secondary fluorescence corrections for epma: using …johnf/fournelle_sf_2005mm.pdf · 2019. 10....
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Secondary Fluorescence Corrections for EPMA: Using PENELOPE
Monte Carlo Simulations
John Fournelle*, Justin Gosses*, Jacques Kelly*, Kathy Staffier*, Jeff Waters**, and Caroline Webber*
* Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 ** Department of Material Science and Engineering, University of Wisconsin, Madison, WI 53706
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The primary volume of x-rays generated is relatively small --dependent on keV and material composition.
Here in geologic material, at 15 keV, it is ~a few microns,
FeTiO3 FeTiO3 Fe3O4
3 microns
15 keV
Example using CASINO
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However …
The x-rays generated in the primary volume can easily travel far outside the original material’s volume — producing SECONDARY FLUORESCENCE (SF) in a different material.
The detector will register those SF x-rays as coming from the primary excitation volume.
FeTiO3 FeTiO3 Fe3O4
15 keV
Fe Ka
Ti Ka Ti Ka
Bremstrahlung
3 microns
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We had a problem… in a specimen in Nb-Pd-Hf-Al bearing phases
Some researchers claimed 10 wt% Nb in 2 phases where our PI suggested Nb should be absent.
- The other researchers did EPMA by EDS at 30 keV, measuring Nb Kα.
- But our lab measured Nb Lα (WDS at 18 keV) and got ~0 wt% Nb.
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We checked out the phase (Pd2HfAl) we found to have zero Nb in, acquiring an EDS spectrum (at 28 keV).
But there was a small Nb Ka peak!
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First thought:
Secondary fluorescence might explain the discrepancy, as
• problematic phases just a short distance from Nb phase
• Pd Ka x-rays strong enough to excite K edge of Nb
But can we prove it?
K edge KaNb 18986 ev 16615 evPd 24350 ev 21177 ev
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Pd2HfAl (no Nb)
Nb
2 ways to address the problem 1. Experimentally: Create a ‘non-diffused couple’ of Nb
against Pd2HfAl, and measure the Nb Kα with distance away from the boundary. (LIF220 crystal needed for WDS -- took some time to acquire). --The data were consistent with secondary fluorescence.
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We acquired a copy of PENELOPE, and began to learn how to run it… on both a WinPC and under MacOS X, using easily accessible G77 compilers.
2. But while waiting to get LIF220 installed on our electron probe, we learned about the PENELOPE program - which we discovered had been shown to successfully reproduce Secondary Fluorescence.
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…but with a little perserverance it became fairly easy.
It wasn’t as easy as running snazzy GUI-front ended programs …
… eventually 5 grad students, some with no programming or command line experience, quickly learned how to run it on their laptops.
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We started with a simple geometry and the default PENELOPE detector (annular) … And reproduced the Nb-Pd2HfAl non-diffused couple data fairly well, but found some slight differences.
Annular detector
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Could geometry -- orientation of the sample relative to the detector -- be causing the discrepancy between the “ideal” annular detector, and the real WDS spectrometer geometry?
PENELOPE annular detector
Actual WDS sample-detector geometry
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We set up distinct experimental (non-diffused couple) and PENELOPE models: one with the Nb side facing the detector, the other 180° away …
… there was ~40% difference!
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Ø Difference in amount of SF could be explained by differences in absorption: higher mac for Nb Ka thru the Pd2HfAl (57) vs thru Nb (20)
Ø This confirmed Secondary Fluorescence as the problem – and showed that PENELOPE is a good tool for simulating the effects of SF -- valuable when it is difficult or impossible to create experimental non-diffused couple.
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Incidently, Penelope can generate an EDS-like spectrum
Synthetic PENELOPE characteristic and
continuum x-rays in non-diffused couple model
EDS acquired spectrum
in Nb-free phase in
phase assemblage
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As an EPMA class project, UW-Madison students simulated various models of interest with PENELOPE on their personal computers.
1. Meteorites: Fe diffusion In Cu particles
2. Trace Ti and Al in quartz
3. Trace Mg in olivine, Fe in plagioclase
4. Pyroxene geothermometry: Ca in opx lamallae in clinopyroxene
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Recall: done Fall 2004
Simplified Model used:
1. Annular detector only
2. Non-diffused couple (infinite half-spaces)
x y
z
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Cu in most stony meteorites occurs as 1-20 µm grains associated with troilite (FeS) and NiFe.
Fe Diffusion in Cu inclusions?
Duke and Brett (1965) considered the concentration of Fe in 10-20 µm Cu grains in a stony meteorite. Their EPMA measurements gave 1-4 wt%.
Cu formed @ 475°C in equilibrium with Fe has <0.2 wt% Fe in solid solution. Secondary fluorescence??? They attempted to show with non-diffused couples.
NiFe
Cu particle
Their EPMA conditions: 25 keV, TOA 52.5° on ARL probe. We calculate Cu Ka x-ray range as <1.5 µm
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Fe Diffusion in Cu PENELOPE simulates 2 wt.% Fe in Cu at 5 µm away from pure Fe (e.g. a 10 µm diameter Cu sphere could show 2 wt% Fe in its center.)
If you are interested in trace levels, SF yields 34 ppm Fe at 100 microns away from the Fe material.
0
1
2
3
4
5
0 10 20 30 40 50
Distance from boundary (um)
Wt
% F
e
wt% Fe
This simulation matches closely recent experimental work (Llovet and Galan, 1996).
PENELOPE allows simulating any takeoff angle (here 52.5°) and keV (25)
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Trace level of Ti and Al in Quartz
EPMA many times used to measure some trace element concentrations in minerals.
… one example is quartz
But is it really in the quartz?: low concentration of Al and Ti measured by EPMA: could this be from SF of Al or Ti-rich phases either within or adjacent to quartz (e.g. rutile needles in quartz)?
Experimental conditions: 20 keV, 40° takeoff angle; electron range in quartz 3-4 microns
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“Ti” in Quartz if there is nearby rutile
The 2 curves represent different paths out of the sample to the detector (different mass absorption values.)
It is clearly possible to get 500-1200 ppm of apparent Ti within 30 microns of the interface.
This is all from continuum x-ray excitation (E0 = 20 keV).
TiO2 SiO2 0
500
1000
1500
2000
2500
5 10 15 20 25 30 35 40 45 50
distance from interface (um)
ppm
Ti ppm Ti thru SiO2
ppm Ti thru TiO2
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“Al” in Quartz near corundum
PENELOPE suggests that you need to be at least 10 microns away from a lateral Al-rich phase to be certain that SF producing less than 100 ppm of apparent Al.
A worst case scenario would be 500 ppm of Al at 5 microns distance.
Al2O3 SiO2
0
250
500
750
1000
4 5 6 7 8 9 10distance from interface (um)
pp
m A
l ppm Al thru Al2O3ppm Al thru SiO2
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Adjacent olivine and plagioclase
What SF can do…for trace levels of Ca in olivine and of Fe in plagioclase
Conditions: 15 keV, 40° take off angle
Plag An80 Olivine Fo90 (no Ca) but 7.6 wt% Fe
(no Fe) but 11.7 wt% Ca
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Trace level of Ca in olivine Plag An80 Olivine Fo90
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0 10 20 30 40 50 60 70 80 90 100
Distance from interface (um)
wt.
% C
a o
r C
aO
wt.% Cawt.% CaO
Secondary fluorescence can easily boost the Ca content particularly within 25 microns of rim adjacent to Ca-bearing phases.
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Correction for secondary fluorescence
clinopyroxene Olivine Fo90
Llovet and Galan (2003) showed the correction for Ca in olivine adjacent to clinopyroxene using PENELOPE simulation:
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Trace level of Fe in plagioclase
EPMA analyses of plagioclase normally have several tenths of wt.% FeO.
How much is due to secondary fluorescence?
> Quite a bit. And if olivine was fayalite (Fe2SiO4), it would be much higher.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 10 20 30 40 50 60 70 80 90 100
Distance from interface (um)
wt.
% F
e or
FeO
*
wt.% Fewt.% FeO*
Model assumptions: 15 kev; olivine has 9.8 wt% FeO (7.6 wt% Fe)
Plag An80 Olivine Fo90
9.8 wt% FeO 7.6 wt% Fe
Fo90:
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Ca in orthopyroxene lamellae within clinopyroxene
Figure 2. Cartoon illustrating model run positions. Negative values indicate a position in the orthopyroxene grain; positive values inicate a position in the clinopyroxene grain.
Modeled electron beam position
OPX CPX
Coexisting compositions of ortho- and clinopyroxenes are used as a geothermometer.
There is only a small amount of Ca in orthopyroxene; we decided to see if PENELOPE could tell the potential for error in Ca content of thin orthopyroxene lamellae, and the resulting error in temperatures.
Clinopyroxene = Ca(Mg,Fe)Si2O6
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Additional Ca from secondary fluorescence of adjacent cpx
500.001
0.01
0.1
1
10
100
40 30 20 10 5 2 -50 -100-40-30-20-10-5-2 -2.5 -7.51 0
PositionClinopyroxene Orthopyroxene
Figure 4. Log plot of wt % Ca from fluoresence in the system cpx-opx.
PENELOPE SIMULATION
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IMPACT ON GEOTHERMOMETRY
PENELOPE SIMULATION
750
740
730
720
710
700
690
680
670
660
6500 -10 -20 -30 -40 -50
Taylor (1998), method 1Taylor (1998), method 2Brey and Kohler (1990), method 1Brey and Kohler (1990), method 2
EXPLANATION
Distance away from clinopyroxene boundary
Figure 6. Plot illustrating change in calculated temperature based on subtracting the effects of fluorescence from orthopyroxene analyses. Method 1 = subtraction of fluorescence before ZAF correction. Method 2 = subtraction of fluorescence after ZAF correction.
uncorrected temperatures
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… and something else
In troubleshooting low totals in chromite grain mounts, the question arose: if there is a several order magnitude size difference between unknowns (small grain separates) and the standard (large), what could result?
Can PENELOPE help?
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Using the new PENELOPE geometry:
Compare a small sample (modelled here) sitting in plastic (epoxy) and a much large standard sitting in
plastic
z
Cr2O3
PMM (plastic)
Sample = 10 um polished sphere embedded in plastic
Standard = 2 mm polished sphere
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z
Cr2O3
PMM (plastic)
Sample = 10 um polished sphere embedded in plastic
Standard = 2 mm polished sphere
Is the lack of “additional” Cr x-ray counts resulting from “normal, within same phase” fluorescence responsible???
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z
Cr2O3
PMM (plastic)
Sample = 10 um polished sphere embedded in plastic
Standard = 2 mm polished sphere
Set up a PENELOPE simulation: Standard of “huge size”, 2 mm; Unknowns of smaller sizes
Accelerating voltage of 20 keV, TOA 40 degrees
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Yes, “missing” fluorescence may cause problems
Standard=2000 µm Cr2O3 Unknown = smaller Cr2O3
A 100 µm grain of pure Cr2O3 will have 1% low Cr K-ratio, and a 10 µm grain will have a K-ratio 2.5% low.
Electron range (K-O): 1.7 micron Cr Ka X-ray range (A-H): 1.6 micron
0.74
0.76
0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
0 10 20 30 40 50
diameter in microns
frac
tion
of
max
x-r
ay in
ten
sity
4
0.7
0.75
0.8
0.85
0.9
0.95
1
1 10 100 1000 10000
diameter in microns
fract
ion
of
max x
-ray in
ten
sity
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In conclusion Secondary fluorescence across phase boundaries has been a difficult issue to
address in the past.
PENELOPE provides a useful tool to evaluate -- and correct -- this secondary
fluorescence.
Gross differences in sizes between standards and unknowns may introduce
unsuspected errors due to “missing” fluorescence