1. Lecture 4a
Common Error in EPMA: Secondary Fluorescence from Outside Primary Excitation Volume
John Fournelle, Ph.D.
Department of Geoscience
University of Wisconsin
Madison, Wisconsin, USA
December 1, 2015 Nanjing

2. Common EPMA needs/wants
Need to measure low ppm of some element
Need to analyze small crystals in volcanic glass)
Need to analyze small inclusions in minerals
Need to determine if there is diffusion profile between two phases
But simple EPMA can produce errors if SECONDARY FLUORESCENCE is not understood

3. “Simple EPMA” assumes….
That all X-rays detected come from the electron beam interaction volume

4. Fluorescence Problems
UW- Madison Geology 777
Fluorescence Problems
Alloy:
90 at% Ni
10 at% Fe
Goldstein et al. p. 142
Secondary fluorescence is an important issue that must be appreciated. Generated X-rays are not scattered nearly as much as incident electrons, and thus the generated X-rays can travel relatively long distances (50 um in Fig 3.49) within the specimen and produce a second generation of X-rays. If the specimen (and standards) are relatively large (=homogeneous), this is not a problem.

5. Fluorescence Across Phase Boundaries
UW- Madison Geology 777
Fluorescence Across Phase Boundaries
Reed 1993, p. 258
However, if minor or trace elements are being analyzed in small grains (Phase 1) and the host phase (2) has high element abundance, an error can be made in the EPMA analysis.

6. Fluorescence across boundaries
UW- Madison Geology 777
Fluorescence across boundaries
“false Co”
“false Cu” Cu Co

7. Secondary Fluorescence Correction
UW- Madison Geology 777
Secondary Fluorescence Correction
An American Mineralogist 2003 article (Llovet & Galan) reports an innovative approach to correcting the secondary fluorescence (SF) in diffusion couples and from adjacent phases. This utilizes a complex Monte Carlo program called PENELOPE that permits complicated geometric models of electron and X-ray behavior in materials. SF can be simulated in a model that represents the actual specimen.

8. Secondary Fluorescence Correction
UW- Madison Geology 777
Secondary Fluorescence Correction
SF can be simulated with Monte Carlo program PENELOPE in a model that represents the actual specimen (e.g. Fig 1).

9. Secondary Fluorescence Correction
UW- Madison Geology 777
Secondary Fluorescence Correction
Correction for SF in Ca in olivine using PENELOPE. The corrected Ca-in-olivine temperatures matched closer the 2 pyroxene thermometer temperatures than when uncorrected. Llovet and Galan, 2003.

10. Ti in Quartz Geothermometer
Quartz fill of rutile needles
Sato and Santosh, 2007, Mineralogical Magazine, 71, 143.
This published study used the Ti measured in the quartz at ~10 microns from the rutile as the ‘true’ Ti content of the quartz. It gave minimum temperatures of 1190°C.

11. Ti in Quartz Geothermometer
Sato and Santosh, 2007, Mineralogical Magazine, 71, 143.
I simulated the quartz with a rutile, at various distances from the interface in the quartz. The simulation matches the published experimental data; my conclusion is that virtually of the Ti measured by EPMA is due to Secondary Fluorescence from the rutile.

12. UW- Madison Geology 777
An SF real story
Another lab reported 10 wt% Nb (below) in what should have been Nb-free phase (by EDS at 30 kV). The issue was small grain size and nearby Nb (right image)
When I ran WDS (18 kV) I found essentially zero Nb -- what is the problem?
The original researchers used Nb Ka because, with EDS, it is impossible to resolve Nb La (it sits between Al Ka, Hf Ma and Pd La).
Above: EDS spectrum on Pd2HfAl 5 um away from Nb. Pd Ka is very efficient at traveling across the border and exiting the Nb
Fournelle, Kim and Perepezko (2005)

13. And I could reproduce their error by WDS – at 28 kV! Using Nb Ka
And we see that also at 28 kV, there is minimal Secondary Fluorescence for Nb La line

14. Experimental Validation of Nb Ka Secondary Fluorescence in Nb-free Pd2HfAl

15. PENELOPE/PENEPMA
Rigorous Monte Carlo program that tracks both electrons and photons
Complex geometries possible
Steep learning curve on original software
Newer interfaces easier to use:
PyPenelope (Philippe Pinard)- full app (go to SourceForge to download)
Probe for EPMA – simplified app for secondary fluorescence (included in CALCZAF/Standard). A tutorial is at the ProbeSoftware Forum.

16. A GUI for the calculation program can be downloaded from http://epmalab.uoregon.edu/calczaf.htm)