Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber. UHV conditions are integral to scientific research. Surface science experiments often require a chemically clean sample surface with the absence of any unwanted adsorbates. Surface analysis tools such as X-ray photoelectron spectroscopy and low energy ion scattering require UHV conditions for the transmission of electron or ion beams. For the same reason, beam pipes in particle accelerators such as the Large Hadron Collider are kept at UHV. Overview Maintaining UHV conditions requires the use of unusual materials for equipment. Useful concepts for UHV include: Sorption of gases Kinetic theory of gases Gas transport and pumping Vacuum pumps and systems Vapour pressure Typically, UHV requires: High pumping speed — possibly multiple vacuum pumps in series and/or parallel Minimized surface area in the chamber High conductance tubing to pumps — short and fat, without obstruction Use of low-outgassing materials such as certain stainless steels Avoid creating pits of trapped gas behind bolts, welding voids, etc. Electropolishing of all metal parts after machining or welding Use of low vapor pressure materials (ceramics, glass, metals, teflon if unbaked) Baking of the system to remove water or hydrocarbons adsorbed to the walls Chilling of chamber walls to cryogenic temperatures during use Avoiding all traces of hydrocarbons, including skin oils in a fingerprint — always use gloves Hydrogen and carbon monoxide are the most common background gases in a well-designed, well-baked UHV system. Both Hydrogen and CO diffuse out from the grain boundaries in stainless steel. Helium could diffuse through the steel and glass from the outside air, but this effect is usually negligible due to the low abundance of He in the atmosphere. Measurement Pressure Measurement of high vacuum is done using a nonabsolute gauge that measures a pressure-related property of the vacuum, for example, its thermal conductivity. See, for example, Pacey. These gauges must be calibrated. The gauges capable of measuring the lowest pressures are magnetic gauges based upon the pressure dependence