Report on future detector requirements at ESRF

Spatial resolution:
Point detectors will continue to be used but 2D detectors can be useful for small angle
scattering and for the study of dynamic structures (q dependence of the diffraction pattern). In
most cases, a spatial resolution of 100 µm would be convenient.
Efficiency:
To reach the desired time resolution, APDs are used. In order to improve the detection
efficiency at high energies, the APD can be chosen thicker and/or tilted to increase the
effective thickness seen by the photons. Nevertheless, this has two drawbacks. First, if the
detector chip collects more photons, it can saturate the electronics. Second, in thick diodes,
the time of propagation of the electrons is not negligible compared to the studied time
structures. Thus, one loses time resolution, which is even worse at high energies than at lower
ones because the lifetime is shorter.
In order to avoid those drawbacks, the efficiency is increased by stacking several diodes one
behind the other and by synchronizing the corresponding electronics.
Dynamic range and linearity:
The detector must be able to count single photons as well as support the high intensity of the
prompt pulse (10 10 to 10 11 photons/s). It can be noticed that measuring the intensity of the
prompt pulse is not needed and that the signal can be gated so that it does not reach the
counter.
Flux on detector:
The expected flux on the detector can reach 10 11 photons/s in the prompt pulse (that is
70 000 photons in the pulse) and ranges from 0.01 photon/s to 10 5 photons/s in the delayed
signal.
Particular operating conditions:
The detector is operating near magnetic fields. Intents have been made to put it inside the
cryostat in order to increase the detection solid angle. Even if up to now the low temperatures
have disturbed the avalanche process in the diodes, the eventuality can be kept in mind.
The detector is mounted on a diffractometre arm.
Required detector
The required detector must be fast, with high count rate, good time resolution and be capable
of making a very fast recovery after high flux exposure.
Existing detectors
The current detectors are APDs connected to fast electronics: (amplifier, constant fraction
discriminator, time to amplitude convertor and then ADC and MCA to record the signal).
For the highest energies, up to 24 APDs are staked in order to improve the efficiency.
Short term possibilities
The chosen technology is likely to remain APD, with improved electronics. Projects for
developing 1D and 2D APD arrays are starting (XNAP project).
Main required improvements
The main expected improvement concerns first the ability of the electronics to resolve high
count rates very rapidly. Then, the aim is to have 2D detectors using the same technology.
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