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Published:01 September 1956

The Neutrino

Naturevolume 178, pages446–449 (1956)Cite this article

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AnErratum to this article was published on 08 September 1956

EACH new discovery of natural science broadens our knowledge and deepens our understanding of the physical universe; but at times these advances raise new and even more fundamental questions than those which they answer. Such was the case with the discovery and investigation of the radioactive process termed 'beta decay'. In this process an atomic nucleus spontaneously emits either a negative or positive electron, and in so doing it becomes a different element with the same mass number but with a nuclear charge different from that of the parent element by one electronic charge. As might be expected, intensive investigation of this interesting alchemy of Nature has shed much light on problems concerning the atomic nucleus. A new question arose at the beginning, however, when it was found that accompanying beta decay there was an unaccountable loss of energy from the decaying nucleus1, and that one could do nothing to the apparatus in which the decay occurred to trap this lost energy2. One possible explanation was that the conservation laws (upon which the entire structure of modern science is built) were not valid when applied to regions of subatomic dimensions. Another novel explanation, but one which would maintain the integrity of the conservation laws, was a proposal by Wolfgang Pauli in 1933 which hypothesized a new and fundamental particle3 to account for the loss of energy from the nucleus. This particle would be emitted by the nucleus simultaneously with the electron, would carry with it no electric charge, but would carry the missing energy and momentum escaping from the laboratory equipment without detection.

The concept of this ghostly particle was used by Enrico Fermi (who named it the 'neutrino') to build his quantitative theory of nuclear beta decay4. As is well known, the theory, with but little modification, has enjoyed increasing success in application to nuclear problems and has itself constituted one of the most convincing arguments in favour of the acceptance of Pauli's proposal. Many additional experimental tests have been devised, however, which have served to strengthen the neutrino hypothesis; and also to provide information as to its properties. The very characteristic of the particle which makes the proposal plausible its ability to carry off energy and momentum without detection has limited these tests to the measurement of the observable details of the decay process itself: the energy spectra, momentum vectors and energy states associated with the emitted electron and with the recoiling daughter nucleus5. So, for example, an upper limit has been set on the rest mass of the neutrino equal to 1/500 of the rest mass of the electron by careful measurement of the beta-energy spectrum from tritium decay near its end point6, and it is commonly assumed that the neutrino rest mass is identically zero.

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References

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