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Explain PIXE

Explain theory behind PIXE

PIXE stands for proton induced X-ray emission. That is the basic idea behind PIXE. You have an particle accelerator that accelerates protons towards an object, or in my case an organism. Quantum theory states that orbiting electrons of an atom must occupy discrete energy levels in order to be stable. This means there are specific electron shells. The proton will knock an electron out of one the inner electron shells. This is depicted in the picture as the pink proton flies off in one direction and the green electron goes in the other. After this event an electron will jump down from higher electron shell to the lower one. This releases energy in the form of x-rays. Every element gives off their own specific x-ray energy level signature. Those x-rays can be detected and then arranged into a spectrum. The x-ray energy would be the x axis and the number of detections of the energy would be the y axis. one could then analyze the spectrum and know what elements are in the object or organism. Because PIXE only effects the electrons, PIXE is non-destructive; it does not change the element because the element is dependent on the number of protons not electrons.



Explain Setup at ND

The accelerators are housed in the Nuclear Structure Lab. There are two accelerators: the FN Tandem Van de Graaff Accelerator and the KN Van de Graaff Accelerator. I only used the FN Accelerator (number 3 on the map). The FN accelerator was installed in 1968. The accelerator works under the same concept of a Van de Graaff generator. The accelerator starts with the ion source (1 and 2 on the NSL map). The ion source sends negative ions in a vacuum towards the accelerator. In the accelerator there is a centralized metal electrode known as the terminal. The terminal is charged to a very high positive potential. The negatively charged ion beam is accelerated towards the positively charged terminal. Just as the ion beam is about to enter the terminal it passes through a thin carbon foil. The carbon foil strips the electrons from the ion beam. The ion beam is now positively charged as it enters the terminal. The terminal is now repelling the positive ion beam out the opposite side of the accelerator. It is a tandem accelerator because there are two points of acceleration. There is before the carbon foil and after the carbon foil. The accelerator is housed in a steel tank as seen in the first picture below. This is so that the high voltage surfaces are isolated from the outside world. This keeps the surfaces of the accelerator from discharging. The system that charges the terminal uses a "Pelletron chain" in the same way that a Van de Graaff generator uses a belt to charge itself. After the leaving the accelerator, the beam can be steered through the use of magnets. The beam is steered through the next room, past it, and into the next room. Here, the beam ends at the place seen in pictures 2,3, and 4. At this point the beams smashes into the sample and the x-rays are detected.



Explain how to run the accerator

My mentor has a key into the lab and a separate key that he uses to start the accelerator. I do not do much in the way of actually running the accelerator. This is not of my own choice. It has to do with the nature of the accelerator being dangerous. It uses massive amounts of electricity and voltage. The accelerator also produces massive amounts of radiation. There are all kinds of precautions they have to take to ensure safety at the lab. I am aloud to uses certain controls at the lab. I can use the controls that move the video camera thats mounted near the samples. We control the beam and accelerator from the control room which is number 11 on the NSL map.

References:

@http://en.wikipedia.org/wiki/Particle-Induced_X-ray_Emission @http://www.mrsec.harvard.edu/cams/PIXE.html @http://physics.nd.edu/NSL.htm @http://www.nd.edu/~nsl/Research_Facilities/FN_site/nsl_tandem.html