Results of checking the hypothesis of a possible 235U/238U displacement in the nature

The hypothesis was tested of a possible shift in the U235/U238 ratio in natural conditions. To do this, a number of methods for determining the U235/U238 ratio were devised and perfected (mass-spectrographic, neutron-activation, and alphagamma radiometric coincidences. The isotope composition was studied in samples taken from various depths (from a few tens of metres to 1,000 metres and more) from two adjacent mineralized tectonic zones of proterozoic age sloping downwards to 1,500 metres. Most of the samples had a constant U235/U238 isotope ratio averaging 0.007256 +- 0.000002. In a number of samples, however, a divergence of the isotope composition between 0.3 and 0.4 % from the constant value was discovered. The variations were observed at various depths. At a depth greater than 1,000 metres in both zones examined there was a deficit of up to 0.44 % of U235 (U235/U238 ratio 0.007225 +- 0.000006). This phenomenon may be explained by a partial ''burning up'' of U235 in the proterozoic as a result of the induced fission of uranium. At a depth of a few hundred metres samples containing a few thousandths of a percent of uranium show a relative surplus of U235 of up to 0.25 % (U235/U238 ratio 0.007376 +- 0.000005). It is possible that in this case there was a diffusion separation of the uranium isotopes when it became difficult for the uranium to penetrate upwards to a great distance along the tectonic zone, or the relative surplus of U235 may have been formed from Pu 239 emerging from deeper down. A more definite explanation for the observed tendency for the relative amount of U235 to diminish in the lower regions of the radioactive zones and to increase by corresponding in the upper regions may be provided by further experimental data. The shifts in the U235/U238 ratio observed also merit further detailed study. (author)