Defocus Beam Effects in X-ray Mapping

[Probeman]

You could say I'm somewhat surprised this isn't as large an effect as I thought it might be, but still it is an effect we should try to avoid. What am I talking about?

Well, some have argued that one should defocus the beam so that the beam diameter matches the pixel size, but it occurs to others that defocusing the beam will produce a greater probability of acquiring intensities from more than one phase.

So I ran a small (32 x 32 pixel) acquisition of a Cu-Al eutectic allow that consists of a Cu-Al alloy and a relatively pure Al phase, first with a fully focused beam and a second acquisition with an 8 um defocused beam where the acquisition size for both was 256 um x 256 microns (256um/32 = 8um).

The results are attached below, but it is clear that there are somewhat more "edge" pixels in the defocused beam acquisitions. Specifically, the 0 um beam size acquisition shows 44% "edge" pixels, while the 8 um defocused beam acquisition shows 51% "edge" pixels. I suspect the relatively small difference between the two acquisitions is because the beam scan is smoothly scanning across the sample surface so in the sense you are stuck with an acquisition area equal to the pixel size- at least in the scan direction!

Now you might say, ok, we're not surprised. But as I said in the beginning, some people have argued that we should be defocusing our beam for x-ray mapping and I think the data shows we should not unless there is a specific reason for doing so, but I can't think of why unless it is for symmetry...

[Kent Ross]

I compared bulk compositions of 3 Al-rich chondrules from an ordinary chondrite, using defocused beam analysis, EDS mapping, quantified using thermofisher software (NSS), and WDS mapping, quantified using calcimage.  I've attached our abstract from the M&M 2019 meeting in Portland OR, and I'm attaching the ppt of my talk. The results for the 3 methods were similar for most elements, but I concluded that the PFE-Calcimage results were best, because it calculates compositions pixel by pixel, unlike the other two methods, which violate the fundamental assumption of matrix corrections, where we are quantifying mixed x-ray counts from more than one phase.

[Kent Ross]

Here's the abstract attachment, from comparison of three methods for determiningg bulk composition.

[Probeman]

There's been some discussion in the Athens EPMA Workshop on the problems with using "defocused beam analysis" or DBA.

Yes, it might seem that one could obtain a quick "average" analysis of a heterogeneous sample, but it is not quantitative for the basic reason that x-ray absorption is a non-linear process, so averaging x-ray intensities is not quantitative. Another way to put it is as stated years ago by Chuck Fiori (NIST), "if the interaction volume is not homogeneous, all bets are off"!

Instead one must acquire x-ray maps as usual, making sure that the beam size is smaller than the features of interest and then one needs to quantify the pixels, correcting for background and matrix effects and then (and only then), one can average the concentrations of the pixels.  Here are some references regarding the problems with "defocus beam analysis":

     Sorbier, L., Rosenberg, E., Merlet, C. & Llovet, X. EPMA of Porous Media: A Monte Carlo Approach. Mikrochim. Acta 132, 189–199 (2000).
    A von der Handt et al., Microscopy and Microanalysis, Volume 29, Issue Supplement 1, 1 August 2023, Pages 844–845, https://doi.org/10.1093/micmic/ozad067.419
    JJ Donovan et al., American Mineralogist: Journal of Earth and Planetary Materials 106 (11) (2021), p. 1717
    J Barkman et al., Microscopy and Microanalysis 19 (S2) (2013), p. 848 . doi:10.1017/S1431927613006235
    P Carpenter et al, 40th Lunar and Planetary Science Conference (2009), p. 2531
    J Berlin et al., Microscopy and Microanalysis, Volume 14, Issue S2, 1 August 2008, Pages 110–111, https://doi.org/10.1017/S1431927608084845
    J Berlin et al., 37th Lunar and Planetary Science Conference (2006), p. 2370.
    DJ Lindstrom (1999) LPS XXX, No. 1917.
    PH Warren, 28th Lunar and Planetary Science Conference (1997) #1406
    MA Nazarov et al. (1982) LPS XIII, 582-583.
    AL Albee et al., Lunar and Planetary Science Conference (1977) 7-9
    AL Albee et al., 8th International Congress on X-ray Optics and Microanalysis (1977), p. 526-537.
    J Bower et al, 8th International Congress on X-ray Optics and Microanalysis (1977), p. 182-184.

Also see this topic:

https://probesoftware.com/smf/index.php?topic=198.0

Note that such averaging only provides an average composition based on the area of each phase.  For some applications we need the mass fraction of each phase, and for that we need to normalize the %area with the phase densities.  See here:

https://probesoftware.com/smf/index.php?topic=1071.msg7095#msg7095