Source: Cosmic rays interact strongly with matter. There is little matter in space, so they manage to reach Earth fairly easily. However, they interact with the air molecules at the edge of the atmosphere, forming pions.
Pions are very unstable and decay, 99.98% of the time according to the following model:
Thus, muons are created.
Properties: Muons have a mass 207 times greater than an electron, and about 1/9 that of a proton. The mean lifetime of a muon is 2.197 microseconds; relativistic effects cause this to be long enough for the particle to reach the Earth (non-relativistically, it would survive for only .67 km). Also, muons have a lot of energy, and can ionize other atoms easily. Interacts with other atoms through all except the strong interaction.
Muons in the Detector: Muons in the detector can cause all kinds of problems, and have to be accounted for. In KamLAND, for example, the following measures have been taken:
· The detector is surrounded by ultrapure water and PMTs. Anything entering the detector will cause Crenkov radiation in the tubes, which will be detected, labeling the muon as a muon.
· When a muon is located, events in the spatial and temporal vicinity of the muon will be negated.
· The other problem is muon spallation – when a cosmic ray busts up a nucleus – which is generally quite easy to detect and can be discarded.
Spallation Energy Spectrum:
Muon Energy Spectrum