What is Medipix3?

Medipix3 is a CMOS pixel detector readout chip designed to be connected to a segmented semiconductor sensor. Like its predecessor, Medipix2, it acts as a camera taking images based on the number of particles which hit the pixels when the electronic shutter is open. However, Medipix3 aims to go much further than Medipix2 permitting colour imaging and dead time free operation.

When a low energy x-ray photon (1keV < E < 20keV) undergoes a photoelectric interaction in a segmented semiconductor the photoelectron created deposits all of its charge within a tiny volume. Under the influence of an electric field the electrons and holes created drift towards the collection electrodes. At the same time as the charge drifts under the electric field it also diffuses in all dimensions. This means that it often happens that although most of the deposited charge is collected in the pixel where the initial hit took place some charge is collected in neighbouring pixels. This in turn means that the observed spectrum of a single pixel is distorted towards lower energies (see image). The Medipix3 pixel readout architecture seeks to mitigate the effect of charge sharing by summing charge between neighbouring pixels and allocating the sum or hit to the individual pixel with the highest collected charge.

[image: energy spectrum at 10keV]

In blue is the spectrum observed by a 55 mm square silicon pixel detector which is uniformly exposed to 10 keV photons. In red is the spectrum seen by a pixel operating in charge summing mode where the output of 4 pixels are added. Other material and energies have also been simulated.

The Medipix3 architecture allows pixels to operate either in single pixel mode or in charge summing mode.

As each Medipix3 pixel will have 2 thresholds and 2 counters the user may configure the chip to work either in simultaneous read/write mode (one counter is read out while the other counts) or in sequential read/write mode with 2 different thresholds.

It is also possible to bump bond only 1 pixel in 4 increasing the sensor pixel pitch from 55 mm to 110 mm while having 8 counters per pixel. This is called spectroscopic mode and permits either 4 separate thresholds in simultaneous read/write mode or 8 thresholds in sequential read/write mode.

In more complicated circumstances (for example, where higher energy photons are used and) where Compton scattering and/or fluorescence takes place in the sensor the charge deposited may be spread over more than 110 mm x 110 mm. Therefore the charge summing mode can also be applied to the lower pixel granularity allowing 110 mm pixel pitch but collecting charge over a volume defined by a 220 mm square and the sensor thickness.