Direct Fusion Method for Determining the Radium Content of Rocks

摘要

New principles for removing radon from rocks and for measuring minute quantities of radon are presented. Powdered rock specimens are boiled without flux by direct heating in an evacuated, graphite‐resistance furnace. The duration of heating is 2½ minutes, which is to be compared with about one hour for the quickest of former methods. The gases from the melt are led to the ionization chamber through an electrostatic ion‐trap. A small insulation drying device in the ionization chamber protects the amber from water vapor. The natural observational limit for measurements of small quantities of radon is defined as the point at which the average ionization from the radon just equals the probable statistical variation in the background ionization due to cosmic and local radiation, and an apparatus is described which permits measuring radon down to this limit. A string electrometer measures only the difference in ionization in two identical ionization chambers, both of which contain the same background gases, but only one of which contains the radon to be measured. A systematic study of the percentage of the total radon removed from biotite granite as a function of (a) temperature, (b) duration of heating, (c) fineness of sample grains, showed that temperature is the only important factor, and that 1800°C effects complete radon removal.