Helium-3

Helium-3 is a light and stable isotope of helium. An isotope is a version of an element that has a different number of particles in its center.

While most helium on Earth is helium-4, helium-3 is unique because it has only three particles in its nucleus. It contains two protons and one neutron. This makes it one of only two stable nuclides that have more protons than neutrons. Understanding helium-3 is important because it behaves differently than other gases and may hold the key to future energy.

The physical behavior of helium-3 is driven by its tiny mass. It has an atomic mass of 3.016 Da, which is lower than helium-4 at 4.0026 Da. Because the atoms are so light, their properties are mostly determined by their zero-point energy. This energy allows helium-3 atoms to overcome certain interactions with less thermal energy than helium-4. This leads to very different boiling points. Pure helium-3 gas boils at 3.19 K, while helium-4 boils at 4.23 K. At its boiling point, helium-3 is also much less dense, at only 59 g/L compared to 125 g/L for helium-4.

One of the most fascinating things about helium-3 is its ability to become a superfluid. A superfluid is a liquid that can flow without any friction or resistance. This happens at an extremely low temperature of 2.491 millikelvins.

Unlike helium-4, which is a boson, helium-3 atoms are fermions. This is because they have an odd number of spin particles, giving them a spin of one half. To become a superfluid, these fermions must form pairs, similar to Cooper pairs in superconductivity. These pairs act like bosons, allowing the liquid to enter a superfluid state.

Scientists have discovered that helium-3 actually has two distinct superfluid phases. These are known as the A-phase and the B-phase. The B-phase occurs at lower temperatures and lower pressures. The A-phase occurs at higher temperatures and higher pressures and is stabilized by magnetic fields.

Because helium-3 is so pure, it is an ideal way to study these complex states of matter. In fact, the purity is so high that all other materials solidify and sink to the bottom, leaving the liquid extremely clean.

The history of helium-3 began with theoretical ideas and ended with Nobel Prizes. In 1934, the Australian physicist Mark Oliphant proposed its existence while at the Cavendish Laboratory. He was studying what happens when fast deuterons collide. In 1939, Luis Alvarez and Robert Cornog successfully isolated the isotope for the first time. Later, in the 1970s, David Lee, Douglas Osheroff, and Robert Coleman Richardson discovered the superfluid phase transitions. Their work earned them the Nobel Prize in Physics in 1996. In 2003, Alexei Abrikosov, Vitaly Ginzburg, and Tony Leggett won a Nobel Prize for refining the understanding of these phases.

Helium-3 is found in several different locations across our planet and beyond. It is a primordial substance, meaning it was trapped in Earth's mantle during the planet's formation. It can escape through hotspot volcanoes like those in Hawaii or through mid-ocean ridges. Some is also produced when cosmic rays hit lithium or through the decay of tritium.

On Earth, it is quite rare. In the atmosphere, it accounts for only about 7.2 parts per trillion. However, the Moon may be a much better source. The solar wind has deposited helium-3 into the upper layer of the lunar regolith, which is the layer of loose dust on the Moon's surface.

Looking toward the future, scientists are interested in helium-3 for nuclear fusion. Fusion is the process of joining atoms together to release massive amounts of energy.

Using helium-3 could be an "aneutronic" process. This means the reaction would not release the dangerous neutron radiation common in traditional fusion. It might also require lower temperatures than other fusion methods. While extracting it from Earth's natural gas is difficult and energy-intensive, the abundance of helium-3 on the Moon makes it a major topic for space exploration and long-term energy research.