(due to) A. van Donkelaar, CSIRO Health Sciences

SYNOPSIS: A non-optical method for centralising the oscillation axis on the beam axis for microbeam crystallography

A simple and reliable method of centring the oscillation axis on the X-ray beam axis involves using a short piece of 100-micron-thick platinum wire mounted on an conventional cryoloop base (available from AXCO) together with the Rigaku/MSC pin-diode X-ray flux meter. The wire, which should have a squarely-cut exposed tip, is placed on the goniometer head and the uppermost tip centred visually on the oscillation axis using the specimen telescope. After centring, lower the goniometer height until the wire tip is well clear of the beam axis.

With the X-ray shutter closed, swing the beamstop out of the beampath and position the pin-diode on the outgoing beam axis away from the coldstream. Open the X-ray shutter manually from the generator control panel and ascertain that the pin-diode is registering the expected beam intensity, then raise the goniometer base height until the wire tip is just above the anticipated beam axis height.

Adjust the oscillation base lateral micrometer translator while observing the pin-diode meter flux reading, looking for a minimum value coinciding with the wire obscuring the beam. If none is found for over the lateral translation range, the wire tip is below the beam axis. Once the wire is located in the beam, rotate the oscillation axis while observing the pin-diode meter. If the meter reading is seen to vary with rotation angle, lower the goniometer head height slightly and re-centre the wire in the beam using the lateral translation adjustment. Repeat this process until a steady minimum value is observed on the pin-diode meter irrespective of oscillation base rotations. Note that lowering the goniometer head too far will result in the minimum reading on the meter increasing as the wire tip leaves the beam.

When the wire tip is successfully centred, lower the goniometer head height until the pin-diode meter reads half the value observed with the wire fully removed. Again, rotate the oscillation axis to check that the wire tip is well centred on the oscillation axis (the meter reading should be constant). The oscillation axis is now centred on the beam axis and the top-edge of the wire tip coincides with the centre of the beam. The specimen telescope may now be centred on the position of the centre of the top edge of the wire. Alternatively, paper copies of a grid representing the specimen telescope graticule can be kept on which the observed beam centre position may be recorded and displayed near the instrument. This is important as some users prefer to locate the beam axis in the telescope (e.g.) one graticule division away from the crosshair centre in each axis, to avoid obscuring the view of small crystals by the crosshairs.

Note: the choice of platinum wire minimises the X-ray scatter to which the operator is exposed. The use of other types of wire is not recommended!