A positron, however, does not endure long after formation. All about it were atoms containing electrons. It could not move for more than a millionth of a second or so before it encountered one of those electrons. When it did, there was an attraction between the two, since they were of opposite electric charge. Briefly they might circle each other (to form a combination called “positronium”) but only very briefly. Then they collided and, since they were opposites, each cancelled the other. The process whereby an electron and a positron met and cancelled is called “mutual annihilation”. Not everything was gone, though. The mass, in disappearing, was converted into the equivalent amount of energy, which made its appearance in the form of one or more gamma rays. (It works the other way, too. A gamma ray of sufficient energy can be transformed into an electron and a positron. This phenomenon, called “pair production”, was observed as early as 1930 but was only properly understood after the discovery of the positron.) Of course, the mass of electrons and positrons is very small and the amount of energy released per electron is not enormously high. Still, Dirac’s original theory of antiparticles was not confined to electrons. By his theory, any particle ought to have some corresponding antiparticle. Corresponding to the proton, for instance, there ought to be an “antiproton”. This would be just as massive as the proton and would carry a negative charge just as large as the proton’s positive charge. An antiproton, however, is 1836 times as massive as a positron. It would take gamma rays or cosmic particles with 1836 times as much energy to form the proton-antiproton pair as would suffice for the electron-positron pair. Cosmic particles of the necessary energies existed but they were rare and the chance of someone being present with a particle detector just as a rare super-energetic cosmic particle happened to form a proton-antiproton pair was very small. [Illustration: _The Bevatron began operation in 1954._] Physicists had to wait until they had succeeded in designing particle accelerators that would produce enough energy to allow the creation of proton-antiproton pairs. This came about in the early 1950s when a device called the “Cosmotron” was built at Brookhaven National Laboratory in Long Island in 1952 and another called the “Bevatron” at the University of California in Berkeley in 1954. Using the Bevatron in 1956, Segrè (the discoverer of technetium who had, by that time, emigrated to the United States), the American physicist Owen Chamberlain (1920- ), and others succeeded in detecting the antiproton.