[GSAS-II] Anomalous SAXS in GSAS2?

[Avakyan L.A.]

Dear GSAS II developers ans users!

I'm using GSAS II for XRD analysis, thanks to Vadim Volochaev's warm 
advice. I found that it is indeed incredibly powerful and very flexible 
software. Thanks for the developing with open source!

Now I'm interested in the fitting of Anomalous Small-Angle X-ray 
Scattering (ASAXS) data of core-shell bimetallic particles, somewhat 
repeating the paper Haug2009 (doi:10.1088/0957-4484/20/50/505705).
How do you think, is it possible to perform this task in GSAS with minor 
changes in codebase?

The general small-angle study is already implemented and we have 
integrated several 2D images in SASD curves of the same sample for 
different photon energies. In general: can I just use sequential fit 
over these patterns?
In more detail some questions appeared:
1) Does (anomalous) atomic factors are calculated independently for each 
dataset? I have a doubts, since "Anomalous density" shown in SASD > 
Substances window seems not to depend on the wavelength (energy).
2) Building of core-shell (spherical) model. As I understand, this 
requires two fit models: spheres of first substance (core) and spherical 
shells of the second substance.
The addition of form-factors and volumes calculation for spherical shell 
seems to be pretty straightforward and I have put them in GSAS code by 
adding functions with simple subtraction of spherical FF/vol functions 
with different radii.
3) Finally, the model should be the same for all SASD curves, with 
changes in data described only by f' and f'' energy dependence. As I can 
see, it is impossible to add constraints of equivalence between SASD 
fitting parameters like it can be done in XRD analysis, is it? It is 
hard for me to estimate the amount of work to implement the SASD 
constraints, is it planned or too complex for current codebase state?

Thank you in advance,
Best wishes,
Leon

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Dr. Leon Avakyan
Ass.Prof. Physics Faculty,
Southern Federal University,
Russia

-- 
Leon Avakyan