deposition. From a solution containing rhenium, subsequent treatment precipitates rhenium sulfide, together with which there precipitates a radioactive element with a 13-minute half-life. In this way separation of the new elements is achieved.

In studying the separated active products it was established that the substance with a 90-minute half-life is not an element, but consists in turn of a mixture of two elements with half-lives of 50–70 min. and 2–3 days. These latter radioactive elements have so far not been separable by chemical methods. Possibly isotopes are involved here.

In the experiments described above one might have feared that, upon prolonged irradiation of uranium with neutrons, UX₁ (half-life 24 days) and UZ (6.7 hours) could be formed in appreciable amounts. However, the atomic checks that were carried out showed that this does not introduce appreciable distortions into the results obtained.

Thus the experiments of Fermi and his collaborators, as well as the experiments of Meitner and Hahn, indicate that elements with atomic numbers greater than 92 probably exist. True, as yet there are too few data to assign a definite atomic number to each of the new radioactive elements.

L. Groshev

ARTIFICIAL RADIOACTIVITY OF GASEOUS NITROGEN INDUCED BY DEUTERONS *

Artificial radioactivity, usually induced by fast particles, is observed on plates of a solid substance. However, in some cases, as Crane and Lauritsen have shown, radioactive centers may be in the gaseous state, diffusing into the gas from the surface of the irradiated plate. In this case, trajectories beginning inside the gas appear if the gas is introduced into a Wilson chamber.

Recently Livingston and McMillan * observed artificial radioactivity induced by a flux of deuterons directly in a gas. These authors established that certain substances—for example, platinum calcined to a white heat in an oxygen flame, and pure and oxidized copper—acquire artificial radioactivity with the same half-life, \(126 \pm 5\) sec., if they are bombarded with fast deuterons (energy about 2 MeV) in a vessel with air. Placing the plates in various gases and irradiating them with deuterons, the authors established that in the case of O₂, H₂, A the radioactivity is practically destroyed, but that it remains almost the same as for air in the case of N₂ and CO₂. True, for the last gas, in place of the 126 sec. period, a 10.4 sec. period characteristic of C is observed. These experiments show that the radioactivity of platinum irradiated in air by deuterons is caused by the radioactivity of gaseous nitrogen.

Further experiments showed that the radioactive atoms arising from nitrogen are very strongly bound to platinum. The authors believe that the strong “sticking” of radioactive atoms to platinum occurs as a consequence of the momentum that the atoms receive in the reaction with the deuteron. Evidence for the correctness of such a mechanism of “sticking” of radioactive atoms is the fact that platinum exhibits radioactivity with a period of 156 sec. if it is irradiated in vacuum, passing deuterons through a dense aluminum foil closing the entrance aperture. The period 156 sec. is characteristic of the artificial radioactivity of aluminum. In this case the radioactive atoms, evidently, can reach the platinum from the aluminum foil only by virtue of the momenta they acquire in nuclear reactions.

For a more direct investigation of the radioactivity of gases, the authors

* Mc. Millan and Livingston, Phys. Rev. 47, 452, 1935.

they filled, at atmospheric pressure, a chamber into which deuterons entered through a window with various gases. After irradiation with deuterons, the gas was collected in a special vessel and then investigated for the presence of radioactivity by its $\gamma$-radiation (this $\gamma$-radiation is the annihilation radiation of positrons, whose presence for activated nitrogen was established from the deflection in a magnetic field of the radiation of a platinum plate irradiated by deuterons in nitrogen).

Investigation of the radioactivity of the gases showed that, indeed, air and nitrogen under the action of deuterons acquire radioactivity with a half-life of 126 sec., and $\mathrm{CO}_2$ with a period of 10.4 sec. It should be noted that the radioactivity occurring here is noticeably weaker than that obtained on a platinum plate bombarded by deuterons in the same gases.

To determine the nature of the atom arising from nitrogen when the latter is bombarded with deuterons, the authors carried out the following experiment. After irradiation with deuterons, nitrogen was mixed with hydrogen and passed through two tubes, the first of which contained platinized asbestos, and the second $\mathrm{CaCl}_2$, and was then collected in a vessel. If the asbestos, which serves as a catalyst for the formation of water from $\mathrm{H}$ and $\mathrm{O}$, is heated to $500^\circ$, then all the activity is found in the tube with $\mathrm{CaCl}_2$, whereas with cold asbestos the activity is found in the vessel. From this one may conclude that the active atom is an oxygen atom.

The proposed reaction is:

\[ {}^{14}\mathrm{N} + {}^{2}\mathrm{H} \to {}^{15}\mathrm{O} + n, \]

\[ \mathrm{O}^{15} \to \mathrm{N}^{15} + e^{+}. \]

The existence of positrons in the decay of activated nitrogen was mentioned above. The presence of neutrons was established by means of an ionization chamber closed with a paraffin plate. A rough calculation shows that, in order of magnitude, the number of neutrons and the number of decaying radioactive atoms coincide.

It is rather difficult to carry out quantitative calculations in the case of radioactive gases. The authors indicate that the intensity of the radioactivity for $\mathrm{N}$ and $\mathrm{C}$ is approximately of the same order; however, this is not confirmed by experiments with plates containing nitrogen.

L. Groshev