100 YEARS AGO

All the experimental evidence so far obtained is now in agreement with the view that the γ rays are an extremely penetrating type of Röntgen rays which have their source in the atom of the radio-active substance at the moment of the expulsion of the β or kathodic particle. For example, I have found that the γ rays from radium always accompany the β rays, and are always proportional in amount to them. In radium the β and γ rays appear only in the third change occurring in the radio-active matter which causes “excited activity,” i.e. in the fourth of the chain of radio-active products which result from the disintegration of the radium atom... The γ rays arise from the disintegrated atom, and are not secondary rays set up by the bombardment of the radium as a whole by the β rays.

E. Rutherford

From Nature 10 March 1904.

50 YEARS AGO

During the past year, evidence has gradually accumulated for the existence of a new type of nuclear excitation which appears to be due to the presence, within a nucleus, of a neutral hyperon, now designated Λ0. It is believed that this particle can be bound to the other nucleons of a nucleus to form a relatively stable structure as measured on a nuclear time-scale, and that the eventual disintegration of such a nucleus is due to the decay of the Λ0-particle. The existence of the Λ0-particle, which until recently was referred to as the ‘heavy neutral V-particle’, was established by experiments on the cosmic radiation with Wilson chambers. It was shown to be the more common of the neutral particles which produce the ‘neutral V-events’ discovered by Rochester and Butler in 1947... These considerations suggest that the Λ0-particle is an excited nucleon in a different sense from that suggested by familiar analogies. We are entering a new field where basically new concepts remain to be established; but it seems reasonable to conjecture that the nucleon is transformed into an excited nucleon as a result of changes in its internal constitution. If so, we are beginning to make a new penetration into what Maxwell called “the strange strata of the material world”, a penetration into the world of the nucleon. It seems that matter is inexhaustible.

C. F. Powell

From Nature 13 March 1954.