The abnormal absorption of heavy elements for hard γ-rays

Abstract

By absorption measurements of the hard γ-rays from ThC", which are the most homogeneous type of γ -rays obtainable, we can now, in the case of light elements, prove the validity of the theoretical scattering formula of Klein and Nishina, and for heavier elements find the existence of an extra-absorption which is not yet accounted for in that formula. There are several factors which might contribute to the abnormally large absorption coefficient of hard γ-rays in heavy elements, and at present we know fairly definitely that at least two of them do exist. They are: (1) the photo-absorption of the shell-electrons; (2) nuclear absorption. L. H. Gray has elsewhere given the evidence for the first effect. The existence of the nuclear absorption in the case of heavy elements has been confirmed by the discovery of a new scattered radiation, that is, a secondary radiation which is other than that predicted by the Klein-Nishina formula. It is not the intensity of the new radiation that establishes the nuclear absorption, but the change of wavelength, which could hardly be explained in any other way. This change of wavelength suggests immediately that the nucleus is perhaps first left in an excited state by the interaction, which might be a disintegration or merely an excitation, and then the emission of one or more new quanta follows. Should such a mechanism exist we should also expect the existence of a nuclear excitation potential or a disintegration potential. Now, the investigation of excitation potentials (or disintegration potentials) requires a continuous range of wavelengths which is not easy to obtain. This can, however, be secured by the use of the scattered rays when a beam of homogeneous γ-rays from ThC" falls on a light scatterer. From Compton-Debye’s theory we have λ = λ 0 + h / mc (1- cos θ), where λ 0 and λ are the wave-length of the primary and scattered rays, and θ is the angle of scattering. In this way we have an available range of λ from 4·7 to 4·7 + 48·5 XU, and we can measure the absorption coefficient of a heavy element for different wave-lengths within this range. As we pass from the short to the longer wave-lengths we should find a sudden change of the absorption coefficient if there exists a sharp excitation (or disintegration) potential, i. e ., if the nuclear absorption suffers a sudden change at a certain wave-length. Although it may be that no sharp nuclear excitation potential exists, a point of view which is not unreasonable when we consider the continuous nature of the β -ray spectra, such experiments are nevertheless important for two reasons: (1) it is interesting to know the variation of the nuclear absorption with λ ; (2) the same experiment gives information on the magnitude of the ordinary photo-effect for short λ . The difficulty of such an experiment lies in the weak intensity of the scattered rays, and in the lack of a high degree of homogeneity, since a narrow range of the scattering angle results in an enormous loss of intensity. By the use of a pressure ionisation chamber and a sensitive Hoffmann electrometer measurements have, however, been made which lead to interesting results.