cosmic ray shower
Nuclear Chemistry

Experiment N-8

Radiation from Space: Cosmic Muons

Gathering & Analyzing Evidence from Cosmic Rays


Cosmic rays is a term that is sometimes used rather loosely to describe radiation coming from outer space.  The origin of cosmic rays in not certain, but likely most originate in our Milky Way Galaxy.  They are essentially atomic nuclei, stripped of any surrounding electrons, and accelerated to very high energies.  Their energies vary widely with a few being amongst the most energetic particles known.  Since most of the ordinary matter in the universe is Hydrogen, the vast majority of cosmic rays are just protons carrying large amounts of kinetic energy.

When the cosmic rays collide with molecules in the outer layers of the Earth's atmosphere, much of the energy is converted into a shower of newly created particles.  Most of these decay very rapidly and their decay products are absorbed by the atmosphere.  Among the longest lived particles are muons, some of which continue in the direction of the Earth's surface.  As charged particles, they are gradually deflected by the relatively weak magnetic field of the Earth.  This deflection depends on their direction of travel since magnetic fields exert a force on charges traveling perpendicular (across) the magnetic field.  So more muons penetrate to the Earth's surface at either of the Earth's magnetic poles than at the Equator.  And the kind of electric charge (+ or - by Ben Franklin's terminology) determines which direction the muons are deflected as they cross the magnetic field.

The muon is an excited form of lepton which after a brief existence decays to a stable electron (with a half life of 2 x 10-6 s).  Because of their high speed, a significant amount of muons have their decay delayed as described by Albert Einstein's theory of Special Relativity.  As a result of their high speed, relatively slow decay rate compared to most of the other particles which were created, and inability to reactive with baryons such as protons and neutrons, we are continually showered with muons at the Earth's surface. Most of them continue to pass through us to decay inside the earth.  As they penetrate through matter, the electromagnetic field interacts with the matter transferring some of their energy.  Only a tiny fraction decay while passing through us, releasing all their remaining energy.  As they some pass through us they create chemically reactive radicals and occasionally cause mutations in our DNA.  Most of those mutations are either repaired by mechanisms in living cells designed to do so, or cause the cell to die and be replaced by other cells.  Very rarely the mutation may start a cancer and even more rarely a mutation in a reproductive cell (egg or sperm) may cause a birth defect, with a portion of those being responsible for evolutionary changes.

Unless we use specialized equipment to augment our senses, we are generally unable to detect any of these cosmic rays.  (Occasionally astronauts have reported flashes in their eyes thought to be caused by cosmic rays.)  A group of high school and college science teachers have formed a group called QuarkNet to operate pairs of muon detectors at schools around North America to gather information about cosmic rays, precisely recording location and time in a database at FermiLab so coincidences can be used to evaluate the energy and direction of the the most powerful cosmic rays which produce showers spread widely enough to be detected at multiple schools.  The plan is to allow students to analyze this data and help learn more about these little understood particles.  A group of physics teachers (including the author) worked at SLAC to assemble some of these muon detectors.  Use this link if you wish to see photographs and description of the building the detectors.


This experiment is being developed in coordination with the QuarkNet group at FermiLab.  Link to the list of e-labs currently providing.


  1. Use this link to access the list of e-labs.
  2. Use the link to go to the Cosmic Ray e-Lab.
  3. Select   Student Home   from the black menu bar.
  4. Log in as a guest.  (You don't need to fill in any blanks.)
  5. Select   Get Started 
  6. After reading the next screeen, select   Let's Go! 
  7. Clicking on either the map or the black menu bar on the next screen to choose where to go from there.

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.



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conceived 17 November 2002
latest revision 3 July 2007
by D Trapp
Mac made