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Detectors |
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Detectors |
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Detectors
Various detectors are considered elsewhere in this course, but as already stated we need to have a view of the whole diffraction system when devising and conducting in-situ non-ambient time-resolved experiments. For this reason a generic global look at detectors is in order at this point. Much of the information used here has been supplied by Dr.Rob Lewis, to whom we are grateful, formerly of the Instrumentation Dept. at the CLRC Daresbury Laboratory, UK. We will concentrate mainly on x-ray detection, as neutrons are covered elsewhere, but some mention where appropriate will be made to neutron detectors.
Detection Mechanisms
Obviously the aim of a detector is to capture diffracted photons in an experiment and to rapidly transmit that information as a suitable signal to the observer/computer with as little time/space distortion as possible. To do this detectors need to exploit some suitable effect produced by the act of capturing a photon:
| A large number of desirable | ( ) |
and undesirable | ( ) |
attributes of detectors could be discussed at length; |
| this is a summary: |
Energy Discrimination: This is the ability of a detector to "sense" the energy of an X-ray photon during detection. Energy discrimination is very useful in diffraction since diffracted X-rays will all have the same energy as the incident X-ray whereas absorbed/scattered X-rays in general will have lower energies. Thus energy discrimination can be used to remove background X-rays and thereby greatly improve the quality of the diffraction pattern.
Dynamic Range: This could be defined as the ratio of the strongest photon intensity that the detector can handle to the smallest intensity that can be perceived above the noise. It is an important item in diffraction since the intensity of different diffraction peaks can vary by orders of magnitude; but all intensities, from weak to strong, need to be recorded. A dynamic range of say 200 would be rather poor (i.e. one which would be able to measure an intensity of either 100 or 0.5 photons per second) whereas a value of >10000 would be rather good.
Maximum Counting Rate; Total Count: These are simply the maximum count rate and total (integrated) counts at which a given detector can operate. For time-resolved work, for example, it might be necessary for good data statistics for parts of the diffraction pattern to be collecting at rates of 1000 photons per second (referred to as cps - counts per second) or the total pattern to be increasing at 104− 106 cps. For the total count rate, one wants to be assured that the detector does not saturate before a sufficiently large number of photons (e.g. 107− 109 counts) have been accumulated.
Point counters and Position Sensitive Detectors: If a detector produces a single output then it is in effect a point counter even if the area/volume over which the photons are captured is large. However if there is some way in which the photons captured at different parts of the detector can be distinguished then one has in effect a position sensitive detector (psd). A psd is inherently more useful than a point counter for powder diffraction: this is because a point counter has to be scanned over the diffraction pattern (2θ angle scanning) and so most photons end up being wasted, with the majority of the diffraction pattern being "in waiting" for the point counter to arrive; by contrast psd's can in principle collect the whole pattern simultaneously without wasting diffracted photons.
Direct and Indirect Detectors: Direct detectors are those in which the X-ray photon is converted directly into a signal, such as with gas detectors or solid state detectors. Indirect detectors are those in which an intermediate step (or steps) has to be involved to convert the X-ray photon event into an observable signal - a classic example is X-ray film where photographic development has to be performed to make the exposed silver halide grains appear black.
Photon counting and Photon integrating modes: The Photon counting mode is one in which the detector, in principle, responds individually to every single photon received - the main features and examples are:
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detector noise usually negligible |
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huge dynamic range |
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simultaneous imaging and wavelength determination |
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no dead time between frames. Very fast framing |
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limited flux capability |
| Detector noise can dominate |
| Limited dynamic range |
| Dead time between frames |
| No wavelength determination |
| High flux capability |
Armed with these definitions and classifications we can now discuss the main features of detectors used for in-situ non-ambient time-resolved powder diffraction experiments.
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© Copyright 1997-2006.
Birkbeck College, University of London.
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Author(s):
Paul Barnes Martin Vickers Rob Lewis |