Transcript Electron probe microanalysis EPMA - UW

UW- Madison Geoscience 777
Revised 2-25-2013
Electron Probe Microanalysis
EPMA
Wavelength Dispersive
Spectrometry (WDS) I
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Generic EMP/SEM
Electron gun
Column/ Electron
optics
Optical microscope
EDS detector
SE,BSE detectors
Vacuum
pumps
Scanning coils
WDS
spectrometers
Faraday current
measurement
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Key points
• X-rays are dispersed by crystal with only one
wavelength (nl) reflected (=diffracted), with only
one wavelength (nl) passed to the detector
• Detector is a gas-filled (sealed or flow-through)
tube where gas is ionized by X-rays, yielding a
massive multiplication factor (‘proportional
counter’)
• X-ray focusing assumes geometry known as the
Rowland Circle
• Key features of WDS are high spectral resolution
and low detection limits
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Spectrometers
An electron microprobe
generally has 3-5
spectrometers, with 1-4
crystals in each. Here, SP4
(spectro #4) of our SX51
(#485) with its cover off.
Crystals
(2 pairs)
Proportional
Counting Tube
(note tubing for gas)
PreAmp
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BSE detectors
alternate
X-ray path
Only a small % of X-rays
reach the spectrometer. They
first must pass thru small
holes (~10-15 mm dia; red
arrows) in the top of the
chamber (above, looking
straight up), then thru the
column windows (below,
SP4).Thus, in our EMP, there
are different vacuum regimes
in the chamber vs the
spectrometer, separated by the
windows (not in JEOLs).
Crystal
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Wavelength Dispersion
Of the small % of X-rays that reach the crystal, only those
that satisfy Braggs Law will be diffracted out of the crystal.
BA’ = A’C = d sinq
for constructive reinforcement of
a wave, the distance BA’ must be
one half the wavelength. Thus,
2d sinq = l and by similar
geometric construction = nl
nl = 2d sinq
Note that exact fractions of
l will satisfy the
conditions for defraction.
Thus, there is a possibility
of “higher order”
(n=2,3,...11,?) interference
in WDS (but there also is
the means electronically to
discriminate against the
interference).
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nl = 2d sinq
BA’ = A’C = d sinq
for constructive reinforcement
of a wave, the distance BA’
must be one half the
wavelength. Thus, 2d sinq = l
and by similar geometric
construction = nl
What is nl ?
This is sometimes difficult to
comprehend. Assume you have
your spectrometer set to one
particular position, which means
for that sinq and that 2d (let’s say
they = 12 Å), there are several
possible signals that the
spectrometer can tune in to at that
position: (1) an x-ray with
wavelength of 12 Å, (2) an x-ray
of 6 Å, (3) an x-ray of 4 Å, (4) an
x-ray of 3 Å, etc --of course, if
and only if such x-rays are being
generated in the sample.
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Lots of Crystals
Over the
course of the
first 30 years
of EPMA, ~50
crystals and
pseudocrystals
have been
used.
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Crystals and PC/LSMs
Consider the order of 2d in Braggs Law: sin q varies from .2-.8, and l
varies over a wide range from hundreds to fractions of an Å. Thus, we
need diffractors that cover a similiar range of 2d, from around 1 Å to
hundreds of Å. In our SX51, we utilize TAP, PET and LIF crystals for
the shorter wavelengths. For longer wavelengths, there are 2 options:
• pseudocrystals (PCs), produced by repeated dipping of a substrate in
water upon which a monolayer (~soap film) floats,progressively adding
layer upon layer, or
• layered synthetic microstructures (LSMs; also LDEs, layered diffracting
elements), produced by sputtering of alternating light and heavy
elements, such as W and Si, or Ni and C.
• Both these are periodic structures that diffract X-rays. The LSMs yield
much higher count rates; however, peaks are much broader, which have
good/bad consequences, discussed later.
• In reality, people interchange the words PC, LS M, LDE, etc. Cameca
uses PC and JEOL uses LDE, for same things.
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Crystals and PCs
on the UW SX51
There is a more precise form of Braggs Law, that
takes into account refraction, which is more
pronounced in the layered synthetic diffractors,
nl = 2d sinq(1-k/n2)
k is refraction factor, n is order of diffraction
Pseudocrystals/LSMs
Goldstein et al, p. 280
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Crystals and PCs: Which to use?
The EPMA user may have some control over which crystal to
use; some element lines can be detected by either of 2
crystals (e.g. Si Ka by PET or TAP, V Ka by PET or LIF),
whereas other elements can only be detected by one (e.g. S
Ka by PET). Each probe has its own (unique?) set of crystals
and the user has to work out the optimal configuration,
taking into account concerns such as
• time and money (spread the elements out)
• interferences vs counting statistics (sharper peaks usually
have lower count rates)
• stability (thermal coefficient of expansion)
Gopon et al. 2013
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TAP
-Low counts
-Good resolution
Crystal
comparison
PC0
-HighÅ counts
-Poor resolution
Å
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Full Width Half Max
Peak (spectral)
resolution is
described by
FWHM
Full Width
Si Ka on TAP
sin q = 27714
P/B= 4862/40=122
FWHM=0.038 Å
Å
Max
4862 cts
Half max
2431 cts
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Acquiring accurate
Si Ka counts is
critical for
geological EPMA.
Always use TAP!
(with rare
exceptions) as the
the wider peak (6x
wider, see FWHM)
is less sensitive to
chemical peak
shifts in silicates
(and higher count
rate is a plus)
The class project in 2002
was to collect data to
compare the efficiency of
different crystals/ PCs for
certain elements.
Crystal comparison
Å
Å
Si Ka on TAP
sin q = 27714
P/B= 4862/40=12
FWHM=0.038 Å
Si Ka on PET
sin q = 81504
P/B= 207/1.3=159
FWHM=0.006 Å
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WDS detector
P10 gas (90% Ar - 10% CH4)
is commonly used as an
ionization medium. The X-ray
enters through the thin
window and 3 things can
occur: (1) the X-ray may pass
thru the gas unabsorbed (esp
for high keV X-rays); (2) it
may produce a trail of ion
pairs (Ar+ + e), with number
of pairs proportional to the
X-ray energy; and (3) if the
X-ray is >3206 eV it can
knock out an Ar K electron,
with L shell electron falling in
its place. There are also 2
possibilities that can result
from this new photon:
(3a) internal conversion of the excess energy
with emission of Auger electron (which can
produce Ar+ + e pairs); or (bc) the Ar Ka X-ray
can escape out thru a window, reducing the
number of Ar+ + e pairs by that amount of
energy (2958 eV)-->creating an Ar-esc. peak
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Detector amplification
Nominally, it takes 16 eV to
produce one Ar+- electron pair,
but the real (effective) value is
28 eV. For Mn Ka (2895 eV),
210 ion pairs are initially
created per X-ray. However,
there is a multiplier effect
(Townsend avalanching). For
our example of Mn Ka, all 210
electrons are accelerated
toward the center anode (which
has a positive voltage [bias] of
1200-2000 v) and produces
many secondary ionizations.
This yields a very large
amplification factor (~105), and
has a large dynamic range (050,000 counts/sec).
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Detector windows
Thin (polypropylene) windows are used for light X-rays (e.g. those
detected by TAP and PC crystals). Since the windows are thin, the gas
pressure must be low (~0.1 atm). And being thin windows, some of the
gas molecules can diffuse out through them -- so the gas is replenished
by having a constant flow. For thicker windows (mylar), 1 atm gas
pressure is used (with higher counts resulting). Sealed detectors with
higher pressure gas (e.g. Xe) are also used by some. (I want one!)
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WDS detector
The bias on the anode
in the gas proportional
counter tube needs to
be adjusted to be in the
proportional range. Too
high bias can produce a
Geiger counter effect.
Too low produces no
amplification.
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WDS pulse processing
The small electron pulses
(charges) generated in the
tube are first amplified in the
pre-amp (top) located just
outside the vacuum on the
outside of the spectrometer,
then sent to the PHA board
where they are amplified
(center) and shaped
(bottom). Each figure is for
one (the same) pulse.
Goldstein et al Fig. 5.10
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Ar-escape peak
Reed, 1993, p. 90
There is a probability that a small
number of Ar Ka X-rays produced
by the incident X-ray (here, Fe
Ka) will escape out of the
counting tube. If this happens,
then those affected Fe Ka X-rays
will have pulses deficient by 2958
eV. Fig 7.8 is an unusual plot of
this (for teaching purposes); what
is normally seen is the Pulse
Height distribution where the
pulses are plotted on an X-axis of
a maximum of 5 or 10 volts.
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Actual PHA
plot for Fe Ka:
note the Arescape peak
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Actual PHA
plot for Si Ka:
there is no Arescape peak.
WHY?
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Higher order reflections
Recall nl = 2d sinq. Higher
order reflections are possible in
your specimens, and must be
avoided to prevent errors in
your analyses.
Reed (1993) reports that LIF
can show a strong second order
peak, up to 10% of the first
order peak.
In 1999, the 777 class examined
the higher order reflections of
Cr Ka lines. On PET, 2nd and
3rd order peaks were present,
and decreased ~an order of
magnitude with increasing n.
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On TAP, up to the 8th
order peak was
present. Here, the
intensity of the odd
numbered orders was
less than the
following even order,
e.g. the 5th order Ka
had 30% fewer
counts than the 6th
order line.
Differential mode of
pulse height analysis
(PHA) may be used
to ignore counting
higher order X-rays.
n
3 4 5 6 7 8
counts 1563 327 75 102 31 97
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Pulse Height Analysis
window
baseline
If a higher order reflection falls
upon a peak (or unavoidable
background) position, the analyst
has the option of using Pulse
Height Analysis, i.e. setting up a
window and not counting any Xrays that have energies greater
than the window’s upper limit.
There is a lower limit (baseline,
usually 0.5 volts). The window
stretches above it (here 4 volts),
and thus a second order reflection should have a pulse height around 6 volts,
and would not be counted.
There are some situations where operation in differential mode can
introduce errors (pulse height depression), so the normal mode is integral. And
if differential is used, it is good to do some tests first to get a feeling of how
comprehensive its filtering action is.
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Integral vs Differential PHA
Analysis of ‘light elements’ such as C is
complicated because of the long
wavelength (44 Å) which means that higher
order reflections of many elements can
interfere. At top, where the PHA is set to
the “count everything” integral mode, the
3rd order reflection of Ni La1 falls very
close to C Ka and adds some to C peak
counts. Note also the 2 and 3 order Fe L
lines. By setting the detector electronics to
the discrimination mode (differential),
bottom, the higher order lines are strongly
(but not totally) suppressed.
Spectrometer scans
Goldstein et al, p.507-8
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Putting it all together:
Spectrometer =
Crystal + Detector moving in a
highly choreographed dance
Dance floor =Rowland Circle
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Rowland Circle
For most efficient detection
of X-rays, 3 points must lie
upon the focusing circle
known as the Rowland
Circle. These points are
• the beam impact point on
the sample (A),
• the active central region of
the crystal (B), and
• the detector -- gas-flow
proportional counter (C).
C
B
A
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Rowland Circle
The loci of 3 points must
always lie on the Rowland
Circle. Starting at the top
position (blue), there is a
shallow angle of the X-ray
beam with the analyzing
crystal. To be able to defract
a longer wavelength X-ray
on the same spectrometer,
the crystal travels a distance
further out, and effectively
the (green) Rowland Circle
“rolls”, pinned by the
beam-specimen interaction
point.