MEASUREMENT OF ISOTOPIC ABUNDANCE RATIO OF ARGON-36, ARGON-38 AND ARGON-40 BY GENERAL-PURPOSE DOUBLE-FOCUSED MASS-SPECTROMETER

Atomic fractions of each isotope in Ar-36-Ar-38-Ar-40 gas mixture near natural abundance were measured by using a general-purpose double-foc used mass spectrometer HITACHI M-80. Because an amount of gas in a sam ple bulb and consequently a height of output peak decreases while taki ng measurements, 30 sets of peak height and time elapsed for each isot ope were measured for calculation of atomic fractions through a least squares method under the assumptions (1) the reduction in heights foll ows an exponential function of time and (2) the rates of reduction are the same among argon isotopes. Application of this method to argon ga s samples extracted from the top and bottom of a thermal diffusion col umn for isotope separation have proved that the method gives data with small dispersion even for Ar-38, and is satisfactory for studying sep arative performances of thermal diffusion column.