Transmutation of the lithium isotope of mass seven by deuterons

Abstract

Recent investigations on the transmutations of the light elements have shown that the mass number 5 is the only one not occupied by a stable element. In a letter to ‘Nature,’ Dr. Oliphant has pointed out that the mass defects of the light elements show a periodic variation, and has deduced from this that the missing element should have a mass of about 5·0125. It is to be expected that the place should be occupied either by an isotope of helium or of lithium of mass five, but more probably by the former. These investigations were initially undertaken to test whether the isotope of helium of mass five arose in certain transmutations. In considering the whole field of artificial transmutations, experience has shown that practically every type of reaction which is energetically possible does take place, but the relative frequency of each reaction is governed by factors which are not yet understood. Among the light elements the nuclei 1 H, 2 D, 3 T, 3 He, 4 He, and 6 Li, 7 Li, 8 Be, etc., all appear as products of nuclear transmutations, and it is reasonable to expect that the nucleus of mass five, if it exists, should also make itself evident. For example, we might expect that the transmutation of 7 Li when bombarded by deuterons should proceed in the alternative ways: 7 Li + 2 D → 4 He + 4 He + 1 n + W 1 (1) → 4 He + 5 He + W 2 (2) → 8 Be + 1 n + W 3 . (3) The bombardment of 7 Li with deuterons is therefore a likely experiment in which to search for a 5 He nucleus. It has been shown that (1) results in a continuous distribution of α- particles with energies ranging from the lowest observable (0·6 m. e-volts) to a maximum of 8·1 m. e-volts. The reality of (2) would be revealed by the presence of two homogeneous groups of particles superimposed on this continuous distribution. The work described in this paper has been done in order to ascertain whether or not the second of the above trans­mutations does occur.