Helium

³He is a product of the radioactive decay of ³H (half-life of 12.31 years). The relative variations in the amount ratio n(³He)/n(³H) can be interpreted in terms of elapsed time. This has been especially useful in aquatic systems, including oceans, lakes, and aquifers, that received large inputs of ³H from precipitation following thermonuclear bomb test periods. ³H- ³He dating provides the elapsed time since a water mass became isolated from the atmosphere in the time range from the mid-1950s to the present. Such studies are important for establishing the sustainability of groundwater resources in shallow aquifers [27], [28]. ⁴He is a product of radioactive decay in the uranium and thorium decay series. As a result, ⁴He concentration is used to estimate the relative ages of minerals and groundwater. In closed systems (systems that do not exchange matter with their surroundings), relative variations in the amount ratio n(⁴He)/n(U) can be interpreted in terms of elapsed time, although other processes can alter the distribution of helium, which is highly mobile in terrestrial environments [29], [30]. ⁴He concentrations commonly increase along groundwater flow paths through a cumulative release from aquifer materials. This rate of accumulation is used to estimate the time since the groundwater was recharged at the surface. The ⁴He accumulation method of groundwater dating is typically used in deeper aquifers, where groundwater is relatively old and the ³H- ³He method cannot be used because of the relatively short half-life of 12.31 years for ³H [30].

³He has a large absorption cross section for neutrons, which makes it especially useful for radioactivity detection [31], [32]. In this application, neutrons produced by the radioactive decay of elements, such as uranium and plutonium, enter the detector, where the reaction ³He (n, p) ³H produces ¹H and ³H atoms. This induces further collisions and the release of electrons, which interact with charged surfaces to generate an electric current. Large amounts of ³He are used to produce neutron detectors in portal monitors for detecting illicit radioactive materials at ports, border crossings, and airports (Fig. IUPAC.2.1). Unfortunately, the isotope ³He is rare and there is a need to incorporate alternative gases for use in neutron detectors. ³He neutron detectors are also used in devices that determine the proportions of water, oil, and gas in wells drilled for energy production. Other important uses of ³He include lasers, gyroscopes used for missile stability and guidance, and cryogenic research (ultra-low temperature, less than 1 K). The global supply of ³He available for research and practical applications has become severely limited in recent years, such that prices have increased substantially and some uses have been curtailed [31], [32]. A major source of ³He is from nuclear weapons containing ³H, recovered when the warheads are reconditioned or dismantled. ³He accumulates in such devices as a radiogenic product of ³H decay. The annual supply of new ³He has decreased with reductions in nuclear arsenals.

³He is used as an inhalant to improve magnetic resonance imaging (MRI) of the lungs [34].