Abstract
New techniques for determining the rate of helium production in igneous rocks by direct counting of the alpharays, combined with the removal and measurement of the helium accumulated in a rock specimen during geologic time, provide a direct physical method of helium age analysis which is free from complicated chemical manipulation, makes possible more rapid measurement of the helium and radioactivity of terrestrial materials, and has only a second-order dependence upon radioactive standards. Equations are derived which relate the true rate of production of helium to the observed rate of alpha-particle emission from a thin source. Age relationships are derived which allow a simple, but accurate, correction to be applied to the approximate age in obtaining the true age of geologically old materials, in which the decay of the parent radioactive elements has been appreciable. Intercheck radioactivity measurements have established the mutual reliability of the alpha-count method and the radon-thoron method of determining the total radioactive content of rocks and ores and have shown that there are no unknown, strong, alphaemitting, radioactive elements in ordinary terrestrial materials. The agreement between the "alpha-helium" and "radon-thoron-helium" ages on the same samples indicates that the accepted decay constants of the parent elements, thorium and uranium, are correct to within the experimental uncertainty of the measurements (\ifmmode\pm\else\textpm\fi{}5 to 10 percent). The application of the alpha-helium method to igneous rocks has revealed that certain mineral constituents allow a significant amount of their radiogenic helium to escape during geologic time. For this reason the revised helium time scale is based on measurements of selected retentive minerals, magnetite being one of the most promising. As a by-product of these researches, the alpha-activity of 81 rock specimens has been found to range from 0.0 to 5.8 with an average of 1.23 alpha-rays per hour per milligram.
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