Artificial Radioactivity Induced by γ Rays

Until recently only two cases of the disintegration of nuclei by photons were known, namely: the disintegration of nuclei of heavy hydrogen and beryllium by the γ rays of Ra and ThC″. In both cases neutrons are emitted as a result of the reaction. Attempts to detect an analogous effect in other elements for the very same photons did not lead to positive results.

On the other hand, in recent years a number of authors have established that, when lithium is irradiated with fast protons, γ radiation arises whose energy reaches 17 MeV.

Recently Bothe and Gentner (Naturwiss. 25, 90 and 126, 1937) attempted to detect the disintegration of nuclei of elements by this radiation. To obtain γ rays they bombarded lithium with protons of about 500 kV energy. Their experiments showed that certain elements (P, Cu, Br, Ag, Sb), under the action of the γ radiation arising in lithium, become radioactive (the β activity was measured by means of a counter with a thin window). Analyzing the decay curves of the intensity of the artificial radioactivity as a function of time, the authors found that some of the investigated elements possess one decay period, while another part of the elements—for example, Br* and Ag—possess two periods; moreover, for Br, Ag, and Sb decay periods were found which should apparently be ascribed to new isotopes of these elements hitherto unknown. In the case of Br, it was also shown that the active element is the isotope Br. One of the periods of active bromine ($T = 18$ min.) coincides with the decay period of the radioactive element that arises when Br is irradiated with neutrons** [$^{79}\mathrm{Br} + {}^{1}\mathrm{n} = {}^{80}\mathrm{Br}$ (18 min.)]. Therefore here one may suppose, with sufficient justification, that the disintegration of Br by a γ quantum proceeds according to the following reaction:

\[ {}^{81}\mathrm{Br} + \gamma \longrightarrow {}^{80}\mathrm{Br} + {}^{1}\mathrm{n}, \]
\[ (18\ \text{min}) \]

i.e. the disintegration process consists in the emission of a neutron; in this case, after emission of the neutron, the nucleus remains in an unstable state, thereby accounting for the presence of radioactivity. In the authors’ opinion, all the cases investigated are compatible with the idea that the incident photons tear neutrons out of the nuclei. The effective cross section for such processes is of the order of $10^{-27}\ \mathrm{cm}^{2}$.

To prove that the artificial radioactivity is produced precisely by γ rays, the authors carried out the following experiments. They measured the activity for Cu and Br at different energies of the protons producing the γ rays. The measurements showed that the activity changes with the proton energy in the same way as the γ radiation, which in this energy region has a sharply expressed resonance maximum.

Thus, there is reason to believe that the artificial radioactivity observed by them is indeed induced by γ rays.

L. Groshev, Moscow

* According to the latest data of Bothe and Gentner (Naturwiss. 25, 284, 1937), in the case of Br three different decay periods are observed (note added in proof).

** When Br is irradiated with neutrons, three decay periods are observed.