Quantitative detection of tiny amounts of helium isotopes in a hydrogen isotope atmosphere using a standard-resolution quadrupole mass spectrometer

Abstract

This article describes a new method to quantitatively detect extremely small amounts of helium (and eventually other inert gases) in complex gaseous mixtures, even in the presence of very high concentrations of hydrogen isotopes, despite using a conventional (i.e., standard-resolution) quadrupole mass spectrometer. This is accomplished by means of two nonevaporable getter (NEG) pumps, which effectively remove all active components in the original gas mixture with particular regard to hydrogen isotopes. Obviously, noble gases are preserved. The gas sample to be analyzed is initially admitted in a first chamber equipped with a NEG pump operating at high temperature (≈450°C), in order to remove most of the active gases quickly. The residual gas mixture is then allowed to expand through a gate valve into the analysis chamber, where a second NEG pump, operating at room temperature, completes the selective pumping action and removes to high degree hydrogen isotopes and other chemically active gases. The turbomolecular pump, which is used to evacuate the chamber, is excluded during the analysis. The results of extensive tests carried out with a fully automated facility built at ENEA Frascati are reported, demonstrating the effectiveness of this method which allows detection of He4∕D2 peak ratios as small as ≈2.5×10−7. During these tests, a quadrupole mass spectrometer which can operate in both standard- and high-resolution modes has been used. This latter operating mode has been selected, when necessary, in order to provide evidence for the absence of deuterium contributions to mass 4.