Secondary γ-rays produced by β-rays

When the cathode rays of a vacuum tube impinge on any material they produce the X-rays, which are not deviated by a magnetic field, and are much more penetrating than the cathode rays which produce them. We might, therefore, expect that when theβ -rays from radioactive substances impinge on a plate, similar penetrating rays would be emitted from the plate. Such a penetrating type of rays, theγ -rays, is almost invariably associated with theβ -rays, but it has generally been thought that theseγ -rays are due to the expulsion of theβ -ray from the radioactive atom. In some cases they are certainly not due to the impact ofβ -rays on external objects, theγ -rays of radium C being an instance of this. Here theγ -rays come; from the radioactive atoms, and in such amount that they effectually mask the possible production ofγ -rays byβ -rays as the experiments of H. Starke show. Starke attempted to find whetherβ -rays did produceγ -rays. He used for this purpose 6 milligrammes of radium bromide contained in a very thin glass tube, which let most of theβ -rays out. Theγ -rays from this ionised the air in an electroscope, the walls of which were thick enough to absorb all theβ -rays. He looked for an increase in the ionisation when various materials were placed just behind the radium. He found practically no difference in the reading, and, from that and a similar experiment in which he deflected theβ -rays away from the electroscope by a magnetic field, concluded that no measurableγ -radiation was caused by theβ -rays of radium C. His experiments show that, ifγ -rays are produced, they form a very small proportion of those coming from the radioactive atom.