URANIUM-235 IN THUCHOLITE

The radioactive mineral thucholite, found among the pegmatites of Ontario, contains, together with uranium and thorium, a considerable amount of carbon. A mineral containing carbon evidently has a volcanic origin. It may be supposed that, if it arose from a mass of sufficiently large size in which the carbon acted as a moderator for fission neutrons, then at one time a chain reaction could have taken place there. In that case the uranium contained in the thucholite should differ from ordinary uranium in its percentage content of U²³⁵. For chemical analysis the thucholite samples were ground and ignited, the losses, including carbon and volatile impurities, amounting to 26.5%. In the analysis for carbon its amount proved to be 21%. The amount of U₃O₈ in the thucholite was 7.2%. In the experiments, uranium oxide U₃O₈ obtained from thucholite, and a control with ordinary uranium, were deposited in a layer of 2 mg/cm² on aluminum foil. α-particles of uranium and fission fragments were recorded with the aid of an air counter. The foil containing the control uranium showed a counting intensity 10% greater than the uranium from thucholite. Uranium fission was induced by neutrons from a (Ra-Be) source of 1 millicurie; the neutrons were moderated in a paraffin block.

Analogous experiments were carried out with the aid of an ionization chamber, inside which samples of foil containing U₃O₈ were placed. The counting intensity of α-particles for the control sample was 92,000 per minute, and for uranium from thucholite—84,000. The neutron source and paraffin were placed outside the chamber. To record the fragment intensity a pulse discriminator was used. The results of the experiments are given in the table.

Table

In terms of uranium content the difference could not have been more than 1%, whereas the α-intensity of the control sample was greater by 10%. This difference can be explained by additional α-emitters in the control sample or by impurities in the U₃O₈ from thucholite that absorbed α-particles. Impurities lowering the α-activity by 10% are insufficient to explain so small a number of fission pulses from uranium from thucholite.

Thus, the results of the experiments are in agreement with the assumption of a lower percentage content of U\(^{235}\).

In the short note being reviewed,*) the authors set forth the factual side, without drawing the conclusions that would follow if further work confirms the present result. Indeed, if at some moment in the Earth’s crust a chain reaction occurred, then the released energy could have caused the dispersal of the mass in which the chain reaction took place, and the formation thereby of ore veins with inclusions of uranium.

In this connection it should be noted that as early as 1939 Academician V. I. Vernadskii proposed an explanation of the Earth’s volcanic activity by the release of atomic energy. Such a process, based on a chain reaction, may also occur not continuously, dying down and flaring up depending on the displacement of the Earth’s strata, the accumulation and decay of the reaction products, and changes of temperature in the process itself. As is known, many centers of volcanic activity on the Earth’s surface have outlets of radioactive springs.

It should be noted that here we encounter the first case of a difference in the isotopic composition of elements under terrestrial conditions.

K. D. Tolstov

*) J. K. Orr, Phys. Rev. 76, No. 1, 155 (1949).