Page 14 - Neutrons for Sciences and Society
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CHAPTER 1 Pre-history
The neutron was discovered in 1932 by James Chadwick (Fig. 1.1 and Fig. 1.2). It is a neutral particle with a mass very close to that of the proton, both of which are constituents of atomic nuclei. The free neutron is produced in certain nuclear reactions. The first, which led to its discovery, was the collision between alpha particles and the nuclei of beryllium:
4He + 9Be → 12C + n
During initial years of study it was this reaction which was used
in the construction of neutron sources. Such a source led to the discovery in 1939 by Hahn and Strassman that the nucleus of uranium3 would undergo fission, induced by capturing a neutron. It was shortly noted that this fission process, in addition to producing energy, was accompanied by the emission of neutrons (generally two or three); theoretically this could
Fig. 1.1: James CHADWICK, Nobel prize winner for physics (1935)
3 More precisely, the nucleus of 235U (0.7% of natural uranium) is the only nuclide existing in any appreciable amount in nature that is fissile with thermal neutrons.
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