The artificial production of nuclear γ-radiation

The study of the γ-radiations emitted by atomic nuclei has greatly increased in interest and importance in recent years owing to the theories connecting these radiations with the intimate structure of the nuclei. Many attempts have been made to excite atomic nuclei to radiation by external stimulation, mainly by bombardment with the α-, β-, γ-rays emitted by radioactive bodies. The first evidence of the artificial stimulation of nuclear γ-radiation was obtained by Slater in 1921, who found that a small amount of penetrating γ-radiation was produced when the elements tin and lead were bombarded by the α-particles emitted from radon. The experiments were very difficult on account of the rapid growth of radium B and C, but the consistency of the results, control experiments with paper substituted for lead, and the absorption coefficients of the radiations, all indicated that the results were trustworthy. Slater estimated that the fraction of the radon α-particles passing through the ead which produced γ-radiation was of the order 1 in 6000; in the case of tin this fraction was about 1 in 12000. Subsequent experiments by other investi­gators, including the present writer, using polonium α-particles instead of radon α-particles, showed definitely the absence of effects of this order of magnitude. Since polonium α-particles have only slightly less energy than radon α-particles, the observations are very difficult to reconcile. Later in this paper a possible explanation of this discrepancy will be suggested. Two years ago the writer obtained evidence, which will be mentioned later, of the production of penetrating y-radiations when aluminium was bombarded by polonium α-particles, and Bothe and Becker have definitely established the production of nuclear γ-radiation in several of the lighter elements by α-particle bombardment. The effects are, however, in every case of a much smaller intensity than those found by Slater. The amount of radiation observed is indeed so small that very strong sources of polonium are essential, and on this account the writer has only recently been able to make a detailed study of the radiations.