Frontend electronics

The limitations on the energy range arising from the readout electronics come from the noise and from saturation. The electronic noise limits the minimum detectable energy for single photons, while saturation limits the maximum detectable signal either for single photons or in total.

• In single photon counting detectors, the minimum threshold cannot be set lower than 3-5 times the electronic noise. If the threshold is set at approximately half of the X-ray energy (see specific documentation about single photon counting detectors), the minimu detectable energy will be about 6-10 times the noise.
  In order to reduce the noise of the frontend electronics different settings can be chosen, but this puts a limit on the maximum incoming flux that can be detected without incurring in pileup (see specific documentation about single photon counting detectors). Figure 2.3 shows an example of the settings used for the MYTHEN detector for different energy ranges and fluxes.
  For state of the art single photon counting detectors, the minimum thrshold can be about 2-3 keV (details depend on the detector and can be further reduced using special settings).

  Concerning saturation, this imposes a maximum value for the comparator threshold. Normally photons of higher energies can still be detected, but without resolution concerning the threshold energy and eventually losing spatial resolution. By changing the settings it is possible to increase the maximum threshold value (normally also noise increases in this case).

• For charge integrating detectors the electronics noise puts a limit on the minimum detectable signal. Therefore if single photon resolution is required, the minimum detectable energy is defined as for single photon counting detectors at about 6-10 times the electronic noise. In case no single photon resolution is required, the electronic noise will put a limit on the sensitivity of the detector i.e. the total accumulated signal needs to be larger than 6-10 times the noise in order to be detected (also about 2-3 keV depending on the detector). It is important to point out that the acquisition time of charge integrating detectors is limited by the leakage current of the sesnors and the noise quadratically sums out. Therefore the signal for low energy photons should be strong enough to be acquired during single frames.

  Concerning saturation, this sets a limit on the total number of photons acquired during the acquistion slot and is normally much larger than the energy released by single X-rays. Dynamic gain switching can strongly increase the dynamic range of the detector up to 10E+4 12 keV photons.

Figure 2.3: Settings to be chosen for the MYTHEN detector as a function of the X-ray energy and radiation intensity.