The absorption in lead of the γ-rays emitted by radium and radium C

Abstract

In a previous paper by Rutherford and the author attention has been drawn to the fact that the two types of γ-radiation emitted by radium B and radium c which are exponentially absorbed by aluminium both show irregular absorption curves when lead is used as the absorbing material. The curve obtained for pure radium C was observed to fall far more rapidly than was to be expected only after traversing a thickness of 1·5cm of lead. The absorption curve in lead of the γ-rays from radium B was obtained by taking the difference between the radium (B+C) and the radium C curves. The results so obtained were not determined with very great accuracy, but they served to show that in this case, to, the absorption is not exponential , and that the absorption coefficient rapidly diminished from about μ =11(cm. -1 ) to μ =2(cm -1 ). The accuracy of the curves did not, however, permit of their complete analysis as in the case of those previously obtained for aluminium. During the course of his work on characteristic radiations Barkla has Pointed out and investigated the anomalous effect on the absorption of a characteristic radiation by an element whose atomic weight is near to that of the element which emits the radiation. His experiments were, however, confined to elements of comparatively low atomic weight. As the atomic weights of radium B and radium C can only differ by a small amount, and as they have atomic numbers differing only by unity, viz., radium B=82 and radium C=83, it seemed of importance to determine accurately the absorption curves in lead, and to examine whether any additional information can be obtained which may indicate whether the radiations emitted by radium B and radium C are characteristic of these elements and fall into the series given by Barkla.