TE Coan Outreach

Pedagogical Muon Lifetime Experiment

Using the readily available (and reliable!) flux of muons at sea level, we measure the muon's lifetime by the standard technique of using a fat cylinder of plastic scintillator to slow and stop some of the muons, and a single photomultiplier tube to view the two scintillator flashes associated with the entry and decay of the stopped muons. The time between the two flashes is then histogrammed and fit with an exponential distribution to extract the lifetime. In the right-hand figure, the scintillator and photomultiplier tube are contained in the black cylinder and the small box contains the measuring electronics.

A muon enters the scintillator, slows and then stops, causing the scintillator to emit a flash of light as the kinetic energy of the muon is transferred to the scintillator. Some time after the muon stops, its decay into an electron and two neutrinos produces another flash of scintillator light. Unlike conventional approaches to time the 2.2 μsec muon lifetime that rely on expensive CAMAC and NIM circuitry often found only in particle physics research labs, we designed and developed timing circuitry that uses the properties of a special integrated circuit (IC), a field programmable gate array (FPGA). This flexible IC can be programmed to behave in many ways and its incorporation into a printed circuit board is relatively inexpensive. The figure to the right is a block diagram of the overall instrument.

The result of our measurement for roughly 23,000 muon decays is shown in the next figure, with the muon lifetime result shown in the upper right hand corner. The result is slightly less than the familiar free space value of because the muons whose decays we observe are measured in matter, and (negative) muons in matter have a second decay channel in addition to their spontaneous decay.

Contact either Thomas Coan or Jingbo Ye for more details.

<email to coan@mail.physics.smu.edu>

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