The excitation of γ -rays by the α -rays of ionium and radiothorium

It has been shown by one of us that theα -rays of radium C, when they impinge upon matter, excite a small but detectable amount ofγ -radiation. Owing to the presence of the intense primaryγ - radiation which accompanies theα -rays of radium C, however, a detailed investigation of this excitedγ -radiation was not possible. With an intense source of ionium placed at our disposal by Prof. Rutherford it has been possible for us, not only to show clearly thatγ -rays are excited by theα -rays of ionium, but also, owing to the absence of products emittingβ andγ -rays, to make a detailed study of the nature of this excitedγ -radiation. The results of these experiments are given in this paper. We have found also thatγ -rays are emitted from radiothorium, and a short account of the work which led to this result is given here also, although, as will be explained later, it was not possible for us to make a systematic study of their nature.