Electron Probe Microanalysis EPMA

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Electron Probe MicroanalysisEPMA
Related Topics Secondary X-ray Fluorescence
(XRF) and Synchrotron Radiation
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Whats the point?

• We utilize the x-rays produced by the electron
  microprobe for many research applications.
• There are other techniques, similar in some ways,
  that are worth discussing, that utilize x-rays
  for secondary x-ray fluorescence. Two in
  particular are
• XRF (X-Ray Fluorescence), where x-rays from a
  sealed tube are used to produce x-rays by
  secondary fluorescence in samples of interest
  (traditionally a macro-technique)
• Synchrotron Radiation, where electrons are
  accelerated in 10s-100s meters diameter rings,
  and then made to produce highly focused beams of
  extremely intense x-rays or light, which are then
  fed into many different types of experiments.
• The benefits of secondary x-ray fluorescence
  include very low detection limits (10s of ppm
  easy in 10 seconds, no backgrounds)

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XRF Basics

• The basics of XRF are very similar to those of
  EPMAwe are dealing with characteristic x-rays
  and continuum x-rays with the exception that we
  are doing secondary fluorescence x-ray
  spectroscopy of our samples using x-rays coming
  out of a sealed tube to excite the atoms in our
  specimen.
• The big difference is that
• there is NO continuum generated in the sample
  (x-rays cant generate the Bremsstrahlung), and
• we are using BOTH characteristic x-rays of the
  sealed tube target (e.g., Cr, Cu, Mo, Rh) AND
  continuum x-rays to generate the characteristic
  x-rays of the atoms in the sample.
• XRF has been a bulk analytical tool (grind up
  50-100 grams of your rock or sample to analyze),
  though recently people are developing micro XRF
  to focus the beam on a 100 mm spot.

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X-ray Sources
The standard X-ray tube (top right) was developed
by Coolidge (at GE) around 1912. It is desirable
to produce the maximum intensity of x-rays a Cu
target tube might be able to deliver 2 kW. The
limiting factor is the heat that the target
(anode) can handle cold water is used to remove
heat. Higher power can be delivered by
dissipating the heat over a larger volume, with a
rotating anode (bottom right). However, this is
not normally used for XRF.
Power in watts current amps x voltage
volts
From Als-Nielsen and McMorrow, p. 31
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X-ray Attentuation
This figure shows the attenuation of the X-rays
in the target (sample). In addition to
photoelectric absorption (producing
characteristic X-rays and photoelectrons Auger
electrons), the original X-rays may be
scattered. There are two kinds of scattering
coherent (Rayleigh) and incoherent (Compton).
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X-ray Scattering
Coherent scattering happens when the X-ray
collides with an atom and deviates without a loss
in energy. An electron in an alternating
electromagnetic field (e.g. X-ray photon), will
oscillate at the same frequency (in all
directions). This is useful for understanding
X-ray diffraction (in depth). Incoherent
scattering is where the incident X-ray loses some
of its energy to the scattering electron. As
total momentum is preserved, the wavelength of
the scattered photon increases by the equation
(in Å) where f is the scatter angle. Since f
is near 90, there will be an addition peak from
the main tube characteristic peak at about 0.024Å
higher wavelength
Coherent
Incoherent
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Compton Scattering Peaks
The top figure shows a wavelength spectrum of the
Mo Ka peak from the x-ray tube. The other 3
figures show the splitting of the primary Mo Ka
peak into a Compton Scattering Peak due to the
incoherent scattering in an Al target, and the
effect of changing the scattering angle.
From Liebhafsky et al, 1972
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Continuum of X-ray Tube in XRF
Secondary fluorescence by x-rays in the sample
does not produce continuum x-rays there. However,
the continuum is produced within the selected
x-ray tube which is the gun in XRF. This
continuum is of interest here as it is useful
for excitation source in XRF. Kramers (1923)
deduced the relationship between continuum
intensity, wavelength and atomic number of the
x-ray source (target) where the x-ray
intensity I is a function of x-ray tube current
i, Z is the mean Z of the target and lmin is the
E0 equivalent.
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Kramers Law and Continuum Intensity

• Some comments
• for maximum XRF counts, you want to maximize
  your current (I) and minimize your lmin which is
  to say 12.4/E0 or run at the highest
  accelerating voltage your x-ray tube can handle
  (40-50 keV)
• obviously, the higher the Z of the target in the
  tube, the higher the counts

From Williams, Fig 2.2

• finally, Kramers Law is sometimes used in EPMA
  for theoretically modelling the Bremsstrahlung
  there

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On spectral presentation XRF
Why do these look so different from our normal
EDS view of a spectrum?????
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XRF spectrometer
An XRF spectrometer is very similar to an
electron microprobe just replace the electron
gun with an x-ray tube located very close to the
specimenboth the characteristic and the
continuum x-rays cause (secondary) fluorescence
of the specimen, and the resulting x-rays are
focused using collimators in either WDS (crystal
counter) or EDS (solid state detector) mode .
Fig 4-1 Williams
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A Currently Marketed XRF (WDS version)
This actual model contains additional
components. There are probably over a dozen
companies building and selling XRFs of various
designs. In fact, two are here in
Madison Bruker-AXS (Siemens) and ThermoNORAN
(microXRF)
From Bruker-AXS brochure
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Sample Prep in XRF
Samples and standards (fine powders) are mixed
with a flux (e.g., a glass disk with 90 LiBO4
for major elements, a pressed pellet with 75
cellulose for traces). The purpose is to
minimize particle size / micro-absorption
effects by producing a more uniform absorption
path for samples made of discrete phases that may
not have been ground down into submicron sizes.
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Correction of XRF Intensity Data

• XRF intensity data (counts) is much simplier to
  correct, compared with EPMA data
• No Z (atomic number) correction
• No F (fluorescence) correction
• Only A (absorption) correction
• Calibration curves are developed for each
  element.

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Synchrotron Radiation (SR) - Defined
Synchrotron particle (electron, proton,
neutron) accelerator. The particle orbits a
track acceleration is produced by an alternating
electric field that is in synchronism with
orbital frequency. SR electromagnetic radiation
(e.g. radio waves, X-rays) generated within a
synchrotron, or through similar natural process
in deep space (e.g. some of strongest celestial
radio sources). Electrons or other charged
particles moving in a strong magnetic field field
are forced to spiral around magnetic lines of
forces. If they travel near speed of light, they
emit, in direction of travel, a sharp beam of
electromagnetic radiation polarized normal to the
direction of magnetic field. Whether radiation
appears as light or radio waves depends on its
frequency, which is determined by the electrons
velocity.
Encyclopedia Britannica, 1974
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Synchrotron Setup
From Als-Nielsen and McMorrow
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Wigglers or Undulators and X-rays
Shown here is the cone of x-rays generated by
positrons moving with near-speed-of-light energy
through an insertion device. The array of
permanent magnets produces a magnetic field that
alternates up and down along the positron path,
causing the particles to bend back and forth
along the horizontal plane. At each bend, the
positrons emit synchrotron radiation in the x-ray
part of the spectrum.
From The Advanced Photon Source at Argonne
National Laboratory, October 1997 brochure
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Synchrotron X-ray Diffraction
In x-ray scattering experiments, an x-ray beam is
passed through a sample, and the intensities and
directions of the scattered x-rays are measured.
The pattern of scattered x-rays is converted by
the computer into information about the
arrangement of atoms in the sample.
From The Advanced Photon Source at Argonne
National Laboratory, October 1997 brochure
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Synchrotron X-ray Microscopy
A monochromator and a pinhole are used to select
the coherent, laser-like part of an x-ray beam
from an APS undulator. This beam is then focused
to a tiny spot by a zone plate and directed at a
sample being studied. As the sample is scanned
back and forth across the beam spot,
the x-rays transmitted through the sample are
recorded in a computer. The data are then
used to develop an image showing the structure
of the sample
From The Advanced Photon Source at Argonne
National Laboratory, October 1997 brochure
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Synchrotron X-ray Spectroscopy
A beam of x-rays passes through a sample and a
measurement is made of the degree to which x-rays
of different energies are absorbed by the sample.
One type of x-ray spectroscopy is called extended
x-ray absorption fine structure, EXAFS. In EXAFS
spectra, weak oscillations indicate the effect of
scattering from neighboring atoms by an electron
ejected from the atom that absorbs an x-ray. This
involves electron scattering effects, rather than
the x-ray scattering effects described in the
previous slide.
The weak oscillations in EXAFS spectra can be
analyzed by computer models to infer the relative
locations of atoms in the structure.
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Advances with X-ray source brightness with time
From Als-Nielsen and McMorrow
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UW-Madison Synchrotron Radiation Center
(Stoughton)
In 1965 construction began on the 240 MeV
electron storage ring Tantalus for advanced
accelerator concepts tests. But before its
completion in 1968, interest in synchrotron
radiation research soared, and changes were made
to accommodate SR. And it then became dedicated
to SR, and here many breakthroughs were made,
e.g., the superiority of the electron storage
ring as a source of SR was first shown. In 1977,
SRC began construction on a new and much larger
SR source, Aladdin (1 GeV
storage ring). The SRC storage ring beamlines are
optimal for ultrahigh vacuum ultraviolet (vuv)
and soft x-ray (sxr) research.
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UW-Madison SRC

• Aladdin was constructed with 36 beam ports,
  and 4 long straight sections for insertion
  devices like undulators and wigglers. There are
  26 beamlines in operation and 5 under
  development.
• The SRC serves the requirements for many
  investigations, including
• high resolution optical absorption spectroscopy
  of solids and gases
• high resolution reflectance spectroscopy of
  solids
• photoinduced luminescence in solids and gases
• photoabsorption, dissociation and ionization
  cross section measurements
• chemisorption and physisorption studies
• modulation spectroscopy

• photoelectron diffraction
• x-ray lithography
• x-ray microscopy
• intrared spectroscopy and microscopy (FT-IR)

1996 literature quote.
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Resources for XRF and Synchrotron
Introduction to X-Ray Spectrometry by K. L.
Williams, 1987, Allen Unwin (covers both XRF
and EPMA) X-Rays, Electrons, and Analytical
Chemistry by Liebhafsky, Pfeiffer, Winslow and
Zemany, 1972, Wiley (title says it all) Elements
of Modern X-Ray Physics by Als-Nielsen and
McMorrow, 2001, Wiley Synchrotron powder
diffraction by L.W. Finger, in Modern Powder
Diffraction (Bish and Post, eds) Reviews in
Mineralogy Vol 20, 1989, Min. Soc. Am.