Understanding the Muon Lifetime Experiment

Primary cosmic rays are particles such as protons and neutrons moving at high energies through the intersteallar medium. Locally, many of these are ejecta from the sun. When these primary cosmic rays come toward earth they encounter atmospheric nuclei at around 30,000 m above the surface. The imapacts cause nuclear reactions which produce pions. The pions decay into muons; this generally occurs at around 9000 m altitude. The muons rain down upon the surface of the earth, travelling at about 0.998c. Many decay on the way down while others reach the surface. A few of those will encounter Jeff and Ed's muon detector. These muons constitute "secondary cosmic rays" and have paths which are indicated by the red arrows on the diragram. Note that this diagram shows a "muon shower" on the Alps...such an event on the Rockies would look much the same!

Most of the cosmic ray muons will pass right through the scintillator portion of the detector. Each one, as it passes though, will excited an atom doped into the scinitllator material. This atoms will then return to ground state, emitting a photon. However, this detector is designed to ignore individual muon "hits" and instead look for a rarer phenomenon.

Some muons will lose all of their kinetic energy upon entering the scintillator. This will cause an atom to absorb energy and then release a photon, just as described above. However, those muons which have lost their kinetic energy will remain in the scintillator until they decay after some time dt into an electron and two neutrinos. (Forget trying to detect neutrinos with this device!) That decay causes an energy release and therefore another photon to be ejected. This is a "double hit" in which two photons are produced in rapid succession at times t and t+dt.

The detector is designed to look for double hits and ignore single hits. Of course there will be some coincidental double hits that occur when two energetic muons just happen to strike the scintillator in rapid succession. This is background that will have to be subtracted out. The detector will record a large number of double hits and send them to a computer. MS Excel is then used to put the hits into time bins; simple statistical analysis will reveal the muon lifetime.

A more thorough diagram of the detector assembly is on the left. This particular diagram shows an oscilloscope as the device used to read data; substitute a data acquisition card and a computer for the oscilloscope.

RETURN
to finding the lifetime of the muon