Determination of alpha activity of natural samples by track detectors

The planar alpha activity was determined of solid solutions of granite, trachyte and basalt containing uranium and thorium. The KODAK LR 115 dielectric track detector was used for the measurements. The detection efficiency was 0.256+-0.O17. The lowest detectable concentration at total irradiation time of 3.9x106 s was 6x10-5% U or 2x10-4% Th which is within the limits of accuracy given by the statistical error of counted tracks and the error in U and Th determination in standards. The lowest detectable level of the two elements may further be reduced either by extending the exposure time or by changing the evaluation method (including non-etched tracks). (J.B.)