Hello there!
I have a question regarding performing structure refinement using TOPAS with synchrotron powder diffraction data. Explained below is my situation and I am hoping to receive a quick response from one of the experts here. Please help!
Situation: I have collected powder diffraction data using a flat image plate detector in a synchrotron beamline. The incident X-rays are 100 um in diameter, gets diffracted from the sample and then the diffracted data is collected by the image plate detector placed behind the sample. I have data for both the sample and a calibrant (a NIST standard for which I have information regarding the lattice parameter and crystallite size). From the collected data, I have generated the "I vs. 2-th" plots to be input into the TOPAS (commercial version) program. Regarding the incident beam/ instrument parameters, I know the following:
Beam size: 100 um in diameter at the source. I also have an estimate of the beam divergence along equatorial and axial direction at the point the beam hits the detector ( 2-th =0). Beam size is approx. 600 um in equatorial and 300 um in axial direction at the point it hits the detector. Also, I know the distance (D) between the sample and the detector (at the point where 2-th=0).
In order to perform Rietveld refinement in TOPAS, I need to setup the correct "Instrument parameters" for synchrotron data but there is not much information available in TOPAS user manual. My idea is to obtain the instrument parameters (especially the contribution from beam divergence in both axial and equatorial directions) by using the calibrant as a standard and then keep these fixed while analyzing my sample. Which instrument convolution function I may use and how to account for the incident beam divergence in TOPAS? Should I use Finger et al. model or any other defined model in TOPAS or should I go for the “additional convolution” feature?
My question is similar to the one in this thread: http://topas.dur.ac.uk/unb/forum.php?req=thread&postid=66.
But I do not understand the need to use “simple axial model” and why

lam ymin_on_ymax 0.0001 la 1.0 lo 0.354134 lh 0.1

is chosen by the user.
My aim is to obtain the emission profile and the instrument broadening by using the NIST standard. Then, I want to keep these FIXED for my sample(s). The above thread also talks about using the additional convolution feature by using:

lor_fwhm = pr1 Tan(Th) + pr2/Cos(Th) ;
 gauss_fwhm = pr3 Tan(Th) + pr4/Cos(Th) ;

But then the users agreed on instrument broadening being negligible. But, I want to device a way in TOPAS to separate out the instrument broadening, if possible -- in addition to obtaining the correct emission profile, if possible. I am almost convinced about using the emission profile as "lam ymin_on_ymax 0.0001 la 1.0 lo 0.354134 lh 0.1" (as given in the above thread), but the instrument broadening part is a mystery to me! Could any of you please help me in this regard? Thanks in advance.
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AB