Electron Probe Microanalysis EPMA

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Electron Probe MicroanalysisEPMA
Revised 10-19-2005
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• Wavelength Dispersive Spectrometry (WDS) I

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Generic EMP/SEM
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Electron gun
Column/ Electron optics
Optical microscope
EDS detector
Scanning coils
SE,BSE detectors
WDS spectrometers
Vacuum pumps
Faraday current measurement
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Key points
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• 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
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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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X-ray path
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BSE detectors alternate
Only a small of X-rays reach the spectrometer.
They first must pass thru small holes (10-15
mmdia 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.
Crystal
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Wavelength Dispersion
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Of the small of X-rays that reach the crystal,
only those that satisfy Braggs Law will be
diffracted out of the crystal.
nl 2d sinq
Note that exact fractions of l will satisfy the
conditions for defraction. Thus, there is a
possibility of higher order (n2,3,...11,?)
interference in WDS (but there also is the means
electronically to discriminate against the
interference).
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What is nl ?
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nl 2d sinq
This is sometimes difficult to comprehend. Assume
you have your spectrometer set to one particular
position, which means for that sinq and that 2d
(lets 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
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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
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• 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 A. 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,
  LSM, LDE, etc. Cameca uses PC and JEOL uses LDE,
  for same things.

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Crystals and PCs on the UW SX51
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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
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Pseudocrystals/LSMs
Goldstein et al, p. 280
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Crystals and PCs Which to use?
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• 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
• interferences vs counting statistics (sharper
  peaks usually have lower count rates)
• stability (thermal coefficient of expansion)

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Crystal comparison
The class project in 2002 was to collect data
that will be put in a chart to compare the
efficiency of different crystals/ PCs for certain
elements.
Å
Å
TAP gives a higher count rate, and wider peak for
Si Ka vs. using PET
Si Ka on TAP sin q 27714 P/B
4862/40122 FWHM0.038Å
Si Ka on PET sin q 81504 P/B
207/1.3159 FWHM0.006Å
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Full Width Half Max
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Max
4862 cts
Peak (spectral) resolution is described by FWHM
Full Width
Half max
2431 cts
counts
Si Ka on TAP sin q 27714 P/B
4862/40122 FWHM0.038Å
Å
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WDS detector
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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 gt3206 eV it can knock out an Ar K
electron, with L shell electron falling in its
place. There are also 3 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) (3b) Ar Ka X-ray itself
can knock out electron of another Ar molecule,
producing Ar e pair or (3c) 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)

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Detector amplification
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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 of1200-2000v) and
produces many secondary ionizations. This yields
a very large amplification factor (105), and has
a large dynamic range (0-50,000 counts/sec).

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Detector windows
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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.
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WDS detector
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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
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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
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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.
Reed, 1993, p. 90
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Actual PHA plot for Fe Ka note the Ar-escape peak
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Actual PHA plot for Si Ka there is no Ar-escape
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. On LIF, 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.
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Pulse Height Analysis
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
X-rays that have energies greater than the
windows 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
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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
danceDance floor Rowland Circle
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Rowland Circle
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• 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).

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Rowland Circle
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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.