Most of the XRD community is presently using 0D point counter or 1D linear PSD (position sensitive detector) in powder diffraction as well as rocking curve analyses. Any mention of the extinct photographic film in such applications is virtually non-existent. In the old days at Rutgers (1979-1984), we used film to image the cross-section of the incident beam at various distances from the exit slit along the detector "track" on the diffractometer plane for such alignment.
1. What is the preferred procedure for aligning the diffractometer axis parallel to the incident beam direction exiting the beam conditioner slit currently?
2. How critical is this alignment for precision in analyses?
3. How would the conventional 0D RSM be affected by this alignment?
Looking for input from those in the XRD community using Bruker, Panalytical, Rigaku, Bede, Jordan Valley, Phillips, GE Diano (Dino:-) or other diffractometers.
Practical examples and data would be really appreciated.
Dear Ravi,
wea re using an Phillips/PanAlytical multi-purpose difftractometer with both a point detector and a 1D linear PSD (X'Celerator). For alignment, we usually scan the direct beam from the X-ray anode without any sample with a slit in front of the point detector. This way, we have access to the "0"-position for both the incident and exit beam. Then we mount the sample and position it in the centre of the beam, i.e. we move the sample upward with both X-ray source and detector at 0° until the direct intensity is only 50%. Then we "rock" the sample, for the Philipps diffractometer this implies that we have to move both sample and detector, because the angle of the sample is fixed for our diffractometer. If the shadowing is not symmetrical, (i.e. maximum intensity at 0°) then we have to correct the sample position. After correction of theta, then a repetition of the z-alignment and eventually a second rocking scan, until positions converge.
Hope this helps, best regards, Dirk
That procedure is what I've seen followed in most diffractometers. However, it doesn't confirm whether the beam direction is coplanar with the diffractometer plane (perpendicular to the diffractometer axis).
Most of the alignment procedures seem to assume the following:
Since the 0D detector acceptance window typically is relatively large compared with the beam size (for point source) it would be impossible to verify this condition, wouldn't it?
Even in the case of a line source this verification will be unattainable, wouldn't it?
With a 2D detector (or dental film) on the other hand, it would be possible to quantify this deviation, if present, by imaging the beam at various slit to detector distance, in my opinion. We found such an anomaly in one of the diffractometers while imaging the incident beam for quantification. In our case the line source actually "moved up" several pixels as the detector was moved closer to the source slit. Initial computation indicates a tilt of the incident beam, a line source about 2 mm X 22 mm, w.r.t. the equatorial plane (represented by the detector arm) of nearly 1o. This deviation can only be corrected by either tipping the diffractometer plane w.r.t. the incident beam appropriately or vice versa, correct?
I would think such a potential relative "tilt" may affect RSM significantly. What's your opinion?
Am I being a bit too stringent with this requirement? Does it even matter for routine linear diffractograms? I'd have to methodically test the effect by changing the SDD (slit/specimen to detector distance) and repeating a bunch of diffractograms and RSM's next opportunity I have. Meanwhile, any of you with unlimited access to a diffractometer and x-ray source can double check and post the results perhaps :-)
The image attached is the incident beam at 140mm and 0mm from the rocking axis. Detector plane at 90o to the diffractometer radius.
https://www.researchgate.net/post/OAT-Oblique_Angle_Topography_Is_this_the_first_in_the_world_Do_you_know_of_anyone_else_in_literature_that_has_used_this_method_for_magnification
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