The Disintegration Constant of Thorium and the Branching Ratio of Thorium C

Abstract

In age determinations of minerals in which thorium predominates, some inconsistencies have appeared when the value used for the disintegration constant of thorium is that adopted by the International Radium-Standard Commission (1930). This value differs from the original Geiger-Rutherford value by 30 percent. The present research was undertaken to determine the disintegration constant of thorium by the method of counting alpha-particles. Easily weighed sources were used. The material was obtained from a thorite from the same lot that was used by Geiger and Rutherford. The thorite contained only a small amount of uranium and correction for the ionium present was made. Various checks on the counting are given. The half-value period of thorium was found to be ${1.39\left(10\right)\mathrm{}}^{10}$ years and the branching ratio, thorium C' to thorium (C+C'), is 0.663. A method of impounding the thoron, yet permitting alpha-particles to escape, is described.