srraju
Hi All,
I am using Caglioti equation to model the LaB6 peaks from x-ray lens. In order to avoid negative values for FWHM, V should be always negative. However due to the strong correlation between U, V and W in the Caglioti equation they are refining to different values for different refinements. I am performing a Pawley refinement by fixing lattice parameter for LaB6.
Can I set V = 0 and can consider only U and W terms? Please advise me if this is valid.
Regards
Raju
srraju
macro UVW(u, uv, v, vv, w, wv)
{
#m_argu u
#m_argu v
#m_argu w
If_Prm_Eqn_Rpt(u, uv, min -1 max 2)
If_Prm_Eqn_Rpt(v, vv, min -1 max 1)
If_Prm_Eqn_Rpt(w, wv, min -1 max 2)
gauss_fwhm = CeV(u, uv) Tan(Th)^2 + CeV(v, vv) Tan(Th) + CeV(w, wv);
}
The equation used in TOPAS standard macros file is different to the cagliotti eqn (No Sqrt in eqn). Also the limits for V should be max 0.
Please advise if the eqn in macro is meant for any special case or can be used for any parallel beam geometry.
any suggestions are gratefully appreciated. thank you.
Raju
johnsoevans
Raju,
You can modify this macro to match the traditional Cagliotti equation or code this directly into your input file. If you're just treating this as an empirical description of peak shape you can try it for any geometry. The important test is whether it fits your data appropriately. If you need a more complex peak shape then look at the various options in the technical reference.
John
srraju
Thanks John,
We have no macros available in TOPAS to simulate the diffraction patterns obtained from polycapillary x-ray lens with analyser foils on diffraction side. So, we thought of using Cagliotti equation. Please advise if there are any updated macros available using FP approach that is missing from the TOPAS v4.1.
I have read the Cheary et al. 2004 NIST paper on parallel beam laboratory diffractometers. The paper discussed the mirror on the incident side but not for the X-ray lens to directly use the equations to write a macro.
Regards
Raju