The SLS Detectors group develops hybrid detectors for synchrotron radiation applications. The detectors must provide high quality data and be reliable for daily users operation at the beamlines. Strip (1D) and Pixel (2D) detectors as well as single photon counting and charge integrating readout ASICs are developed in-house or in collaboration with other institution, as described in details in the project pages.
All names of the SLS detectors refere to Swiss mountains: Mythen close to Schwyz, Gotthard connecting North and South Switzerland, Eiger, Moench and Jungfrau in the Bernais Oberland.
AGIPD is part of a larger collaboration and therefore does not comply to this tradition.
Hereafter a short overview of the main properties of the detectors is given.
Hybrids detectors consist of a sensor absorbing the radiation and the frontend electronics tranforming the charge into a digitizable signal. The two parts are distinct and can therefore be optimized separately, depending on the applications. The interconnection technology between sensor and readout electronics also plays an extremely important role and the SLS Detectors Group is able to produce wire and bump bonded detectors with a high yeld over a large number of pixels.
All SLS Detectors are optimized for silicon sensors of standard thickness (0.3-1mm), which provide an absorption efficiency above 50% for X-rays in the energy range produced by most beamlines of the SLS (6-15 keV). Connecting the ReadOut electronics to sensors of different materials can optimize the carachteristics of the detectors for specific applications, as described in the research section.
Single Photon Counting ReadOut
The single photon counting detectors developed by the SLS Detectors group are MYTHEN (1D) and EIGER (2D).
Single-photon-counting detectors are sensitive to single photons and the only limitation on the fluctuations of the number of counts is given by the Poisson-like statistics of the X-ray quanta. The digitized signal does not carry any information concerning the energy of the X-rays and all photons with an energy larger than the threshold are counted as one bit. This means that the choice of the correct comparator threshold level is critical in order to count all photons of inteserest and suppress the electronic noise and the background of fluorescence light eventually produced by the sample.
The minimum detectable photon energy is limited by the electronic noise and the dynamic range is defined by the bit depth of the counters. Since a photon-counting detector is readout-noise free, an even larger dynamic range can be achieved by summing separate frames without increasing the uncertainties. The improved dynamic range given by the possibility of detecting single photons and by the absence of saturation makes photon-counting systems optimal for experiments where small signals must be detected, e.g. for thin or weakly scattering samples.
In the case of photon-counting systems a deviation from the linearity on the number of counts occurs at high photon fluxes because of the pile up of the analog signal generated by the X-rays absorbed in a very short time in the same strip. This is not a problem for most experiments, however some synchrotron radiation applications (e.g. single crystal diffraction) still suffer from this limitation. Moreoever the single photon counting technique is completely incompatible with X-ray Free Electron Lasers: the signal produced by a single bunch would cross the comparator threshold only once in a very short time and therefore all channels would not be able to count more than one. For this reason charge integrating detector are also being developed.
Charge Integrating ReadOut
The charge integrating detectors developed by the SLS Detectors Group are GOTTHARD (1D), AGIPD, JUNGFRAU and MOENCH (2D).
Charge intergrating detectors continuously integrate the charge colelcted from the sensor. This means that there is no minimum detectable photon energy, provided that the photons are absorbed in the sensor and no single photon resolution is required. Still, normally the dynamic range of the frontend electronics, i.e. the ratio between the signal that produces saturation and the electronic noise, is limited. For this reason the detectors developed by the SLS Detectors group rely on the principle of automatic gain switching.
Automatic gain switching
The automatic gain switching is a method that allows to expand the dynamic range of a charge intergating detector while keeping the electronic noise always well below the Poisson limit given by the number of photons. This means that the detector can provide single photon resolution without incurring into saturation up to 10E4 photons per channel.
Before the measurement the amplifier is in reset and the gain is set to high. When the reset is released the charge starts to be integrated on the feedback capacitor. If the output of the amplifier reaches the threshold, a 2nd or 3rd capacitor is switched in, thus lowering the output. At the end of the measurement the analog and digital (gain) information are readout. With the help of a channel-by-channel calibration curve the number of photons absorbed in the channel is reconstructed. The switching works for continuous (e.g. Syncrotron radiation) as well as instantaneous charge releases (e.g. XFEL pulses).
A gain switching detector with infinite frame rate would be the ideal detector since it could provide single photon resolution, large dynamic range and even energy information (at low rates). Still, the amount of data produced is extremely large and some analysis and compression would be required before storing them.