Positron, Discovery of

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The positron, the antiparticle of the electron, was discovered in two steps. The first and crucial one was by Carl D. Anderson, who in 1932 concluded the existence of a positive particle of electronic mass (positive electron) from the tracks left by cosmic rays in a cloud chamber and in 1936 was awarded the Nobel Prize in Physics for it. The second step entailed the production of positive electrons by means of radioactive sources and the identification of Anderson's particle with the antielectron, whose existence had been suggested by Paul A. M. Dirac in 1931. The whole process took some fifteen months, from Anderson's first communication to Science in September 1932 to Dirac's Nobel lecture on his "Theory of Electrons and Positrons" in December 1933.

Beginning in 1930, upon completing his Ph.D. at the California Institute of Technology (Pasadena), Anderson joined in Robert A. Millikan's long-lasting research program on cosmic radiation, regarded since the early 1910s as a very energetic gamma radiation of extraterrestrial origin. Millikan expected to provide new evidence that the energy of incoming cosmic photons corresponded to the mass defect of light atoms as built from hydrogen, as he polemically thought.

Rather than measuring the absorption of primary cosmic rays by means of an ionization chamber, as was common practice, Anderson undertook the study of secondary particles by means of a cloud chamber that fitted into the powerful electromagnet of Caltech's Guggenheim Aeronautical Laboratory. In the cloud chamber, the sudden expansion of a vapor-saturated container prompted the formation of droplets on the ions left by an ionizing particle in its path. Particles were thus visualized as cloud tracks that could be photographed. If the chamber is placed in a magnetic field, the curvature, range, and ionization density along the tracks provide information about the particle mass and velocity as long as the tracks were clearly visible and not affected by turbulence.

By November 1931, Anderson had a dozen good pictures that showed as many positive as negative particles and frequent instances of a "simultaneous ejection." Anderson, who like most physicists stood by the two-particle paradigm that held matter to consist of just electrons and protons, attributed the positive tracks to the one positive particle known at the time, the proton, and pointed to nuclear disintegration as their probable origin. The ionization density of positive tracks, however, was consistent with a particle much lighter than the proton, and Anderson first thought they might well be electrons traveling upward.

Anderson next inserted a lead plate across the chamber. A particle crossing the plate would lose energy, and the increase in the track's curvature would reveal the direction of movement. Through August 1932 Anderson took new photographs and analyzed several hundred tracks. Three of them showed events that were either due to a light positive particle going through the plate or the simultaneous ejection of an electron and a small-positive. This was the basis of Anderson's communication to Science announcing "the possible existence of a positive electron" (1932).

Anderson did not relate the new particle to Dirac's antielectron, nor did he refer to Dirac's hole theory as a likely mechanism of production. Dirac's ideas were not widely known nor generally accepted at the time, and Anderson referred instead to Millikan's ideas about the cosmic genesis of elements. The discovery of the positive electron was not prompted by the search for antimatter; indeed, the positive electron was originally not an antiparticle at all.

The new particle was met with caution because of its cosmic descent—the nature of cosmic rays was disputed—and the paucity of visual evidence—Anderson had not published any picture. Anderson's note, however, did not go unnoticed. At the Cavendish Laboratory in Cambridge, UK, Patrick M. S. Blackett and Giuseppe P. S. Occhialini had been working since 1931 on a cloud chamber controlled by an electronic coincidence device. The expansion of the chamber was triggered by the simultaneous discharge of two Geiger-Müller counters, one above and one below the chamber. Cosmic ray particles were thus made "to take their own cloud photographs" (Blackett and Occhialini 1932, 363) and 70 percent of their pictures, as compared with Anderson's 2 percent, showed significant events. Blackett and Occhialini attributed 14 tracks "almost certainly" to positive electrons. They presented their case before the Royal Society on February 16, 1933—an event hailed by the press and reported by Science Service, after consulting Anderson, as the "New Particle of Matter Christened 'Positron'" (1933). Dirac's hole theory was referred to by the Cavendish experimentalists as a likely mechanism of production. However, the relationship was not spelled out, and all positive electrons observed so far proceeded from cosmic radiation. Influential physicists such as Niels Bohr and Wolfgang Pauli remained skeptical on both counts.

At the main European centers for radioactive research—the Cavendish in Cambridge, the Institut du Radium in Paris, and the Kaiser-Wilhelm-Institut für Chemie in Berlin—physicists set out to produce positrons in the laboratory by means of radioactive sources, which were far better known and more serviceable than cosmic rays. By late March it was clear that the radiation from a beryllium target exposed to a polonium source—the radiation used to produce neutrons—was also able to produce positrons in lead; early in May several laboratories reported that positrons were ejected from a lead target exposed to high-energy gamma rays, such as those from ThC (208Tl). All the while theoretical physicists, including Rudolf Peierls, Max Delbrück, J. Robert Oppenheimer, and Milton S. Plesset, were trying to make sense of the behavior of positrons by means of Dirac's theory of the electron. Experimental and theoretical developments were assembled in a number of scientific meetings in the fall of 1933, including above all the Seventh Solvay Conference, held in Brussels late in October 1933. By the end of the year, Anderson's particle had been unambiguously identified with Dirac's antielectron, and the positron was born.

The positron thus became the first antiparticle to be discovered. The manipulation of positrons provided early evidence that matter could be created and annihilated—direct confirmation of Einstein's mass-energy relationship. The theoretical analysis of electron-positron interactions helped clarify the stance of early quantum electrodynamics—the quantum theory of the electromagnetic field—and played a key role in the formulation of renormalized quantum electrodynamics in the late 1940s, especially in Richard Feynman's version. Together with the neutron, the positron broke the simple dual paradigm of matter and paved the way for elementary particle physics.

See also:Anderson, Carl D.; Antimatter; Dirac, Paul


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