Medipix3 read-out chip operation
Each pixel of the Medipix3 (MPX3) chip has two thresholds and two 12-bit counters, which can be alternatively merged to one 24-bit counter, providing enhanced dynamic range.
In principle, two main modes of operation are available:
- Single Pixel Mode (SPM): every pixel works independently of its neighbours, analogous to Timepix chips.
- Charge Summing Mode (CSM): two thresholds are used for the energy discrimination. The algorithm is as follows: the detected quantum of radiation generates an electric charge in the sensor. The charge is typically collected to two or more pixels and creates an electric signal in each pixel. The signal is compared to the first threshold in every pixel. Signals below the threshold are cut off (similar to the SPM). The signal/charge of 4 neighbouring pixels is reconstructed (summed) and assigned to the pixel with the largest charge deposition. The result is stored in the first counter and it is called SPM arbitrated (SPMa). In second step, the result is then compared to the second threshold, signals below the threshold are omitted. The result is called CSM and it is stored in the second counter.
Both modes can be moreover operated in several ranges which define the minimal and maximal achievable threshold. The range setting can be found in literature under the term gain mode.
Modes supported in Pixet (WidePIX MPX3 device with 55 μm pixel pitch)
Modes of operation:
SPM 1 CH (Single Pixel Mode, one channel) - one counter is in use, one energy threshold (energy channel) is available, measurement output: one 12-bit or 24-bit image representing the number of events per pixel.
SPM 2 CH (Single Pixel Mode, two channels) - two counters are in use, two energy thresholds (energy channels) are available, measurement output: two 12-bit images representing the number of events per pixel.
CSM (Charge Summing Mode) - two energy thresholds are in use, measurement output: one 12-bit or 24-bit image representing the number of events per pixel after charge summing procedure (second counter).
Ranges (gain modes):
Broad (SLGM) – the largest threshold range.
Narrow (HGM) – better energy resolution at lower energies.
Super Narrow (SHGM) – very limited threshold range, the best energy resolution at lower
energies, better signal-to-noise ratio.
!! Combination CSM + Super Narrow range is not supported with silicon detectors.
For an experienced user, other modes and combinations are accessible via advanced setting in Pixet. They may have no calibration parameters.