Report on future detector requirements at ESRF

Dynamic range:
It is necessary to record the signal both before and after the absorption edge with a signal to
noise ratio better than 10 3 , in order to detect correctly both pre-edge features and EXAFS
oscillations (with an amplitude down to 1% of the edge). After the edge, where the signal is
the lowest, 10 6 photons/pixel are necessary to reach the required SNR of 10 3 .
The intensity ratio Ibefore edge/Iafter edge can reach 1000 in the most absorbing samples (absorption
edge of 3 in the classical logarithmic representation). Thus, the required dynamic range does
not exceed 12 bits. Nevertheless, the sensitivity range of the detector must extend from
10 6 photons/pixel (the lowest signals after edge, strong absorption in sample and in sample
environment) up to more than 10 10 photons/pixel (for I0 measurement in the same acquisition
conditions than the absorption spectrum).
Linearity:
For EXAFS, linearity is an important parameter, in particular for dichroism experiments, and
the detector should be linear in the whole dynamic range.
Flux on detector:
The flux on detector is expected to be of the same order of magnitude as it is now on ID24 or
at maximum multiplied by 3. That is 10 8 to 10 13 photons/s on the whole detector, depending
on the sample and sample environment absorption. Due to the energy profile given by the
undulator, the intensity is not distributed uniformly on the detector: central pixels receive
around 50 times more photons than border pixels.
It is worth notice that good quality EXAFS spectra can be obtained with 50 integrations of
100 ps each in the 4-bunch mode and that equivalent data are expected with integration times
of few µs in the uniform mode.
Particular operating conditions:
Direct detection would improve the overall efficiency of the system. In that case, particular
care should be taken concerning the radiation damage of the detector.
Required detector
The needed detector is a 1D device with short integration time and fast readout. The use of a
1D streak camera is envisaged in order to observe phenomena occurring in the duration of a
photon bunch in stroboscopic mode.
Existing detectors
ID24 is currently using a setup composed of a scintillator, a diaphragm and a FReLoN CCD
camera, reading only 32, 64 or 128 of the 2048 available lines.
Short term possibilities
A collaboration is envisaged with STFC in order to customise and use a fast Ge microstrip
detector. In that case, the spatial resolution of the detector has to be increased for the highest
part of the energy range: charge sharing seems to deteriorate the resulting energy resolution of
the spectra.
Main required improvements
The main expected improvement concerns the detection speed (integration and readout times).
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