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Anderson, Carl David

discovered particles meson positron

(1905–91) US physicist: discovered the positron and the muon.

Anderson, the only son of Swedish immigrants, was educated in Los Angeles and at the California Institute of Technology, where he remained for the rest of his career.

Anderson discovered the positron accidentally in 1932 (its existence had been predicted by in 1928). As a result, Dirac’s relativistic quantum mechanics and theory of the electron were rapidly accepted and it became clear that other antiparticles existed. Anderson shared the 1936 Nobel Prize for physics with for this discovery.

Anderson discovered the positron while studying cosmic rays, which he did by photographing their tracks in a cloud chamber in order to find the energy spectrum of secondary electrons produced by the rays. A lead plate divided the chamber so that the direction of movement of the particles could be deduced (they are slowed or stopped by the lead). Also, a magnetic field was applied to deflect particles in different directions according to their charge and by an amount related to their mass. Many positive particles were seen which were not protons; they were too light and produced too little ionization. Anderson identified their mass as about that of an electron, concluding that these were positive electrons, or positrons. The discovery was confirmed by and the following year.

Anderson discovered another elementary particle within the same year, again by observing cosmic ray tracks. It had unit negative charge and was 130 times as heavy as an electron, and seemed a possible confirmation of theory of a particle communicating the strong nuclear force (now called a pi-meson or pion). However a series of experiments by Anderson in 1935 revealed that it was not and the role of this mu-meson (or muon), as it is now called, remained unclear. The true pi-meson was first found by in 1947. Positrons are inherently stable, but as they are antiparticles of electrons the two annihilate each other. Mesons are intrinsically unstable and decay rapidly.

Anderson,“Cat”(William Alonzo),jazz [next] [back] Anderson,“Buddy”(Bernard Hartwell)

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