The diffractometer (Stoe StadiVari) has four angular degrees of freedom, three sample-orienting (Chi, Omega and Phi) and one detector-orienting (2-theta), and is situated on a set of Aerotech heavy duty high-precision stages to align the center of rotation of the goniometer onto the X-ray beam. It is also equipped with a motorized XYZ stage (named as STOE XYZ) that gives us extra three angular freedom for sample alignment.
Here we briefly describe the typical sample alignment process:
Rotate the OMEGA to 40° so that the sample chamber is perpendicular to the viewing zoom camera. Find the sample chamber with the viewing camera. Center the sample's image by changing the STOE X, Y and Z motor positions. Focus the image of the sample by changing the STOE X. Align the sample chamber's image to the center of the viewing camera by changing the STOE X, Y and Z.
Then rotate the OMEGA angle to 0° so that the sample chamber is perpendicular to the incident X-ray beam. Scan the DAC position in both AEROTECH horizontal and vertical directions perpendicular to the incident X-ray to find the center of sample chamber, while collecting transmitted beam intensity data with a photodiode detector placed behind the sample.
Rotate the sample using the goniometer PHI axis by a few degrees, and repeat the AEROTECH Y transmission scan. Repeat the scan twice, at both positive and negative PHI offsets (like 2-5 degrees). Calculate the sample position along the incident X-ray's direction with the following equation:
Where STOE ΔX is the distance along the incident X-ray's direction that the sample is displaced from the center of the instrument, ΔPhi is the change in the PHI angle between scans, ΔS+ and ΔS- are the positional offsets of X-ray's transmission profile when the PHI angle is tilted by +ΔPHI and -ΔPHI. NOTE: Several iterations of scans can be made to improve the accuracy of sample positioning. Transmission scans are on AEROTECH Y but STOE X needs to be adjusted. So we’re talking about different motors with different mechanical properties. That’s why we have SF (Scale Factor) in the equation which is optimized as 19 through our commissioning experiments.
