Professor:%20Scott%20M.%20LaBrake - PowerPoint PPT Presentation

Title: Professor:%20Scott%20M.%20LaBrake


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Professor Scott M. LaBrake Email
labrakes_at_union.edu Course Physics 300
Spring 2013 Office NWSE N308, N008b
Phone 388-6053 6562 Office Hours MWF
930am 1130am Th 830am 1130am and by
appointment. Actually you probably can just
stop by anytime, Im usually around. Web
http//minerva.union.edu/labrakes
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Proton Induced X-Ray Emission (PIXE) Spectroscopy

• Modern materials analysis technique that uses a
  moderately energetic particle beam to eject
  electrons from a target material.
• Characteristic x-rays are produced from each of
  the elements in the target material.
• PIXE is a non-destructive technique and the
  analyses are sensitive for elements in the range
  of Na to U. (To complete the periodic table we
  use other complementary analysis techniques such
  as PESA, PIGE, and RBS.)
• PIXE is used in fields from biology, medicine,
  forensics, art conservation, archeometry,
  materials analysis and environmental pollution.
• In this experiment well learn the PIXE method
  and apply this nuclear physics technique to the
  analysis of atmospheric aerosols and
  environmental pollution.

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The NEC Tandem 1.1 MV Pelletron Particle
Accelerator
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The NEC Tandem 1.1 MV Pelletron Particle
Accelerator
Target Manipulator
PIXE chamber
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Charge-exchange collision to produce negative
ions.
Positively charged particles are pulled to right
by negatively biasing the quartz bottle. The
positive ions pass through a small opening into a
Rb vapor where they pick up a negative
charge. Then these negatively charged ions are
accelerated toward the center of a high pressure
tank which has a large positive potential.
H
H-
The negative ions are passed through a N gas and
are stripped of their excess charge and are
accelerated away from the positive terminal held
at 1.1MV. Protons are accelerated once towards
the center of the tank (for the extra e-) and
once away (for the lone proton) for 2.2MeV worth
of KE. Alpha particles are accelerated once
towards the center of the tank (for the extra e-)
and once away (for each proton) for 3.3MeV worth
of KE. Both the proton and the alpha particle are
non-relativistic.
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The NEC Tandem 1.1 MV Pelletron Particle
Accelerator
This is the particle accelerators column. In
the center is the terminal shell where the 1.1MV
worth of potential is developed (with respect to
the ends at ground.) The rings are used to
create a potential gradient so that the potential
builds up (or is removed) in a uniform (linear)
fashion.
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The NEC Tandem 1.1 MV Pelletron Particle
Accelerator
This is the high energy column showing the
Pellets (which give the Pelletron its name) and
the charging system. The terminal shell is
usually over the top of the wheel at the right.
As the proton passes through the terminal shell,
Nitrogen gas is bled into the beampipe at the
center of the terminal shell. Through the same
charge exchange collision process the extra
electrons are stripped from the proton.
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PIXE Proton Induced X-ray Emission
PIXE is a non destructive materials Analysis
tool. X-Ray Emission Incident particle interacts
with target ejecting electrons. Electrons are
ejected from shells of atoms in the
sample vacancy is filled by an electron from a
higher shell emitting a characteristic x-ray in
the transition. Data Acquisition X-ray detector
and software to record energy spectrum Need to
calibrate x-ray detector with 241Am.
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PIXE Proton Induced X-ray Emission

• We identify the elements based on their x-ray
  emission spectra.
• Each element has its own unique set of x-ray
  emission lines allowing us to fingerprint them.
• PIXE is sensitive to elements in the range of Na
  to U.
• The light elements are determined by their
  K-series x-rays, while heavy elements are
  determined by their L-series x-rays.
• When an electron transitions between states, the
  emitted photon energy is given by

• If the electron is ejected from the ground state
  (or the K-shell) an electron from a higher
  orbital will fill the vacancy that was created.
• The lowest energy (highest probability) is called
  the a-transition. The next higher energy (lower
  probability) transition is called the
  b-transition.
• We identify the elements based on identifying
  usually the Ka transition (and if lucky the Kb
  also.) We usually are not this lucky And if the
  element is heavy we look for the corresponding L
  series transitions.

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PIXE Proton Induced X-ray Emission

• Each state has an associated kinetic energy (due
  to the electrons motion about the nucleus), and
  a potential energy (due to the separation of
  electric charges between the nucleus and the
  electron in its orbital).
• For a K series transition, the transitioning
  electron is screened from the full nuclear charge
  due to the remaining electron in the ground
  state.
• Thus the transitioning electron sees the nuclear
  charge (Ze) and one electron (-e) or a total
  charge of (Z - 1)e.
• Applying the Bohr theory and writing the kinetic
  and potential energies in terms of the the
  quantized orbits, one can write the energy of the
  emitted photon when the electron transitions from
  an upper to a lower energy state as
• From the energy of the emitted x-ray (knowing the
  transitions nupper and nlower) we could calculate
  Z. This is really difficult.
• The transitions are tabulated as a function of Z.
  Calibration of the energy scale in the emitted
  spectra (using standards) allows us to use the
  energy of a given x-ray from the spectrum
  generated to look up Z.

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PIXE Proton Induced X-ray Emission
On the left a photograph of a typical aerosol
impaction foil. The foils are made out of
Kapton and are approximately 6.3mm thick. These
are put in the impactor and air is drawn through
the nine stages at a rate of 1 L/min for about
48hours.
The photograph on the right is a Kapton impaction
foil after it has been run in the impactor. The
hard part about determining concentrations in
ng/cm3 is determining the area of the impaction
spot and the area of the beam that hits the
Kapton foil.
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PIXE Proton Induced X-ray Emission

• Nine stage cascade impactor.
• Air is drawn up through the bottom and as the air
  moves up through the impactor particles of
  different sizes are separated out.
• The farther up the impactor the air goes the
  smaller the particle sizes that are trapped.
• The largest particles are trapped at the bottom
  of the impactor.
• On the far right this is the impactor assembly
  ready to collect samples.

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• Left is a photograph of the target ladder
  assembly.
• Three targets are able to be run at a time.
• Below is a photograph of the target ladder
  assembly in place with the x-ray detector nose on
  the right hand side. The beam enters from the
  right and proceeds to the right.

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Typical atmospheric aerosol sample with the peaks
labeled. Our analysis software is GUPIX and
GUPIX will calculate the concentrations for you
provided youve calibrated your spectra and GUPIX
using standards.
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PIXE The Experiment
This experiment is an application of nuclear
physics to environmental physics. Here well use
a 2.2MeV proton beam to eject electrons from the
target and from the x-ray spectrum determine the
elemental composition of the target. For the lab
experiment, were going to run 3 or 4 standards
(Cu, Fe, Au, and/or Pb) and determine the
experimental value of H by making a plot of H vs
Z. To determine H for each standard youll need
to use the data analysis software GUPIX which is
explained fully in the lab manual. In order to
use GUPIX youll need to tell the program the
energy calibration parameters (channel number as
a function of energy) of your x-ray detector.
There is a table in the lab manual to do
this. Then your task is to determine the
elemental composition and concentration of one
(perhaps two) atmospheric aerosol samples.
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PIXE The Experiment
Some general information For the lab write-up
follow the lab guidelines. Less is more in terms
of actual writing! Now that doesnt mean write
a page and thats better, you need to express
your thoughts and ideas and make sure that they
are clear to the reader. Pretend that the
reader is someone who has never seen or done the
experiment before. Due Dates for the labs
The lab report is due on or before noon on
Friday, May 24, 2013. Lateness 10 percent per
day late, including weekends. You may submit
electronic copies of the lab report and any
supporting materials that you have. Make sure
it is complete when you submit the report or its
considered late.