Transcript PIXE - Union College

Professor: Scott M. LaBrake
Email:
[email protected]
Course: Physics 300 – Spring 2013
Office:
NWSE N308, N008b
Phone:
388-6053 & 6562
Office Hours:
MWF 9:30am – 11:30am
Th 8:30am – 11:30am
and by appointment.
Actually you probably can just stop by anytime, I’m usually around.
Web:
http://minerva.union.edu/labrakes
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 we’ll learn the PIXE method and apply this nuclear physics
technique to the analysis of atmospheric aerosols and environmental pollution.
The NEC Tandem 1.1 MV Pelletron Particle Accelerator
Photograph of the Union College Pelletron Accelerator showing the major components.
The NEC Tandem 1.1 MV Pelletron Particle Accelerator
Target Manipulator
PIXE chamber
Photograph of the accelerator endstation. The proton beam enters
through the right end passes through the target in the center of the
multi-way cross and exits out of the pipe on the left end. The 3-axis
target manipulator is mounted on the top of the scattering chamber.
Charge-exchange collision to produce negative ions.
Positively charged particles are
pulled to right by negatively
biasing the quartz bottle.
H+
H-
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.
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.
The NEC Tandem 1.1 MV Pelletron Particle Accelerator
This is the particle accelerator’s 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.
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.
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.
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 xrays, while heavy elements are determined by their Lseries x-rays.
•When an electron transitions between states, the emitted
photon energy is given by:
E xray  E upper state  E lower state
Possible x-ray transitions for an arbitrary
atom of nuclear charge Z.
•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.
PIXE – Proton Induced X-ray Emission
•Each state has an associated kinetic energy (due to the electron’s motion about the
nucleus), and a potential energy (due to the separation of electric charges between the
nucleus and the electron in it’s 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:
E xray
 1
1 
 E upper state  E lower state  13.57eV  Z 1  2  2 
nlower nupper 
2
•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.
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.
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.
•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.
YZ
CZ 
YT  H  Q   T

Typical atmospheric aerosol sample with the peaks labeled.
Our analysis software is GUPIX and GUPIX will calculate the concentrations for you provided
you’ve calibrated your spectra and GUPIX using standards.
PIXE – The Experiment
This experiment is an application of nuclear physics to environmental physics.
Here we’ll 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, we’re 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 you’ll need to use the data analysis software
GUPIX which is explained fully in the lab manual.
In order to use GUPIX you’ll 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.
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 doesn’t mean write a page and that’s 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.