Three papers have recently appeared describing investigations of the absorption of the very hard γ-rays from thorium C˝ in a number of elements, and in each case the conclusion was drawn that the results indicate that these very high energy quanta are capable of interacting with certain atomic nuclei. The three sets of experimental results do not agree in detail, but in each case the trend of the variation of absorption coefficient with atomic number is very different form the Z3 law characteristic of photoelectric absorption. Moreover, the difference between the absorption coefficients per electron in heavy and light elements, which has hitherto been attributed to photoelectric absorption, is nearly as great for the hard thorium C˝ γ-rays (hv=2·65. 106 ε-volts), as for the much softer redium C γ-rays (hv ~ 1·8. 106 ε-volts), in marked contrast with the rapid variation with wave-length observed in the softer γ-ray region of the spectrum. Very little is at present know concerning the magnitude of the photo-eletric effect in the γ-rays region, and since absorption coefficients measured in the usual manner are the sum of two terms referring respectively to scattering and to photoelectric absorption, it is essential for the further elucidation of the problem to study the actual scattered radiation.
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L. H. Gray (1931) studied this question.