix-1 was the first device of the Medipix family. It had 64x64 pixels of 170 µm pitch. Pixels contained one comparator (threshold) with 3-bit per-pixel offset adjustment. The minimum threshold was ~5.5 keV. The counter depth was 15-bit. The maximum count rate per pixel was 2 MHz per pixel.

Medipix-2 is the successor of Medipix-1. The pixel pitch was reduced to 55 µm and the pixel array is of 256x256 pixels. Each pixel has two discrimination levels (upper and lower threshold) each adjustable individually in pixels using a 3-bit offset. The maximum count rate is about 100 kHz per pixel (however in pixels with 9x smaller area compared to Medipix-1).

Medipix-2 MXR is an improved version of Medipix-2 device with better temperature stability, pixel counter overflow protection, increased radiation hardness and many other improvements.

Timepix is device conceptually originating from Medipix-2. It adds two more modes to the pixels, in addition to counting of detected signals: Time-over-Threshold (TOT) and Time-of-Arrival (TOA). The detected pulse height is recorded in the pixel counter in the TOT mode. The TOA mode measures time between trigger and arrival of the radiation into each pixel.

Medipix-3 is the latest generation of photon counting devices for X-ray imaging. The pixel pitch remains the same (55 µm) as well as the pixel array size (256x256). It has better energy resolution through real time correction of charge sharing. It also has multiple counters per pixel that can be used in several different modes. This allows for continuous readout and up to eight energy thresholds.

Timepix-3 is a successor of the Timepix chip. One of the biggest distinguishing changes is the approach to the data readout. All previous chips utilized the frame-based readout, i.e. the whole pixel matrix was read out at once. Timepix-3 has event-based readout where values recorded in pixels are read out immediately after the hit together with coordinates of the hit pixel. The chip therefore generates a continuous stream of data rather than a sequence of frames. The next major difference compared to the previous Timepix chip is the ability to measure the hit amplitude simultaneously with the time of arrival. Other parameters such as energy and timing resolution were also improved compared to the original Timepix chip.

The digital data recorded by Medipix/Timepix devices are transferred to a computer via readout electronics. The readout electronics is also responsible for setup and control of the detector parameters. Several readout systems were developed within the Medipix collaboration

• Medipix-1: Early 90s.
• Medipix-2: Late 90s.
• Medipix-3: Collaboration formed 2006.
• Medipix-4: Collaboration formed 2016.

• Medipix collaboration home page
• Medipix3 collaborators
• Medipix3 page on CERN’s Knowledge and Technology Transfer website