Q correction

What is the purpose of q correction?

The q correction is to correct the sample misalighnment as well as the diffractometer misalighnment itself. I quote below the email reply from Sang Soo Lee (ANL), who kindly answered my questions about this topic.

" The correction was developed to adjust misalignments in 2-theta angle when the X-ray beam is not exactly passing through the center of rotation. When you think of this height difference, as expressed as c_off, as shown in the top line of the screenshot you sent me (which Paul derived), you will find that the actual two-theta angle is not exactly the same as the angle recorded in Spec. As you may expect, the correction is more important for data measured at lower angles and its effect gets smaller as the angles get larger. Later, (I think it is true) I realize that such an "arm-zero" correction is needed also for the correction of Qc (because Q = 0 at the critical angle), as actual Q is expressed something like ~= sqrt(Q_meas^2-Qc^2). This correction is not the same as what is shown in the first line but how it influences q is approximately the same as what c_off does. If this Qc correction is more substantial than the height offset, the derived c_off value may not correspond to the actual physical height difference.

In summary, the arm-zero correction is needed but the equation shown in the first line of the script is not necessarily the only solution for the correction. For example, I find a simple expression, Q = scale*(Q_meas-Q_offset) works as well as the one in the first line. Q_offset is typically in the level of Qc. This expression fails when the Q_meas is as small as Q_offset where you need a rigorous calculation. This almost never happens for CTR data. For example, the Qc for silica is 0.03 A^-1 and I assume that our typical CTR starts at Q = ~0.2 A^-1.

It becomes a rather long answer but I hope it helps. A bottom line is that one needs two parameters, scale and zero-offset, to align all Bragg peaks correctly for CTR analysis." Sang Soo Lee

Q correction procedure

The q correction algorithm is based on Sang Soo's reply. Q correction should be done before you start fitting the model.The procedure to do q correction in SuPerRod is as follows

1. expand qcorrection panel --> click the QCorrection button (on the top right of main GUI)

2. select the rod for which you want to do qcorrection from the 'Data set' comboBox. Fill the R_tth(mm), which is the distance in mm from the rotation center (also called pivot point) of diffractometer to the primary beam position on detector when all motor angels are zero.

3. Provide the current L for one of the Bragg peaks on this rod (you need to do this sequence for all Bragg L's from left to right)

4. Provide the value of current scale: should be a value very close to 1 (eg. 0.9999, 1.0001)

5. press Show button

6. Watch the data points close to Bragg L that you have specified before. A good q correction should feature a smooth transition from left side to right side.

   If this is not the case, change the scale value (press show after change) accordingly until this condition is fullfilled.

7. Once you are happy with the q correction for the specified Bragg L, hit Append and move on.

8. Now you should repeat the previous steps (2--> 7) for a different Bragg L.

9. Once you finish q corrections for all Bragg L's on the selected rod, hit Fit button. You should see the whole rod, which has been corrected using fitted q correction parameters. Smooth transition should be seen for all Bragg L's. If this is the case, you can do the final step (hit Apply Q Correcton button).

Do the same steps for other rods. But don't forget to hit Reset button before you start a new rod.