Helium-3 (³He) is one of the two stable isotopes of helium. The other is the much more common Helium-4 (⁴He). Neither of these isotopes should be confused with He II, which is the second liquid phase of ⁴He. He II was discussed in this column in the Spring 2010 edition of Cold Facts. ³He, whose nucleus contains two protons and one neutron, has a number of important uses in cryogenics. ³He also has important applications in neutron detectors and this application, coupled with its rarity, has led to significant shortages of this isotope in recent years.

In cryogenics, ³He plays an important role in sub-Kelvin cooling. ³He has a higher vapor pressure than ⁴He and thus cooling to roughly 400 mK can be accomplished by reducing the pressure above a saturated bath of ³He. This approach using ⁴He becomes impractical below approximately 1.2K. Even more significant is the use of ³He/⁴He mixtures in dilution refrigerators to provide cooling down to the tens of mK level. (Dilution refrigerators were described in this column in the Winter 2012 issue of Cold Facts.) Studies have also shown that there may be advantages in using ³He as a working fluid in pulse tube or Gifford-McMahon cryocoolers.

³He will become a superfluid at sufficiently low temperatures. However, the physical mechanism behind the super-fluidity in ³He is very different than that in ⁴He. Superfluidity in ³He is better described by a mechanism similar to the BCS theory that describes low temperature superconductors than by the Bose-Einstein condensation that explains superfluidity in ⁴He. There are, in fact, two superfluid liquid phases in ³He with transition temperatures at 2.65 mK and 1.8 mK respectively. Fundamental studies of the properties of ³He and its superfluid phases are an important area of research in condensed matter physics.

Naturally occurring ³He makes up roughly 0.1 parts per million of all helium on Earth. However, ³He is a product of the radioactive decay of ³H (or tritium) and thus ³He can be produced by artificially creating tritium and allowing it to decay. This is the source of almost all ³He in use today. A very significant non-cryogenic use of ³He is its application as a medium for neutron detection for both scientific and Homeland Security applications.

Recent growth in these applications, particularly in Homeland Security, has made ³He frequently too expensive and scarce for cryogenic applications. Solutions to this problem are being discussed, including developing other materials for neutron detection so that ³He may be kept for cryogenic applications.

A good overview of ³He physics and applications may be found in “Helium Cryogenics” by S. W. Van Sciver, Springer (2012) while details on the superfluid phases of ³He are given in “The Superfluid Phases of Helium-3” by D. Vollhardt and P. Woelfie, CRC Press (1990). Applications of ³He in small cryocoolers are discussed in “Optimization Calculations for a 30 Hz 4K Regenerator with Helium-3 as a Working Fluid” by R. Radebaugh, Y. Huang, A. O’Gallagher and J. Gary in Adv. Cryo. Engr. Vol. 55B (2010) and “Performance Limits of Pulse Tube Cryocoolers Using ³He” by P. Kittel, Adv. Cryo. Engr. Vol. 53B (2008).