Purdue PHYS 34200 - Scattering of Photons from Free Electrons (12 pages)

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Scattering of Photons from Free Electrons



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Scattering of Photons from Free Electrons

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12
School:
Purdue University
Course:
Phys 34200 - Modern Physics

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RR Oct 2001 SS Dec 2001 Physics 342 Laboratory Scattering of Photons from Free Electrons Compton Scattering Objective To measure the energy of high energy photons scattered from electrons in a brass rod as a function of the scattering angle References 1 A H Compton Phys Rev 21 715 1923 and Phys Rev 22 409 1923 2 A C Melissinos Experiments in Modern Physics Academic Press New York 1966 p 252 65 3 K Krane Modern Physics 2nd Ed Wiley and Sons New York 1996 p 83 87 Apparatus A low energy 22 keV and weak 15 C portable Cd109 source a higher energy 662 keV strong 5 mC Cs137 source in a cylindrical lead shield a NaI T scintillator mounted on a Photo Multiplier Tube PMT an iron shield surrounding the PMT a Canberra 1024 channel PC based Multi Channel Analyzer MCA a high voltage 1 5 kV power supply a brass cylindrical rod for use as a scattering target a carriage to rotate the scintillator and PMT assembly at a fixed distance around the target Introduction In 1923 Compton considered the problem of high energy photons rays scattering from solids Experimentally he found that low energy few MeV ray monochromatic photons scattered by metals change their frequency and that the frequency change depends on the scattering angle This proved to be problematic since at that time light scattering was understood in terms of diffraction in which the scattered diffracted wave does NOT change frequency Compton s experiments and his theoretical analysis of them came to be know as Compton scattering Historically his experiments are important because they provided further compelling evidence that photons do behave as particles which obey conservation of momentum and energy laws Compton was awarded the Noble prize in 1927 for his seminal work Compton s experiment can be understood by considering the interaction of the incident photons with the electrons that comprise a metal Because metals are good conductors of electricity some fraction of the electrons associated with each atom in the metal



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