Cosmic rays are energetic particles from outer space that penetrate and interact with the atmosphere, producing more particles. One of these particles that reaches the surface of the Earth is the muon. A muon is a “heavy” lepton (the same family as electrons) with a mean lifetime of 2.2 microseconds. Muons lose their energy continuously as they penetrate materials, like detectors, and some of them stop and decay inside the detector. I am measuring the mean lifetime using a sodium iodide (NaI) scintillation detector and nuclear instrumentation modules (NIMs).