Alpha-spectrometric determination of uranium and thorium in geological samples

Abstract

Alpha-spectrometric technique is described as an independent method for determining micro-contents of U²³⁸ and Th²³². It is based on the comparison of alpha spectral activity of U²³⁸ and Th²³² in geological samples with that contributed by the U²³²·Th²²⁸ spike of known concentration.

The experiment procedure consists of dissolving fine powdered samples with acid and adding a given amount of U²³²-Th²²⁸ spike. Uranium and thorium are then separated by ion exchange. After purification, each aliquot is mounted onto a separate stainless steel disk for measurement by alpha spectrometer, which consists essentially of a silicon surface barrier detector and a low noise amplification system connected to a multi-channel analyzer. After corrections for the background, tail and other factors, the desired U²³² and Th²³² concentrations can be calculated.

The data obtained by the alpha-spectrometric method using U²³²-Th²²⁸ spike are compared with colorimetric determinations. Excellent agreement is obtained between the two sets of results. The coefficients of correlation are about 0.98 for U²³⁸ and 0.97 for Th²³².

The accuracy of uranium and thorium analyses by this method depends primarily upon the counting statistics of U²³², U²³⁸ and Th²²⁸ and, to a certain extent, upon the calibration of U²³²-Th²²⁸ spike. Errors in uranium and thorium concentrations obtained by this method are generally within 2 to 5%.