MYTHEN detector correction for transmission geometry instruments

djurdjijadzodan

Dear experts,

I need your help in understanding how to implement all the necessary corrections regarding the Mythen detector for XRDs collected at instruments of transmission geometry [1].

I have been given XRDs of LaB6 and other powders collected (not by me) at the STOE transmission diffractometer system (STADI P). Provided that I know the exact specification of the detector [2,3], I am wondering about the following from the publication [1]:

Question 1. The main macro for the capillary displacement has been written at the end of [1]. How do we determine the initial dl and dv values to be used in the macro? Are those values initially being "set" (or read on the computer connected to the instrument) by the experimenter (and those values should then be fixed in the macro upon refinements), or should one use some approximate small dl and dv values in the macro and then let both values to be refined (at once or)?

Question 2. What do the 2theta zero offset values in the table 1 of [1] represent? Is it the "Zero_shift (@, value)" refined value retrieved upon refinements for the chosen dl and dv values? Or if we use the Mythen correction, we don't need to include also the zero shift correction?

Question 3. On the page 610 of [1], the authors started discussing about addressing the cylindrical wobbling of the capillary and provided the formulas. Is there any defined macro already implemented in the TOPAS regarding the effect, or is it to be manually added by the user? I haven't managed to find it myself, but maybe I haven't used the right keywords.

Question 4. In addition, in the [1] it has been emphasized that the "Finger_et_al(capillary hight, 8)" correction still also need to be used (the value 8 as "length of detector (receiving slit) in axial direction", I assume. I am wondering if we could also use the "Slit_width(0.05)" correction as well using the value 0.05 (if the detector strip size is 50 Mm) as "receiving/detector width" value? I have difficulty imagining all small detector parts despite the illustrations from the references below.

Question 5. I have seen at the https://topas.awh.durham.ac.uk/doku.php?id=emission_profile_modelling&s[]=mythen an example with some codes used on some Mythen data. However, my knowledge is still extremely limited and I couldn't have understood the codes. Do you have any specific additional literature advice regarding the specific example on the website?

I am sorry for super basic questions.

Thank you in advance.

Kind regards,
Djurdjija

[1] https://doi.org/10.1524/zkri.2010.1345
[2] https://doi.org/10.1016/S0168-9002(02)02045-4
[3] https://www.stoe.com/application-reports/dectris-mythen-1k-strip-detector-ultra-high-resolution-measurements/

rowlesmr

1:
Start from 0,0, refine it, and make sure the values make sense. Do you know how big your beam was vertically? The verticial displacement shouldn't be more than that number.

2:
The 2theta zero offset values represent a shift in the 2th = 0 position of the detector; ie does the channel which thinks it is at 0° 2th really at 0° 2 th? The mythen correction is a specimen displacement correction; the zero offset is a detector alignment correction.

Take the second row of table 1. The sample is deliberately placed -0.3 mm away from its ideal position laterally, and 0.0 mm vertically. The refined value of the specimen position is -0.27, -0.04 mm (which is pretty close to where it actually is), with a refined 2Th zero error of 0.003° 2Th (which is less than one pixel). If the refinement is done without the specimen displacement correction, the Rwp/GoF is 2.29/1.50, whereas with the correction, the same values are 1.54/1.03.

3:
I don't know of any wobble macros.

4:
Yes. the Slit_width macro is the "top-hat function describingthe pixel spread" mentioned in the first paragraph in the second column on p. 618.

5:
Not really, it's more an experience thing, I'm sorry to say. To build an emission profile, you need a specimen whose cell edge you know the position of. You can then refine the wavelength to match the peak position. To then build up the full emission profile, you need to properly account for all other peak-shape related factors (crystallite size and microstrain, axial divergence, slit_width, capillary size, specimen LAC, detector radius, and probably some others I've forgotten), and the remainder should be from the emission profile. Just as an anecdote, the PD beamline at the Australian Synchrotron always had a significant gaussian width component for all of my data.

Are you using Topas 7? You should also use the new capillary peak shape convolutions to model the effect of the capillary on the peakshape. There is a very good paper describing how it works: https://espace.curtin.edu.au/bitstream/handle/20.500.11937/58859/257287.pdf

HTH

Matthew