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Yukawa, Hideki

nuclear force meson discovered

(1907–81) Japanese physicist: first described the strong nuclear force and predicted the pi-meson (pion).

Yukawa studied at Kyoto University and took his degree there in 1929. He moved to Osaka University to take his doctorate but returned to Kyoto for the remainder of his career, becoming professor of theoretical physics in 1939.

When he was 27, Yukawa developed his theory of nuclear forces. In 1932 had discovered the neutron; Yukawa proposed a strong short-range force between protons or neutrons, which overcame electrical repulsion between the protons in the nucleus without influencing the electrons in the atom. This nuclear ‘exchange’ force involves the exchange of a particle between the nucleons (nuclear constituents) and from the short range of the force (less than 10 –12 m) Yukawa inferred that its mass was about 200 times that of an electron.

In 1936 discovered a particle of the correct mass and called it the mu-meson (now, muon); but it did not interact with nucleons sufficiently strongly to correspond with Yukawa’s prediction. In 1947 and his co-workers discovered another meson (the pi-meson, now pion) in cosmic rays which did correspond with Yukawa’s proposed particle and so established his theory of the strong nuclear force. During the next 10 years many types of pion and muon were discovered, most with short lifetimes. They are now considered (as are protons and neutrons) to be composed of quarks; and the forces in nuclei to be due to interactions between quarks and gluons.

Yukawa successfully predicted (in 1936) that nuclei may absorb one of the innermost electrons (in the 1 K shell) and such K capture by a nucleus was soon observed. He was the first Japanese to be awarded a Nobel Prize, in 1949.

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