Author Topic: Defocus Beam Effects in X-ray Mapping  (Read 2322 times)

Probeman

  • Emeritus
  • *****
  • Posts: 1955
  • Never sleeps...
    • John Donovan
Defocus Beam Effects in X-ray Mapping
« on: October 07, 2014, 01:20:37 pm »
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...
« Last Edit: October 07, 2014, 01:32:37 pm by Probeman »
The only stupid question is the one not asked!

Kent Ross

  • Student
  • *
  • Posts: 2
Re: Defocus Beam Effects in X-ray Mapping
« Reply #1 on: August 16, 2019, 12:38:21 pm »
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

  • Student
  • *
  • Posts: 2
Re: Defocus Beam Effects in X-ray Mapping
« Reply #2 on: August 16, 2019, 12:41:08 pm »
Here's the abstract attachment, from comparison of three methods for determiningg bulk composition.