Hi everyone,
To perform my analyses, I use an AIRTIGHT SPECIMEN HOLDER made of PMMA. This dome generates a low-angle peak (likely partially due to scattering) in my diffraction pattern and background noise that is difficult to correct, even by removing the main part of the peak. Therefore, I am looking to model this phase to improve the modeling of the analyzable phases.
There doesn't seem to be a suitable .CIF file available in COD.
What is the procedure for conducting refinement in these conditions? Should I treat these peaks as 'true diffraction peaks' or as scattered signals?
Dear jean marie Le Roux ,
I do not think, that your background pattern is due to the 'dome'.
Your patter looks like that of PMMA, but significantly shifted to lower 2theta.
Any scatter signal (XRD) from far above the nominal sample surface level, as for example from the 'dome', will have XRD pattern significantly shifted to higher 2theta (you may draw a simple sketch to confirm the direction of that shift by yourself...).
Any scatter( XRD) from below the nomial surface level of your sample will show up being shifted to lower 2theta. So your background signal will arise from the support (PMMA assumed) of your sample.
You may confirm this conclusion by scanning the sample holder (without sample) and without the 'dome' on top and compare with your pattern presented above.
After that we may discuss how to estimate/model your background by modelling the transmission of your sample by taking the linear x-ray attenuation coeffcient 'µ' of your sample, its thickness 'd' and the diffraction angle 'theta' into account. But do not worry, because µ and d are quite unknown yet. Just try to set up an equation for the x-ray transmission*) of your sample with respect to your geometry. We will later see, how to proceed on...
*) the sample support is below your net sample;
a part of
Presentation Penetration Depth For General X-Ray Set-Up
may help you...
Best regards
G.M.
Dear Gerhard martens,
Thank you for your response,
The analyses were conducted at a synchrotron facility with a wavelength of 0.729 angstroms. The diffraction pattern corresponds to an analysis zone where there are no samples present. Therefore, we can estimate that the only contribution obtained comes from the dome and the sample holder.
The attached diffraction patterns demonstrate the evolution of the diffraction patterns during the filiation process. The measurements were performed using an incident beam with a very small dimension of approximately 75µm x 125µm and an incidence angle of 7°.
I have changed the x-axis unit to make it independent of the wavelength.
I hope those clarifications make my problematics clearer. Thanks for your help.
Best regards,
JM.
Dear jean marie Le Roux ,
thanks for feeding back.
I see now, that the 0,7.. A wavelength is the reason for the significant decrease of the 2theta peak angle(s) of PMMA compared to Cu K-alpha XRD pattern...
For modelling your background, you may take your measured background (taken at a 'point' having no net sample material present) and multiply that background by your estimated sample transmission. You may use µ and d of the sample as fitting parameters. The explicite formula depends on your actual geometry; i.e. '2theta' being parallel or normal to the incident 7° of the primary beam.
Because of the fact, that you have only quite tiny peaks*) in addition to the background, I assume that you gave got only a tiny amount of material; so its x-ray attenuation is quite small and the transmission may be approximated by a constant factor ( i.e. being independent on the scatter angle).
So a very simple approach is to use a scaling factor f (f < 1) for estimating the background in the sample pattern. Just try...
But for the 'full matrix spot' pattern, I see, that the constant factor approach will fail...
*) at least for the 'First matrix peaks'
Best regards
G.M.
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