PHYS 242 Lecture 5: PHYS242_Lecture_07-09

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For a change we now will discuss a real experiment as performed in. 1963 by frisch and smith, two us american physicists. This experiment is concerned with the measurement of cosmic ray muons. Muons ( ) are very similar to electrons but ~200 times more massive and instable. Like for radioactive decay the probability for a muon to decay is the same at any time. This leads to an exponential decay law: (cid:1840)(cid:4666)(cid:1872)(cid:4667)(cid:3404)(cid:1840)(cid:2868)exp(cid:4666)(cid:3398)(cid:1872)/(cid:2028)(cid:4667) where (cid:1872) is the time, (cid:1840)(cid:4666)(cid:1872)(cid:4667) is the number of muons at time (cid:1872), (cid:1840)(cid:2868) is the initial number of muons and (cid:2028)(cid:3404)(cid:884). (cid:884) s is their mean live time. This decay law follows from fundamental properties of the decaying particle and as such should be according to einstein"s second postulate identical in all inertial reference frames. Therefore the decay can be used to measure the time: (14) (cid:1872)(cid:3404)(cid:3398)(cid:2028)ln(cid:4666)(cid:1840) (cid:1840)(cid:2868) (cid:4667) (15) (cid:4667)(cid:3406)(cid:885)(cid:882) vertical distance from top of mt.

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