Utilization of α-autoradiography of rocks in the investigation of the radioactive equilibrium

The α-autoradiography of rocks and minerals permits the determination of their uranium or equivalent uranium contents. This is done by counting the number of α-rays emitted in a cm2 per second on a nuclear emulsion and by substituting the obtained number in the appropriate equation. However, the derived equations for this purpose are based on the assumption of the existence of a radiactive equilibrium. In other words, they take into account the emission of 8 α-rays in the 238U series. But if one of the decay members or more of this series are totally or partially missing, the equivalent uranium contents found by this method will be remarkably lower than the true ones. Thus a comparison between the contents determined by autoradiography and those by other methods (e.g. fluorimetry) allows the indication of the existence of a radioactive disequilibrium. Various autoradiographic studies made pointed out the existence of a severe radioactive disequilibrium in certain phosphates and autunites. This was demonstrated after performing the following studies: 1) Measurement of equivalent uranium contents by α-autoradiography. 2) Correction of these as a function of the thorium contents measured by γ-spectrometry. 3) Determination of the uranium contents by fluorimetry. 4) Making a comparison between the results in (2) and (3), which permits to decide whether there is a radioactive disequilibrium or not. 5) The disequilibrium when present, can be further confirmed by α-spectrometry. (Auth.)