Web measurement of muon decay

This page provides a way to determine the muon lifetime. The muon is an elementary particle, one of the three leptons in the Standard Model. It does not decay very fast, because the only decay mechanism occurs by the weak interaction.

There would not be any muons to detect if they were not continually produced. This happens by the impact of cosmic rays on the atmosphere. The "rays" are mostly protons and other nuclei with very high energies. At heights of over ten kilometers this gives rise to muons that travel to the earths surface with very high kinetic energies, almost at the speed of light (300 meter per microsecond). These muons penetrate the atmosphere and also buildings.

Still, the muons are charged particles and they loose energy by ionisation along their trail. Somewhere they must come to a stop. Sometimes this happens inside our detector and this one can see on our web-enabled oscilloscope.

The horizontal scale is one microsecond per division. The green trace shows the signal of a photomultiplier tube coupled to a piece of plastic scintillator. The oscilloscope is set to trigger on this signal when a second event occurs within 9 µs of a preceding event. The offset makes that the second peak is displayed 4 µs to the right of the center. Almost always this is a peak that is caused by muon decay (detected by the ionization trail of the resulting electron or positron).

The preceding green peak is due to the ionization trail of the muon when it enters the plastic. The time difference is the decay time. Decay is a random process. It will take several events before one can make an estimate of the muon lifetime. Decays inside the block scintillation material are rare: an hour can go without events, so be patient.

The yellow trace of the oscilloscope shows the signal from a detector located above the detector in which the decay is measured. Often the muon that is detected in the plastic will have traversed this detector on its way from the upper atmosphere. It is wider because sodium iodide i a slower scintillator material than the plastic detector. When the muon path is not vertical this peak will be absent. Sometimes the NaI detector may show other peaks.