The ion beam struck plates made of the substance under investigation, mounted on a rotating disk, which made it possible to study a number of elements under one and the same conditions. The correct position of the beam was established by the luminescence it produced when it struck a fluorescing screen. The radiation arising in the plates was led out of the tube through a thin mica window \((1.3\ \mathrm{mg}/\mathrm{cm}^2)\). Its intensity was measured with an ionization chamber.

In order to show that the radiation under study is produced by the mercury ions themselves, and not by any other factors, the author carried out a number of control experiments. First of all it was shown that it is not produced by X-rays that might arise when electrons strike the metallic parts of the tube accelerating the ions. For this purpose a thin metallic foil was placed in the path of the ion beam, completely stopping the ions but “transparent” to X-rays. When the ion beam was screened off, no radiation was observed. Consequently, the X-rays arising in the tube play no role. By applying electric and magnetic fields it was also shown that the radiation is not produced by electrons that might reach the emitting plates. Thus the cause of the radiation is the mercury ions. The radiation under study, as shown by absorption experiments in its various media, is electromagnetic radiation, at least for the most part.

To determine the wavelength of this radiation the author measured its absorption coefficient in aluminum and in air. These measurements made it possible to conclude that the radiation under investigation is the characteristic X-radiation of the substance of the plate upon which the mercury ions fall. The fact that the radiation is absent for some elements, but is observed for elements with both larger and smaller atomic numbers, confirms the conclusion given above.

Unfortunately, the author’s apparatus allowed measurements to be made only with X-rays that were not too soft. Therefore no general survey over a large number of elements can be made. However, the basic regularities appear sufficiently clearly.

The experimental data show that in light elements, for example in Al, \(K\)-radiation is excited, whose intensity gradually decreases with increasing atomic number \(Z\). At a certain \(Z\), \(L\)-radiation appears (Br, Mo, Ag). Its intensity, in turn, decreases with increasing atomic number. At large \(Z\), \(M\)-radiation is observed (Pb, Hg). The same regularities were observed earlier by Fränz and Bothe, who investigated the X-radiation arising when a substance is bombarded with \(\alpha\)-particles.

Coates showed that the intensity of the X-radiation arising when a substance is bombarded with mercury ions increases strongly with increasing ion energy. It was established in this connection that for ion energies less than \(3 \cdot 10^5\) e-V no radiation is present.

To explain the observed radiation the author assumes that the nuclei of the mercury ion and of the atom of the irradiated substance approach to distances sufficient to produce changes in their molecules. In the disintegration of such a molecule, electrons of some shell may be redistributed in such a way that in the atom of the irradiated substance or in the mercury ion one or several electrons will be lacking. When these vacant levels are filled, X-radiation appears.

L. Groshev

Literature

1. Coates, Phys. Rev., 46, 542, 1934.