Medipix
Medipix is a family of photon counting and particle tracking pixel detectors developed by an international collaboration, hosted by CERN.
Design
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. 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.
Timepix devices offer two more modes of operation in addition to the counting. The first one is so called “Time-over-Threshold” mode. 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 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 it is possible to measure over frequency-bands of the incoming radiation; thus enabling spectroscopic x-ray imaging.
Medipix-2, Timepix, and Medipix-3 are all 256×256 pixels, each 0.055 mm square, forming a total area 14.08 mm × 14.08 mm. Larger area detectors can be created by bump-bonding many 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.
Comparison with existing technologies
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:| Film emulsions | Charge integrating devices | Photon counting pixel detectors | |
| Operation principle | Change of chemical or physical properties after interaction with radiation. Needs special treatment. | Ionizing radiation creates light and subsequently an electric charge that is collected and integrated in pixels. | Ionizing radiation creates charge directly in the sensor. The charge is compared with threshold and counted digitally in pixels. |
| Advantages | Very high resolution, low noise, cheap. | High spatial resolution. Low price. | Good spatial resolution, high read-out speed, no noise, no dark current, unlimited dynamic scale, energy discrimination |
| Disadvantages | Nonlinear response, limited dynamic scale, needs processing | Dark current, noise, limited dynamic scale | High price. |
Versions
Medipix-1 was the first device of the Medipix family. It had 64x64 pixels of 170 μm pitch. Pixels contained one comparator 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 each adjustable individually in pixels using a 3-bit offset. The maximum count rate is about 100 kHz per pixel.
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 and Time-of-Arrival. 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 as well as the pixel array size. 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 used 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.
Timepix-4 is the successor of the Timepix-3 chip. It has general stronger specifications for instance its time-of-arrival resolution is 195 ps, 8 times faster than Timepix-3, it also has a larger pixelmatrix of 512x448 pixels and can handle 8 times higher data rates.