Construction of the calibrator.



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Construction of the calibrator.

The apparatus is made up of a detector part and a goniometer set-up which holds the silicon calibration crystal. The detector consists of two optically isolated slabs of plastic scintillator (NE technology Pilot U) mounted one above the other in an aluminium housing with a window facing the calibrator crystal to accept backwardly diffracted X-rays. Each bar of scintillator is coupled, using optical grease, to the front window of a photomultiplier tube (Thorn EMI 9814B). This arrangement is shown schematically in Fig. gif).

  
Figure: Schematic diagram showing the calibrator crystal and detector. (The diagram was partially reproduced from [93].)

  
Figure: Photograph of calibrator mounted on the front of the optical bench. X-rays travel into the hutch through the beryllium window at the right of the picture and directly into the entrance window in the calibrator detector housing.

The detector and the goniometer are both mounted on a base which is fixed to the front of the optical bench before the beam collimator and crystal diffraction apparatus. When used for EXAFS work the calibrator is normally mounted behind the experiment using the residual beam which is left after passing through the last ionisation chamber. This set up is however inconvenient for diffraction work. Therefore the calibration crystal in this apparatus has a hole drilled through it to allow a 'rod' of X-rays to travel onwards into the collimator and onto the sample. The outer regions of the beam are then used for calibration purposes. Fig. gif shows the diffraction equipment and the calibrator mounted on the optical bench.

Figure gif is a close up photograph of the rear of the goniometer. The construction allows the crystal to be rotated around the vertical axis ( adjustment) and horizontal axis ( adjustment) with a step increment of . This is in fact a finer increment than is actually required, probably by a factor of ten. This overkill results in increased backlash of the gearing and a longer driving time, neither of which have been an inconvenience so far during operation of this equipment.

  
Figure: Close up view of the calibration crystal goniometer from behind. The crystal, which is cut with a strain relief groove, is fixed at one corner with two-component glue to an aluminium plate which is in turn mounted to the front end of the inner of the two concentric aluminium cylinders visible in the centre of the photograph. In the initial stages of setting up the inner most cylinder could be adjusted laterally with respect to the outer cylinder to allow for centering of the crystal's hole in the rotation axes of the goniometer. The outer cylinder was then rotated to the correct position so that the reciprocal lattice vector of the crystal pointed vertically upwards. The two orthogonal micrometers make contact with the ends of lever arms and are driven by stepper motor/gear/clutch assemblies. This combination gives an angular precision of .

Since the apparatus is mounted in front of the focus there is a small possibility that the crystal intercepts only those X-rays which are contained in the outer edges of the horizontal fan of radiation and which are reflected off the monochromator crystal at slightly different angles to X-rays in the centre of the horizontal fan. Fig. gif shows the geometry of two such beams. The angle of incidence of the beam striking the edge of the crystal is given by

 

where is the angle of incidence of a beam striking the centre of the crystal and is the horizontal angle between the two beams A and B subtended at the source. Using Bragg's law along with equation gif it is possible to obtain an expression for the fractional energy change in the reflected beam due to its not striking the centreline of the monochromator crystal,

The source to monochromator distance on X31 is and the monochromator crystal half width is . The maximum value of due to the effect of horizontal divergence only is and we can therefore assume that the presence of a hole in the calibration crystal has little or no effect on the accuracy of the calibration procedure.

  
Figure: Diagram showing the geometry of two X-ray beams A and B in different regions of the horizontal fan. The two beams strike the monochromator crystal at slightly different angle implying that the reflected beams have differing energies.



next up previous contents index
Next: Operation of the Up: Absolute energy calibration Previous: Theory of the



Gwyndaf Evans
Fri Oct 7 15:42:16 MET 1994