Measurement of Isotopic Abundance Ratio of Argon-36, -38 and -40 by General-Purpose Double-Focuse Mass Spectrometer

Abstract

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