What Is Helium?

Helium is a chemical element (atomic number 2, symbol He) belonging to the noble gas group of the periodic table. It is the second most abundant element in the observable universe, yet rare on Earth because its atoms are light enough to escape the atmosphere over geological time. Helium is colorless, odorless, and chemically inert under virtually all conditions, forming no stable compounds. It has the lowest boiling point of any substance (4.22 K at standard pressure), a property that makes it indispensable as a coolant in applications requiring temperatures close to absolute zero.

Helium on Earth is extracted almost entirely from natural gas wells where it has accumulated through the radioactive decay of uranium and thorium in underground rock formations. It draws scientific and engineering interest from two distinct directions: as a cryogen for superconducting systems and quantum experiments, and as a trace gas whose behavior at very low temperatures challenges classical physics, exhibiting superfluid properties that have no analogue in other substances.

Physical and Thermodynamic Properties

Helium remains liquid at atmospheric pressure down to absolute zero, unlike any other element, which will solidify if cooled sufficiently at atmospheric pressure. Below 2.17 K, liquid helium transitions into helium-II, a superfluid phase in which viscosity effectively vanishes and thermal conductivity becomes extraordinarily high. This superfluid helium can flow through microscopic channels without friction and climb the walls of open containers, phenomena that are direct manifestations of quantum mechanics on a macroscopic scale. The two isotopes, helium-4 (the common form) and the rarer helium-3, exhibit different superfluid transition temperatures and are studied separately in low-temperature physics. These properties are well documented in the low-temperature physics literature, with foundational work published through Physical Review and Physical Review Letters at the American Physical Society.

Cryogenic and Superconducting Applications

The primary industrial use of liquid helium is as a cryogen to maintain superconducting magnets at operating temperature. Superconductivity in conventional materials such as niobium-titanium and niobium-tin requires temperatures in the range of 1.8 to 9 K, which only helium cooling can sustain reliably over long periods. Magnetic resonance imaging (MRI) systems each require several hundred liters of liquid helium to keep the superconducting coils at 4 K, enabling the strong, stable magnetic fields that produce diagnostic images. High-energy physics facilities represent the largest single users of liquid helium: CERN's Large Hadron Collider cryogenic system requires 120 tonnes of helium to maintain its superconducting dipole magnets at 1.9 K across 27 kilometres of tunnel. Research-grade dilution refrigerators using helium-3/helium-4 mixtures reach temperatures below 10 millikelvin, enabling experiments in quantum computing and condensed-matter physics.

Helium in Scientific Instruments and Industrial Processes

Beyond cryogenics, helium serves as a carrier gas in gas chromatography because of its low molecular weight, high thermal conductivity, and chemical inertness, which together produce sharp separation of analyte peaks. Pressurized helium is used to purge and pressurize fuel tanks in rockets, preventing combustion-sensitive propellants from contacting air or moisture. In semiconductor manufacturing, helium backside cooling of silicon wafers during ion implantation and etching controls temperature uniformity across the wafer surface. The NIST Chemistry WebBook provides thermodynamic and spectroscopic data for helium that underpin calibration of instruments in manufacturing and research laboratories.

Applications

Helium has applications in a wide range of fields, including:

• Superconducting magnet systems for MRI scanners and particle accelerators
• Quantum computing hardware operating at millikelvin temperatures
• Gas chromatography and analytical chemistry
• Pressurization of liquid-propellant rocket fuel systems
• Arc welding shielding gas for reactive metals
• Leak detection in vacuum and pressure systems