Diffractometers

The apparatus used to measure diffraction data goes by various names; here we call it a diffractometer. It typically comprises some sort of "collimation" system consisting of an aperture or slit pair to give a final limit to the size of the X-ray beam, a mechanism to hold the specimen crystal precisely in the beam and to move it in some controlled way to sample diffraction space (crystal orienter), and a detector to record the diffracted X-rays. The sort of diffractometer used with conventional X-ray sources is typically a commercial self-contained unit. Those used at synchrotron sources usually contain commercial off-the-

shelf components, but often the assembly somehow manifests the personality of the builder. The collimation system must be chosen carefully, and we will discuss this further in Subheading 4. The crystal orienter is often a single axis, set to rotate perpendicular to the X-ray beam, to enable the crystal to be oscillated through a small angle in the beam during data collection (1). Occasionally, especially at a SR source, one might find a three-axis system wherein two oblique (or orthogonal) axes are used to put the crystal in some particular orientation before having the whole assembly rotated around the third axis.

The most important component of the diffractometer is the X-ray detector. The original X-ray detector for crystallography was X-ray film (a two-dimensional [2D] detector). For some time in the mid-1960s through the 1980s a movable X-ray counter rather like a Geiger counter (usually crystal scintillation or gas-filled proportional counter) was placed behind a small aperture (a zero-dimensional detector). The crystal and detector were moved systematically to bring individual reflections into a diffracting position one at a time so one could measure the diffraction intensity. Finally in the 1980s electronic 2D detectors of various sorts were devised, and these are what we mostly see today. In the mid-1980s a commercial firm produced a fairly popular video-based detector (the "FAST"), which depended on phosphors turning X-rays to visible light for the video system to detect (6,7). About the same time two other firms popularized xenon-filled multiwire proportional counters ([8-10]; reviewed in ref. 11). The video detector was used successfully at both synchrotron sources and on conventional X-ray generators. The multiwire detectors were used on conventional sources only. Both of these early electronic 2D detectors were responsible for many important structures. Soon after, a system based on "storage phosphors" was developed ([12]; reviewed in ref. 13). These phosphors (often europium-doped barium halide) would absorb an X-ray photon and then would go into a meta-stable excited state that would be stable for a long time. The stored energy was released with the scanning of the phosphor plate with a red-light laser, and the blue light that was emitted could be measured. These devices were hugely successful because they were much more sensitive than X-ray film, the sensitive surface and dynamic range were bigger than that of the video system, and they had finer resolution elements (pixels) than the multiwire systems. They could be automated to work reliably for years and had a high dynamic range. Essentially every SR source used these for a time, and they still can be found in both conventional and SR laboratories.

The most abundant 2D electronic detector now seen at SR sources is based on a solid-state electronic imager known as a charge-coupled device or CCD (reviewed in ref. 14). This is closely akin to the imagers used in modern digital cameras. The most common application of these is shown schematically in Fig. 11. A light-emitting phosphor is bonded to the large end of a tapered fiber optic device, and a CCD chip is bonded to the small end. The large (several tens to a

Fig. 12. The Brandeis B4 four-module CCD-based detector. The front-left surface would be covered with a phosphor. One can see the originally cylindrical surfaces of the tapered fiber optic before it was cut to a square. Readout electronics are in the metal cylinders. (Reproduced from ref. 15. with permission from IUCr.)

Fig. 12. The Brandeis B4 four-module CCD-based detector. The front-left surface would be covered with a phosphor. One can see the originally cylindrical surfaces of the tapered fiber optic before it was cut to a square. Readout electronics are in the metal cylinders. (Reproduced from ref. 15. with permission from IUCr.)

few hundred millimeters) diffraction pattern is mapped with adequate efficiency onto the small (a few tens of millimeters) semiconductor chip. Great effort has been made to make these chips nearly perfect in their imaging (no dead pixels), to keep the read-out noise low, and to provide rapid readout. In order to make large-surface area detectors, several of these modules are connected together to act as a single, large detector. An example of the insides of such a detector is shown in Fig. 12 (15). The front surface of this detector is 20 x 20 cm.

Was this article helpful?

0 0

Post a comment