For physicists, helium is often synonymous with low-temperature physics — both as a cryogen and a research material itself. It is possible to reach a base temperature of 4.2 K using liquid 4He; a 3He cryostat can reach 0.3 K with pumping; put the two isotopes together in a dilution refrigerator and temperatures down to 2 mK become routinely obtainable. Both helium species also support superfluid phases: superfluid 3He is especially interesting as a quantum fluid; a supersolid phase is sought in 4He.

For years, there was no question of the availability of 3He, and prices were nearly constant at US$100 per litre. 3He is a decay product of tritium, an isotope of hydrogen, which was produced in abundance during the nuclear arms race. Although the US has since cut back on tritium production, its stockpile was capable of meeting the annual 3He demand of roughly 8,000 litres. 4He exists in large underground deposits around the world. (For the full story on 4He, see W. P. Halperin's Commentary on page 467 of this issue).

But after 11 September 2001, in the interest of national security, the US government started using 3He-based neutron detectors ('radiation portal monitors') to uncover any potential bomb-making components entering the country. That increased demand, coupled with a large order from the Oak Ridge National Laboratory in 2008, suddenly revealed that the US Department of Energy was allocating 3He faster than it could be produced. In 2008, the demand reached 80,000 litres, becoming unsustainable (D. A. Shea and D. Morgan, Congressional Research Service, 2010;

By September 2009, according to the report by Shea and Morgan, the US Departments of Energy, Homeland Security and Defense together decided to stop using 3He in radiation portal monitors and that further allocations would be determined by three criteria: “whether alternatives to 3He exist for the planned application; whether the application increases national or homeland security; and whether the required 3He is needed to complete prior investments in infrastructure”.

The allocation of 3He has become a tricky balancing act. Although the importance of national security cannot be disputed, investment in scientific infrastructure would be wasted if equipment can't be operated. We must find alternatives, reduce or postpone demand and locate new sources.

There is plenty of 3He in the Universe: the gas giants have a large supply from the original solar nebula and there are reserves on the Moon. In practical terms, heavy-water reactors from which tritium is regularly removed and stored are promising sources. Helium is also found in natural gas.

Scientists are also adapting, using cryogen-free technology that (although prone to vibration) is compatible with dilution refrigerators and superconducting magnets. This is one of many innovations that will address — and hopefully solve — the helium problem.