The earth’s atmosphere is daily being bombarded by Cosmic Rays, consisting of high energy particles that travel from the sun or more distance places in the universe. The hits create showers of secondary particles, muons for example, that we can detect with our muon detector!
The muon detector consists of two plastic scintillators connected to corresponding photomultipliers, or PMTs. A particle that hits the scintillator can either deposit some of its energy into the scintillator or be completely absorbed. In both cases the scintillator will emit light that is caught by the PMT who in turn converts it to an electric signal. The detector treats it as a hit when there are coincident signals from each of the scintillators.
Why two scintillators?
The detector doesn’t only catch particle showers formed in the atmosphere but also radiation with lower energy. This low energy radiation will probably be stopped by the one of the scintillators, while radiation with higher energy will likely go through both. In this way we can be a bit more certain that we are measuring the right particles!