Helium isotope ratio measurement by a single focusing mass spectrometer with large incident and exit angles

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Abstract

A single focusing mass spectrometer (rm = 6 cm, φm = 130°) with large incident and exit angles (55°) has been developed to measure 3He/4He ratios. A resolving power of about 600 at 5% of peak height was attained in order to separate 3He ions from those of H3 and HD. The sensitivity for 3He was about 0.3 A (cm3 STP)−1 by use of a secondary electron multiplier, which was high enough to measure the 3He/4He ratio of terrestrial gas samples. The mass discrimination factor of the system was calculated using an in-house standard prepared by mixing known amounts of pure 3He and 4He. The 3He/4He ratio of the atmosphere was measured by repeated runs. The value of (1.35 ± 0.05) × 10−6 agrees well with literature values within experimental error margin. Since the whole measuring system is significantly compact and light enough to load on an automobile, it is applicable to field studies related to earthquake prediction and volcanic eruption.

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