Cosmic rays were first discovered by the Austrian-born physicist Victor Hess in 1912. Making a balloon flight, he measured the level of ionising radiation in the atmosphere and found that it increased with altitude. He concluded that the radiation was non-terrestrial in origin.
Later, people realised that cosmic rays were charged particles because they were affected by a magnetic field. In fact, the field of particle physics originated in cosmic-ray research: the first exotic particles identified – the muon, pion, positron, lambda and kaon – were all discovered in cosmic rays.
Today, high-energy cosmic rays are still used to make unique particlephysics measurements at energies well above those attained in laboratory accelerators.
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