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Ultra-high vacuum refers to pressures lower than 10−7 pascal or 100 nanopascals (one ten-millionth of a pascal). By comparison, atmospheric pressure is 101.3 kPa (kilopascals), more than a billion times greater, the pressure inside a light bulb is about 1 pascal, and the pressure in the walls of a thermos is about 0.1 pascals. Even outer space in the area around Earth isn't an ultra-high vacuum, as it has a pressure of about 100 micropascals, a thousand times greater than in an ultra-high vacuum. In an ultra-high vacuum, the mean free path of each gas molecule is 40 km, so these molecules will collide many times with the walls of their chamber before colliding with each other.
Ultra-high vacuum is primarily used for surface analytic techniques, such as Auger electron spectroscopy, x-ray photoelectron spectroscopy, secondary ion mass spectrometry, thermal desorption spectroscopy, angle resolved photoemission spectroscopy, and thin film growth techniques requiring high purity, such as molecular beam epitaxy and UHV chemical vapor deposition. Ultra-high vacuum is also used in particle accelerators to create an empty beam path.
Creating an ultra-high vacuum requires extraordinary measures. Special chamber designs minimize surface area, high-speed pumps, including parallel pumps, must be used, high conductance tubing is used for pumps, pits of trapped gas (as in bolt threads) must be eliminated, chamber walls must be cooled to cryogenic temperatures to avoid sublimation of gases trapped in nanoscopic pockets, all metal parts must be electropolished, low-outgassing materials such as stainless steel must be used, and the system must be baked at 250 °C to 400 °C (482 °F to 752 °F) to remove hydrocarbon or water traces. Outgassing — the slow intrusion of gas molecules through tiny cracks in the chamber — can be a major problem. Some chambers may be incapable of producing an ultra-high vacuum because of the way they were fabricated, and the hardware must be thrown out and replaced. For all these reasons, achieving ultra-high vacuum can be expensive and difficult.
Although ultra-high vacuum may seem extreme, some environments are an even better vacuum, including the surface of the Moon and interstellar space. Some regions of space, such as the Boötes void, are so rarefied that there is only one atom per cubic meter.