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 xray 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 xray 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.