Recently, I did some refinements with GSAS program to get lattice parameters for some nanomaterials. The data were collected at X17B1 of NSLS with two different distances between image detector and sample. I noticed that the lattice parameters were different with different distances for the same sample. Also, I tried to refine the data with and without asymmetry correction; this also resulted in the different lattice parameters. (I used the same instrumental parameter file for two different distances). I also tried to refine the data collected at other beamlines, the cell parameter was also different. I am a bit confused with all these changes. Any comments from you all will be highly appreciated! Thanks,
If you used the Finger, Cox & Jephcoat (FCJ) asymmetry, it should not work for any 2D detector. The physics of the method starts with a finite-sized detector opening perpendicular to the plane defined by the incident and diffracted beam. If your algorithm for integrating around the diffraction ring on the 2D detector is really lame, you might get some asymmetry - think of a line segment tangent to the ring and plotting the total intensity intercepted by the line as the distance from the center is changed. That is very nearly the FCJ effect, except that FCJ moves the line segment on an arc of constant radius, which is the case for a point detector.
Larry
