Conceptual Tour of Muon Physics
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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.
particles most easily studied by this method are
, and their availability at earth's
surface is due to the interaction of 'cosmic rays' with the earth's upper atmosphere.
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
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
s) to muons and neutrinos.
latter are very hard to detect, but the muons are charged (charge
) and they will
therefore interact with matter electromagnetically.
Happily the muons' mass (about 105
, or about 207 m
) 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.
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.
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