Muon_conceptual_introduction - A Conceptual Tour of 'Muon...

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Conceptual Tour of Muon Physics 1 of 4 David Van Baak, 7/17/09 A Conceptual Tour of 'Muon Physics' It's rather surprising that everyone, everywhere, has always at hand a totally reliable and maintenance-free source of unstable elementary particles for experimental study. The particles most easily studied by this method are muons , and their availability at earth's surface is due to the interaction of 'cosmic rays' with the earth's upper atmosphere. This write-up will introduce you to the simplest techniques of detecting these muons, and a method for acquiring the exponential decay curve for a sample of muons. So, what are 'cosmic rays'? They are mostly protons, accelerated to high energies by mechanisms still unknown, somewhere in the Galaxy. A fraction of this resident population of high-energy (many GeV) protons continually rains down onto earth from space. Their free flight is interrupted when the protons encounter the nuclei of atoms in earth's upper atmosphere, and such encounters induce nuclear reactions (for example, producing radioactive carbon-14 out of nitrogen-14 nuclei). But some of those reactions also yield unstable pions, which decay (in order 10 -8 s) to muons and neutrinos. The latter are very hard to detect, but the muons are charged (charge ± 1e for μ ± ) and they will therefore interact with matter electromagnetically. Happily the muons' mass (about 105 MeV/c 2 , or about 207 m e ) gives them a rather long range in the atmosphere, and many survive to reach sea level. So there is, everywhere on earth's surface, a steady flux of muons, of typical energy 2 GeV, arriving at a convenient though low rate. And a fraction of these muons will interact, electromagnetically, with any matter they encounter. In TeachSpin's 'Muon Physics' apparatus, we have them interact with a cylinder of plastic scintillator. The typical high-energy muon passes right through the scintillator, but in doing so, it causes some ionization, and deposits about 50 MeV of its energy in the scintillator. And some fraction of that energy gets converted to photons of light (that's what scintillator material is good at), and some fraction of that light reaches a photomultiplier tube, which converts the brief flash of light to a detectable pulse of electrons. [Happily, the vastly more frequent events are due to background radiation of the earth. These ambient beta and gamma rays, represent events of initial energy 1 MeV or less, and create much weaker light pulses, easily discriminated against.] So the scintillator/photomultiplier assembly, when properly configured, produces several electronic events every second, almost all of which are due to muons passing through the
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This note was uploaded on 12/13/2011 for the course PHYS 309 taught by Professor Staff during the Spring '10 term at South Carolina.

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Muon_conceptual_introduction - A Conceptual Tour of 'Muon...

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