Mode (4): Use of Line/Area Detectors

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Mode (4): Use of Line/Area Detectors

The powder diffraction modes discussed so far all rely on using an electronic X-ray detector which scans around the sample, the 2θ angle typically covering say 10 to 90 degrees. While the detector is positioned at any given 2θ angle within this range, the X-rays being diffracted at other angles are not being detected. This represents huge wastage and inefficiency since at any one instant most of the diffraction information is being "thrown away" as it were. For this reason a lot of effort has been directed into building position sensitive detectors. These can detect diffracted X-ray photons over a range of 2θ angles simultaneously, the 2θ angle of each detected photon being determined by its arrival position on the detector. Such position sensitive detectors are known as line or area detectors depending on whether they collect in one or two dimensions respectively (see below).

Current position sensitive detectors are generally of two types: an electronic form or an image plate. The electronic form is ultimately the more convenient since information across the whole diffraction pattern is fed directly into the computer. Examples of this technology will be discussed in more detail later in the course.

During the 1990's we saw something of a throwback to the old methods of photographic film, except the device was called an image plate and does not require photographic development in the conventional sense. Image plates are essentially a Japanese invention consisting of a Eu-doped phosphor powder on a plastic sheet, the Eu²⁺ becoming excited to Eu³⁺ by an incoming X-ray photon and remaining so for some days until read out by a laser. The Eu³⁺ loses an electron by a two-stage process, one of which emits detectable light. Therefore the measured light intensity is proportional to the incident X-ray flux. The slowest part of the process is the "reading" of the image which is done typically in 1-10 minutes. Although still not intrinsically as convenient as a completely electronic system, the component steps of image plate processing are now sufficiently fast and automatic to make the whole process a viable alternative. The image plate can be one-dimensional, as a thin strip, though it is usually much more useful as a two-dimensional "flat" plate (thus viewing the whole of the Debye-Scherrer rings, as they are known) or as a two-dimensional "curved" plate which can be moved so as to continuously collect different diffraction patterns with time. These concepts will become clearer later during the course but in the meantime one can gain some idea of the function of image plates from the illustrations below, which show two extreme examples: the first pattern is of rather poor quality, but for good reason in that it has been obtained from a minute amount of material being compressed between diamond anvils, and hence shows texture effects, i.e. a poor signal to noise ratio and somewhat "spotty" rings. However, good quality information can still be obtained from such two-dimensional patterns by going around the Debye-Scherrer rings and adding up all the contributions to improve the overall counting statistics - this is often referred to as "integrating around the rings" and will be re-visited in a later section on high pressure powder diffraction. By contrast the second pattern (not to the same scale) has been obtained under ideal conditions and displays sharp continuous rings.

Courtesy of Dr. Malcolm McMahon, Edinburgh University and Dr. Mark Roberts, Daresbury Laboratory.

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