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The Perse School, Cambridge, UK
Cosmic rays interact with the molecules in the upper atmosphere and in the process generate a type of subatomic particle called a muon. The muon is not a stable particle — it undergoes an exponential decay with a mean lifetime of 2.2µs. Even travelling at speeds very close to the speed of light, muons should only be able to travel about 460m before their number halves. Consequently, virtually no muons should be able to travel the distance from the upper atmosphere to the earth’s surface. However, under the theory of Special Relativity, these muons travelling at speeds near the speed of light experience a signiﬁcant time dilation eﬀect. This means that compared to an observer’s clock, the muons’ clocks tick much slower; so they can travel further (as measured in the observer’s frame of reference) in each mean lifetime. I carried out an experiment to detect these cosmic-ray muons, and thus observe a particle that can only reach the detector because of a relativistic eﬀect. I constructed a cosmic-ray telescope, using two Geiger-Muller tubes to give the detector directionality. I built an apparatus using a Raspberry Pi to carry out coincidence detection, and used it to take readings of the ﬂux of muons at diﬀerent angles to the vertical. Each reading was taken for 24 hours to improve the accuracy, and the data very closely matched the theoretical model for the muon ﬂux at diﬀerent angles to the vertical. I also took readings of the muon ﬂux at three diﬀerent altitudes up to 524m, and by seeing that the ﬂux increased with altitude conﬁrmed that I was detecting radiation of extraterrestrial origin.