Bragg counter

Figure 18.3 Schematic diagram of tin ion chamber that drifts the ionization perpendicular to the particle s path is shown. In this case the anode is segmented and the relative rate of ionization along the path can be determined. The device also contains a Frisch grid between the anode and chamber to improve the pulse-shape response of the device, (c) The schematic version of a detector that drifts the ionization along the particle s path, called a Bragg counter, is shown. The time distribution of the output signal will contain information on the relative rate of ionization all along the particle s path.

Fig. 3.10. Schematic view of Bragg counter spectrometer (BCS) with grided gaseous ionization chamber as AE detector with semiconductor detector as F-detector forming AE — E teiescope...

The spectrometer is set to the appropriate Bragg angle 0 of the requisite characteristic wavelength, and only these X-rays will reach the detector and be counted. The detector employed is the gas proportional counter, which can operate at much faster count rates than the EDS crystal detector. 

So far, we have seen that if we measure the Bragg angle of the reflections and successfully index them, then we get information on the size of the unit cell and, if it possesses any translational symmetry elements, also on the symmetry. In addition, we have seen that the intensity of each reflection is different and this too can be measured. In early photographic work, the relative intensities of the spots on the film were assessed by eye with reference to a standard, and later a scanning microdensitometer was used. In modern diffractometers, the beam is intercepted by a detector, either a charge coupled device (CCD) plate or a scintillation counter, and the intensity of each reflection is recorded electronically. 

Still another method of determining crystal orientation involves the use of the diffractometer and a procedure radically different from that of either Laue method. With the essentially monochromatic radiation used in the diffractometer, a single crystal will produce a reflection only when its orientation is such that a certain set of reflecting planes is inclined to the incident beam at an angle 0 which satisfies the Bragg law for that set of planes and the characteristic radiation employed. But when the counter, fixed in position at the corresponding angle 20, discloses that a... 

In Fig. 8-38(a), fixed crystal M, which should be as nearly perfect as possible, is set to reflect Ktx radiation in the direction of crystal C, which is to be examined. Crystal C is then rotated ( rocked ) through the Bragg angle 6g, while the beam reflected by it is measured in a fixed counter with a wide slit. The resulting curve of intensity vs. 0 is called a rocking curve and the instrument itself a double-crystal diffractometer. 

In one type of spectrometer, called single-channel, the analyzing crystal and counter are mechanically coupled, as in a diffractometer. Thus, when the crystal is set at a particular Bragg angle 0, the counter is automatically set at the corresponding angle 26. With the counter connected to a ratemeter and recorder, the whole spectrum can be continuously scanned and recorded. 

Fleischmann and co-workers have carried out a number of in-situ x-ray diffraction studies at roughened electrodes.Their experiments are different from those previously discussed in that instead of a synchrotron source they employ a conventional x-ray tube ( 1.5 kW) with a position-sensitive proportional counter for the measurement of the diffracted intensity. They have carried out experiments in both transmission (Laue) and reflection (Bragg) modes. 

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