Ultra-high vacuum has pressures below 10-7 pascals and is used for surface analytical techniques and particle accelerators. Achieving it requires special measures such as minimizing surface area, using high-speed pumps, and eliminating trapped gas. It can be expensive and difficult, but some environments are even better vacuum.
Ultra-high vacuum refers to pressures below 10-7 pascals 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 pascal. Even outer space in the area around the Earth is not 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 with the walls of their chamber many times before colliding with each other.
Ultra-high vacuum is mainly used for surface analytical techniques, such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, thermal desorption spectroscopy, angular resolution photoemission spectroscopy and thin film growth techniques that require high purity, such as beam epitaxy and UHV chemical vapor deposition. Ultra high vacuum is also used in particle accelerators to create a vacuum beam path.
Creating an ultra-vacuum requires extraordinary measures. Special chamber designs minimize surface area, high speed pumps including parallel pumps should be used, high conductance tubing is used for pumps, pits of trapped gas (as in bolt threads) should be eliminated, the walls of the chamber must be cooled to cryogenic temperatures to avoid sublimation of gases trapped in the nanoscopic pockets, all metal parts must be electropolished, low-gas materials such as stainless steel must be used, and the system must be fired at 250°C to 400°C (482°F to 752°F) to remove traces of hydrocarbons or water. Outgassing – the slow intrusion of gas molecules through tiny cracks in the chamber – can be a big problem. Some chambers may not be able to produce an ultra high vacuum due to the way they are manufactured and the hardware needs to be thrown out and replaced. For all of these reasons, achieving an ultra-high vacuum can be expensive and difficult.
While ultra-high vacuum may seem extreme, some environments are even better vacuum, including the surface of the Moon and interstellar space. Some regions of space, such as the Bootes vacuum, are so rarefied that there is only one atom per cubic metre.
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