Uranium Z and the problem of nuclear isomerism.

Abstract

It is well known that the idea of the possible isotopic complexity of the elements arose first as the result of investigations into the chemical behaviour of substances produced one from another in the course of successive radioactive transformations. Soon after this idea of isotopy had been generally accepted, Soddy (1917, 1919) suggested a further basis of classification which he believed might also be required in certain cases. According to this suggestion, nuclei which were indistinguishable in respect of charge and mass might still exhibit distinct radioactive properties, or might differ in “any new property concerned with the nucleus of the atom rather than its external shell”. He proposed in effect to classify distinct isotopic species as isobaric or heterobaric, depending upon whether the same or different mass numbers had to be assigned to them. In particular he suggested that the disintegrations of the branch products thorium C' and thorium C", which certainly result in isobaric species of the same atomic number, might in fact give rise to isotopes which were radioactively distinct. This suggestion was tested experimentally by S. Meyer (1918), but was not substantiated. Nevertheless, three years later, uranium Z was discovered by Hahn (1921)—and its chemical and radioactive properties interpreted by him as evidence for nuclear isomerism, in this case with uranium X 2 . Subsequent work by Hahn (1923), Guy and Russell (1923) and Walling (1931) confirmed this interpretation. Uranium Z and uranium X 2 were certainly isotopic, and as both were produced, as it seemed directly, from uranium X 1 the most natural assumption to make was that they were also isobaric. No further examples were found, however, and no attempt at a theoretical description of the phenomenon was made for some time. Then, in 1934, Gamow put forward an explanation based upon the idea of an additional fundamental particle, the negative proton, and, from the experimental side, the discovery of artificial radioactivity opened up entirely new possibilities of isotopy. Now, if v. Weizsäcker’s (1936) explanation of nuclear isomerism is generally preferred to Gamow's, at least it appears certain that several instances of the phenomenon are open to experimental study (Szilard and Chalmers 1935; Bothe and Gentner 1937; Meitner, Hahn and Strassmann 1937; Snell 1937). For that reason we have thought it important to begin such a study by a more detailed investigation than has hitherto been made of the radiations from uranium Z and the relations between this body and its neighbours in the series. The present paper describes the results of this investigation. So far as they have been taken they are distinctly favourable to v. Weizsäcker’s hypothesis; they suggest, therefore, that a precise statement of the hypothesis should be made the basis of our discussion.