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Liquid helium
The chemical element helium exists in a liquid form only at the extremely low temperature of −269 °C (about 4 K or −452.2 °F). Its boiling point and critical point depend on which isotope of helium is present: the common isotope helium-4 or the rare isotope helium-3. These are the only two stable isotopes of helium. See the table below for the values of these physical quantities. The density of liquid helium-4 at its boiling point and a pressure of one atmosphere (101.3 kilopascals) is about 0.125 grams per cm3, or about 1/8th the density of liquid water.〔(【引用サイトリンク】 The Observed Properties of Liquid Helium at the Saturated Vapor Pressure )〕
==Liquefaction==
Helium was first liquefied on July 10, 1908, by the Dutch physicist Heike Kamerlingh Onnes in the Netherlands.〔Wilks, p. 7〕 At that time, helium-3 was unknown because the mass spectrometer had not yet been invented. In more recent decades, some liquid helium has been used as a cryogenic refrigerant, and liquid helium is produced commercially for use in superconducting magnets such as those used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), Magnetoencephalography (MEG), and experiments in physics, such as low temperature Mössbauer spectroscopy.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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