Helium ( ) is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Next to hydrogen, it is the second most abundant element in the universe and accounts for 24% of the elemental mass of our galaxy.

An unknown yellow spectral line signature in sunlight was first observed during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with the discovery of the element with Norman Lockyer, who observed the same eclipse and was the first to propose that the line was due to a new element, which he named helium. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, which is by far the largest supplier of the gas.

Helium is used in cryogenics (its largest single use, absorbing about a quarter of production), particularly in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses- as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers- account for half of the gas produced. A well-known but minor use is as a lifting gas in balloons and airships. As with any gas with differing density from air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of the two fluid phases of helium-4 (helium I and helium II), is important to researchers studying quantum mechanics (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, that temperatures near absolute zero produce in matter. Helium is the second lightest element and is the second most abundant in the observable universe, being present in the universe in masses more than 12 times those of all the heavier elements combined. Its abundance is similar to this figure in our own Sun and in Jupiter. This is due to the very high binding energy (per nucleon) of helium-4 with respect to the next three elements after helium (lithium, beryllium, and boron). This helium-4 binding energy also accounts for its commonality as a product in both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have been formed during the Big Bang. Some new helium is being created currently as a result of the nuclear fusion of hydrogen in stars greater than 0.5 solar masses.

On Earth, the lightness of helium has caused its evaporation from the gas and dust cloud from which the planet condensed, and it is thus relatively rare—0.00052% by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium), as the alpha particles emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations up to 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.

History

Scientific discoveries

The first evidence of helium was observed on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D₃ Fraunhofer line because it was near the known D₁ and D₂ lines of sodium. He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios)." In 1882, Italian physicist Luigi Palmieri detected helium on Earth, for the first time, through its D₃ spectral line, when he analyzed the lava of Mount Vesuvius.

On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare earth elements) with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D₃ line observed in the spectrum of the Sun. These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. Helium was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen. His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science.

In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate the thin glass wall of an evacuated tube, then creating a discharge in the tube to study the spectra of the new gas inside. In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. He tried to solidify it by further reducing the temperature but failed because helium does not have a triple point temperature at which the solid, liquid, and gas phases are at equilibrium. Onnes' student Willem Hendrik Keesom was eventually able to solidify 1 cm³ of helium in 1926 by applying additional external pressure.

In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. This phenomenon is related to Bose-Einstein condensation. In 1972, the same phenomenon was observed in helium-3, but at temperatures much closer to absolute zero, by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson. The phenomenon in helium-3 is thought to be related to pairing of helium-3 fermions to make bosons, in analogy to Cooper pairs of electrons producing superconductivity.

Extraction and use

After an oil drilling operation in 1903 in Dexter, Kansas, produced a gas geyser that would not burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas consisted of, by volume, 72% nitrogen, 15% methane (a combustible percentage only with sufficient oxygen), 1% hydrogen, and 12% an unidentifiable gas. With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium. This showed that despite its overall rarity on Earth, helium was concentrated in large quantities under the American Great Plains, available for extraction as a byproduct of natural gas. The greatest reserves of helium were in the Hugoton and nearby gas fields in southwest Kansas and the panhandles of Texas and Oklahoma.

This enabled the United States to become the world's leading supplier of helium. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium plants during World War I. The goal was to supply barrage balloons with the non-flammable, lighter-than-air gas. A total of 5,700 m³ (200,000 cubic feet) of 92% helium was produced in the program even though less than a cubic meter of the gas had previously been obtained. Some of this gas was used in the world's first helium-filled airship, the U.S. Navy's C-7, which flew its maiden voyage from Hampton Roads, Virginia, to Bolling Field in Washington, D.C., on December 1, 1921.

Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. This use increased demand during World War II, as well as demands for shielded arc welding. The helium mass spectrometer was also vital in the atomic bomb Manhattan Project.

The government of the United States set up the National Helium Reserve in 1925 at Amarillo, Texas, with the goal of supplying military airships in time of war and commercial airships in peacetime. Because of a US military embargo against Germany that restricted helium supplies, the Hindenburg, like all German Zeppelins, was forced to use hydrogen as the lift gas. Helium use following World War II was depressed but the reserve was expanded in the 1950s to ensure a supply of liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption.

After the "Helium Acts Amendments of 1960" (Public Law 86–777), the U.S. Bureau of Mines arranged for five private plants to recover helium from natural gas. For this helium conservation program, the Bureau built a pipeline from Bushton, Kansas, to connect those plants with the government's partially depleted Cliffside gas field, near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gas field until needed, when it then was further purified.

By 1995, a billion cubic meters of the gas had been collected and the reserve was US$1.4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve. The resulting "Helium Privatization Act of 1996" (Public Law 104–273) directed the United States Department of the Interior to start emptying the reserve by 2005.

Helium produced between 1930 and 1945 was about 98.3% pure (2% nitrogen), which was adequate for airships. In 1945, a small amount of 99.9% helium was produced for welding use. By 1949, commercial quantities of Grade A 99.95% helium were available.

For many years the United States produced over 90% of commercially usable helium in the world, while extraction plants in Canada, Poland, Russia, and other nations produced the remainder. In the mid-1990s, a new plant in Arzew, Algeria, producing 17 million cubic meters (600 million cubic feet) began operation, with enough production to cover all of Europe's demand. Meanwhile, by 2000, the consumption of helium within the U.S. had risen to above 15 million kg per year. In 2004–2006, two additional plants, one in Ras Laffan, Qatar, and the other in Skikda, Algeria, were built, but as of early 2007, Ras Laffan is functioning at 50%, and Skikda has yet to start up. Algeria quickly became the second leading producer of helium. Through this time, both helium consumption and the costs of producing helium increased. In the 2002 to 2007 period helium prices doubled, and during 2008 alone the major suppliers raised prices about 50%.

Characteristics

The helium atom

Helium in quantum mechanics

In the perspective of quantum mechanics, helium is the second simplest atom to model, following the hydrogen atom. Helium is composed of two electrons in atomic orbitals surrounding a nucleus containing two protons along with some neutrons. As in Newtonian mechanics, no system consisting of more than two particles can be solved with an exact analytical mathematical approach (see 3-body problem) and helium is no exception. Thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electrons. Such computational chemistry methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps. In such models it is found that each electron in helium partly screens the nucleus from the other, so that the effective nuclear charge Z which each electron sees, is about 1.69 units, not the 2 charges of a classic "bare" helium nucleus.

The related stability of the helium-4 nucleus and electron shell

The nucleus of the helium-4 atom is identical with an alpha particle. High energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud. This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin. Adding another of any of these particles would require angular momentum and would release substantially less energy (in fact, no nucleus with five nucleons is stable). This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature.

For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements.

In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions involving both heavy-particle emission, and fusion. Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a very small fraction, compared with the highly favorable helium-4. The stability of helium-4 is the reason hydrogen is converted to helium-4 (not deuterium or helium-3 or heavier elements) in the Sun. It is also partly responsible for the fact that the alpha particle is by far the most common type of baryonic particle to be ejected from atomic nuclei; in other words, alpha decay is far more common than cluster decay.

The unusual stability of the helium-4 nucleus is also important cosmologically: it explains the fact that in the first few minutes after the Big Bang, as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. So tight was helium-4 binding that helium-4 production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and also leaving few to form heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding per nucleon is stronger than in any of these elements (see nucleogenesis and binding energy) and thus no energetic drive was available, once helium had been formed, to make elements 3, 4 and 5. It was barely energetically favorable for helium to fuse into the next element with a lower energy per nucleon, carbon. However, due to lack of intermediate elements, this process requires three helium nuclei striking each other nearly simultaneously (see triple alpha process). There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.

All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that isn't hydrogen.

Gas and plasma phases

Helium is the least reactive noble gas after neon and thus the second least reactive of all elements; it is inert and monatomic in all standard conditions. Because of helium's relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen. For similar reasons, and also due to the small size of helium atoms, helium's diffusion rate through solids is three times that of air and around 65% that of hydrogen.

Helium is the least water soluble monatomic gas, and one of the least water soluble of any gas (CF₄, SF₆, and C₄F₈ have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x₂/10⁻⁵, respectively, versus helium's 0.70797 x₂/10⁻⁵), and helium's index of refraction is closer to unity than that of any other gas. Helium has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule-Thomson inversion temperature (of about 32 to 50 K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion cooling.

Most extraterrestrial helium is found in a plasma state, with properties quite different from those of atomic helium. In a plasma, helium's electrons are not bound to its nucleus, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized hydrogen, the particles interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora.

Solid and liquid phases

Unlike any other element, helium will remain liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 25 bar (2.5 MPa) of pressure. It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure, but it is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30%. With a bulk modulus on the order of 50 MPa it is 50 times more compressible than water. Solid helium has a density of 0.214 ± 0.006 g/cm³ at 1.15 K and 66 atm; the projected density at 0 K and 25 bar (2.5 MPa) is 0.187 ± 0.009 g/cm³.

Helium I state

Below its boiling point of 4.22 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium-4 exists in a normal colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium doesn't boil, and it expands as the temperature is lowered further.

Helium I has a gas-like index of refraction of 1.026 which makes its surface so hard to see that floats of styrofoam are often used to show where the surface is. This colorless liquid has a very low viscosity and a density of 0.145–0.125 g/mL (between about 0 and 4 K, respectively), which is only one-fourth the value expected from classical physics. Quantum mechanics is needed to explain this property and thus both types of liquid helium are called quantum fluids, meaning they display atomic properties on a macroscopic scale. This may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion (thermal energy) from masking the atomic properties.

Helium II state

Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; heat input instead causes evaporation of the liquid directly to gas. Helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope.

Helium II is a superfluid, a quantum mechanical state of matter with strange properties. For example, when it flows through capillaries as thin as 10⁻⁷ to 10⁻⁸ m it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the two-fluid model for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.

In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.

The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper. This is because heat conduction occurs by an exceptional quantum mechanism. Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air. When heat is introduced, it moves at 20 meters per second at 1.8 K through helium II as waves in a phenomenon known as second sound.

Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm-thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin. As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as gravity waves in shallow water, but rather than gravity, the restoring force is the van der Waals force. These waves are known as third sound.

Isotopes

There are eight known isotopes of helium, but only helium-3 and helium-4 are stable. In the Earth's atmosphere, there is one atom for every million atoms. Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes. The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.

Helium-3 is present on Earth only in trace amounts; most of it since Earth's formation, though some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten, and these ratios can be used to investigate the origin of rocks and the composition of the Earth's mantle. is much more abundant in stars, as a product of nuclear fusion. Thus in the interstellar medium, the proportion of to is around 100 times higher than on Earth. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon's surface contains helium-3 at concentrations on the order of 0.01 ppm, a lot higher than the ca. 5 ppt found in the Earth's atmosphere. A number of people, starting with Gerald Kulcinski in 1986, have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion.

Liquid helium-4 can be cooled to about 1 kelvin using evaporative cooling in a 1-K pot. Similar cooling of helium-3, which has a lower boiling point, can achieve about in a helium-3 refrigerator. Equal mixtures of liquid and below separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions). Dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins.

It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is helium-5 with a half-life of . Helium-6 decays by emitting a beta particle and has a half-life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are created in certain nuclear reactions. Helium-6 and helium-8 are known to exhibit a nuclear halo. Helium-2 (two protons, no neutrons) is a radioisotope that decays by proton emission into protium (hydrogen), with a half-life of .

Compounds

Helium has a valence of zero and is chemically unreactive under all normal conditions. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds, known as excimers, with tungsten, iodine, fluorine, sulfur and phosphorus when it is subjected to an electric glow discharge, to electron bombardment, or else is a plasma for another reason. The molecular compounds HeNe, HgHe₁₀, and WHe₂, and the molecular ions , , , and have been created this way. This technique has also allowed the production of the neutral molecule He₂, which has a large number of band systems, and HgHe, which is apparently held together only by polarization forces. Theoretically, other true compounds may also be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000. Calculations show that two new compounds containing a helium-oxygen bond could be stable. Two new molecular species, predicted using theory, CsFHeO and N(CH₃)₄FHeO, are derivatives of a metastable [F– HeO] anion first theorized in 2005 by a group from Taiwan. If confirmed by experiment, such compounds will end helium's chemical inertness, and the only remaining inert element will be neon.

Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure. The endohedral fullerene molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. If helium-3 is used, it can be readily observed by helium nuclear magnetic resonance spectroscopy. Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances have distinct properties and a definite composition, like all stoichiometric chemical compounds.

Occurrence and production

Natural abundance

Helium is the second most abundant element in the known Universe (after hydrogen), constituting 23% of its baryonic mass. The vast majority of helium was formed by Big Bang nucleosynthesis one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models. In stars, it is formed by the nuclear fusion of hydrogen in proton-proton chain reactions and the CNO cycle, part of stellar nucleosynthesis.

In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million. The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes. In the Earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.

Nearly all helium on Earth is a result of radioactive decay, and thus an Earthly helium balloon is essentially a bag of retired alpha particles. Helium is found in large amounts in minerals of uranium and thorium, including cleveite, pitchblende, carnotite and monazite, because they emit alpha particles (helium nuclei, He²⁺) to which electrons immediately combine as soon as the particle is stopped by the rock. In this way an estimated 3000 metric tons of helium are generated per year throughout the lithosphere. In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. Because helium is trapped in the subsurface under conditions that also trap natural gas, the greatest natural concentrations of helium on the planet are found in natural gas, from which most commercial helium is extracted. The concentration varies in a broad range from a few ppm up to over 7% in a small gas field in San Juan County, New Mexico.

Modern extraction and distribution

For large-scale use, helium is extracted by fractional distillation from natural gas, which contains up to 7% helium. Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure Grade-A helium. The principal impurity in Grade-A helium is neon. In a final production step, most of the helium that is produced is liquefied via a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers.

In 2008, approximately 169 million standard cubic meters (SCM) of helium were extracted from natural gas or withdrawn from helium reserves with approximately 78% from the United States, 10% from Algeria, and most of the remainder from Russia, Poland and Qatar. In the United States, most helium is extracted from natural gas of the Hugoton and nearby gas fields in Kansas, Oklahoma, and Texas. Much of this gas was once sent by pipeline to the National Helium Reserve, but since 2005 this reserve is presently being depleted and sold off.

Diffusion of crude natural gas through special semipermeable membranes and other barriers is another method to recover and purify helium. , the U.S. had proven helium reserves, in such gas well complexes, of about 147 billion standard cubic feet (4.2 billion SCM). At rates of use at that time (72 million SCM per year in the U.S.; see pie chart below) this is enough helium for about 58 years of U.S. use, and less than this (perhaps 80% of the time) at world use rates, although factors in saving and processing impact effective reserve numbers. It is estimated that the resource base for yet-unproven helium in natural gas in the U.S. is 31–53 trillion SCM, about 1000 times the proven reserves.

Helium must be extracted from natural gas because it is present in air at only a fraction of that of neon, yet the demand for it is far higher. It is estimated that if all neon production were retooled to save helium, that 0.1% of the world's helium demands would be satisfied. Similarly, only 1% of the world's helium demands could be satisfied by re-tooling all air distillation plants. Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but economically, this is a completely non-viable method of production.

Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small containers called Dewars which hold up to 1,000 liters of helium, or in large ISO containers which have nominal capacities as large as 42 m³ (around 11,000 U.S. gallons). In gaseous form, small quantities of helium are supplied in high pressure cylinders holding up to 8 m³ (approx. 282 standard cubic feet), while large quantities of high pressure gas are supplied in tube trailers which have capacities of up to 4,860 m³ (approx. 172,000 standard cubic feet).

Conservation advocates

According to helium conservationists like Robert Coleman Richardson, the free market price of helium has contributed to "wasteful" usage (e.g. for helium balloons). Prices in the 2000s have been lowered by U.S. Congress' decision to sell off the country's large helium stockpile by 2015. According to Richardson, the current price needs to be multiplied by 20 to eliminate the excessive wasting of helium.

Applications

Helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness. Of the 2008 world helium total production of about 32 million kg (193 million standard cubic meters) helium per year, the largest use (about 22% of the total in 2008) is in cryogenic applications, most of which involves cooling the superconducting magnets in medical MRI scanners. Other major uses (totalling to about 78% of use in 1996) were pressurizing and purging systems, maintenance of controlled atmospheres, and welding. Other uses by category were relatively minor fractions.

Controlled atmospheres

Helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, and in gas chromatography, because it is inert. Because of its inertness, thermally and calorically perfect nature, high speed of sound, and high value of the heat capacity ratio, it is also useful in supersonic wind tunnels and impulse facilities.

Gas tungsten arc welding

Helium is used as a shielding gas in arc welding processes on materials that at welding temperatures are contaminated and weakened by air or nitrogen. A number of inert shelding gases are used in gas tungsten arc welding, but helium is used instead of cheaper argon especially for welding materials that have higher heat conductivity, like aluminum or copper.

Minor uses

Industrial leak detection

One industrial application for helium is leak detection. Because helium diffuses through solids three times faster than air, it is used as a tracer gas to detect leaks in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers. The tested object is placed in a chamber, which is then evacuated and filled with helium. The helium that escapes through the leaks is detected by a sensitive device (helium mass spectrometer), even at the leak rates as small as 10⁻⁹ mbar·L/s (10⁻¹⁰ Pa·m³/s). The measurement procedure is normally automatic and is called helium integral test. A simpler procedure is to fill the tested object with helium and to manually search for leaks with a hand-held device.

Helium leaks through cracks should not be confused with gas permeation through a bulk material. While helium has documented permeation constants (thus a calculable permeation rate) through glasses, ceramics, and synthetic materials, inert gases such as helium will not permeate most bulk metals.

Flight

Because it is lighter than air, airships and balloons are inflated with helium for lift. While hydrogen gas is approximately 7% more buoyant, helium has the advantage of being non-flammable (in addition to being fire retardant). While balloons are perhaps the most well-known use of helium, they are a minor part of all helium use. Another minor use is in rocketry, where helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo program needed about 370,000 m³ (13 million cubic feet) of helium to launch.

Minor commercial and recreational uses

For its low solubility in nervous tissue, helium mixtures such as trimix, heliox and heliair are used for deep diving to reduce the effects of narcosis. At depths below small amounts of hydrogen are added to a helium-oxygen mixture to counter the effects of high pressure nervous syndrome. At these depths the low density of helium is found to considerably reduce the effort of breathing.

Helium-neon lasers, a type of low-powered gas laser producing a red beam, had various practical applications which included barcode readers and laser pointers, before they were almost universally replaced by cheaper diode lasers.

For its inertness and high thermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a heat-transfer medium in some gas-cooled nuclear reactors.

Helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic refrigeration due to the resulting high heat capacity ratio and low Prandtl number. The inertness of helium has environmental advantages over conventional refrigeration systems which contribute to ozone depletion or global warming.

Scientific uses

The use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes, due to its extremely low index of refraction. This method is especially used in solar telescopes where a vacuum tight telescope tube would be too heavy.

Helium is a commonly used carrier gas for gas chromatography.

The age of rocks and minerals that contain uranium and thorium can be estimated by measuring the level of helium with a process known as helium dating.

Helium at low temperatures is used in cryogenics, and in certain crygenics applications. As examples of applications, liquid helium is used to cool certain metals to the extremely low temperatures required for superconductivity, such as in superconducting magnets for magnetic resonance imaging. The Large Hadron Collider at CERN uses 96 metric tons of liquid helium to maintain the temperature at 1.9 kelvin.

Safety

Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. If enough helium is inhaled that oxygen needed for normal respiration is replaced, asphyxia is possible. The safety issues for cryogenic helium are similar to those of liquid nitrogen; its extremely low temperatures can result in cold burns and the liquid-to-gas expansion ratio can cause explosions if no pressure-relief devices are installed.

Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature.

Biological effects

The speed of sound in helium is nearly three times the speed of sound in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the pitches of the resonant frequencies of the vocal tract. This causes a reedy, duck-like vocal quality. (The opposite effect, lowering frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride or xenon.)

Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration. Breathing pure helium continuously causes death by asphyxiation within minutes. Inhaling helium directly from pressurized cylinders is extremely dangerous, as the high flow rate can result in barotrauma, fatally rupturing lung tissue. However, death caused by helium is rare, with only two fatalities reported between 2000 and 2004 in the United States. However, there were two cases in 2010, one in the USA in January and another in Northern Ireland in November.

At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high-pressure nervous syndrome, a sort of reverse-anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem.

See also

Abiogenic petroleum origin

Helium-3 propulsion

Leidenfrost effect

Quantum solid

Superfluid

Tracer-gas leak testing method

References

Bibliography

External links

;General

U.S. Government' Bureau of Land Management: Sources, Refinement, and Shortage. With some History of Helium.

U.S. Geological Survey Publicationson Helium beginning 1996

It's Elemental – Helium

Chemistry in its element podcast (MP3) from the Royal Society of Chemistry's Chemistry World: Helium

;More detail

Helium (University of Nottingham)

Helium at the Helsinki University of Technology; includes pressure-temperature phase diagrams for helium-3 and helium-4

Lancaster University, Ultra Low Temperature Physics – includes a summary of some low temperature techniques

;Miscellaneous

Physics in Speech with audio samples that demonstrate the unchanged voice pitch

Article about helium and other noble gases

Category:Chemical elements Category:Noble gases Category:Coolants Category:Helium Category:Airship technology Category:Nuclear reactor coolants

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Name	Vin Diesel
Birth name	Mark Sinclair Vincent
Birth date	July 18, 1967
Birth place	New York City, U.S.
Occupation	Actor, director, producer, screenwriter
Years active	1990–present
Partner	Paloma Jimenez
Website	VinDiesel.com }}

Vin Diesel (born Mark Sinclair Vincent; July 18, 1967) is an American actor, writer, director and producer. He became known in the early 2000s, appearing in several successful Hollywood films, including The Fast and the Furious and xXx. He founded the production companies One Race Films, Tigon Studios and Racetrack Records.

Early life

Vin Diesel was born in New York City, the son of Delora, a psychiatrist and astrologer. Diesel has Italian and black ancestry. He has described himself as "definitely a person of color" and stated that he is "of ambiguous ethnicity – Italian and a lot of other stuff". Diesel has never met his biological father, and was raised by his African-American father, Irving, an acting instructor and theatre manager. He made his stage debut at age seven when he appeared in the children's play Dinosaur Door, written by Barbara Garson. The play was produced at Theater for the New City in New York's Greenwich Village. His involvement in the play came about when he, his brother and some friends had broken into Theater for the New City's space on Jane Street with the intent to vandalize it. They were confronted by the theater's artistic director, Crystal Field, who instead of calling the police, handed them scripts and offered them parts in the upcoming show. He remained involved with the theatre throughout adolescence, going on to attend the city's Hunter College, where his creative writing studies led him to begin screenwriting. Diesel became an active film-maker in the early 1990s, first earning notice for the short film Multi-Facial, which was selected for screening at the 1995 Cannes Festival. Diesel has identified himself as a "multi-faceted" actor as a result of early difficulties finding roles due to his mixed heritage.

In an interview on Late Night with Conan O'Brien, he said that he changed his name to "Vin Diesel" while working as a bouncer at the New York City nightclub Tunnel, because in that business one's real name is not usually given out. The name "Vin" is simply a shortened version of "Vincent". He received the nickname "Diesel" from his friends who said he ran off diesel fuel, referring to his non-stop energy.

He has a twin brother, Paul, a younger brother, Tim, and a sister, Samantha.

Career

Diesel's first film role was a brief uncredited appearance in the 1990 film Awakenings. He then produced, directed, and starred in the 1994 short film Multi-Facial, a short semi-autobiographical film which follows a struggling actor stuck in the audition process, because he is regarded as either "too black" or "too white", or not black or white enough. He made his first feature-length film, 1997's Strays, an urban drama in which he was self-cast as a gang boss whose love for a woman inspires him to try to change his ways. Written, directed and produced by Diesel, the film was selected for competition at the 1997 Sundance Festival, leading to an MTV deal to turn it into a series.

He was then cast in Steven Spielberg's 1998 Oscar-winning film Saving Private Ryan on the poignancy of his performance in Multi-Facial. In 1999 he earned critical acclaim for his voice work as the title character in the animation film The Iron Giant. He followed it up with a major role in the Business drama Boiler Room (2000) and then got his breakthrough role as the anti-hero Riddick in the science-fiction film Pitch Black (2000). He attained action hero super stardom with the box office hits, the street racing action film The Fast and the Furious (2001), starring opposite Paul Walker, and the action thriller xXx (2002).

In 2004, he reprised his role as Pitch Black's Riddick in The Chronicles of Riddick which was a box office failure considering the large budget. In 2005 he played a lighthearted role in the comedy film The Pacifier, which became a box office success. In 2006 he chose a dramatic role playing real-life mobster Jack DiNorscio in Find Me Guilty. Although he received critical acclaim for his performance, the film did poorly at the box office. Later that year he made a cameo appearance in The Fast and the Furious: Tokyo Drift, reprising his role from The Fast and The Furious. Diesel was originally offered the lead in 2 Fast 2 Furious but turned it down. He was also offered the chance to reprise his role from xXx in xXx: State of the Union but turned it down as well. In March 2006, Diesel stated that he was working on a sequel to The Chronicles of Riddick which as of 2011 is still in pre-production stages.

In 2007, he was set to produce and star as Agent 47 in the film adaptation of the video game Hitman, but eventually pulled back and served as executive producer on the film instead. In 2008 he starred in the science-fiction action thriller Babylon A.D..

He returned to the The Fast and The Furious series, alongside all the actors from the original 2001 film, in Fast & Furious, which released in April 2009. In 2011, he once again returned, for the fifth film in the series, titled Fast Five.

Personal life

Diesel is noted for his recognizable deep voice; he has said that his voice broke at around age 15, giving him a mature sounding voice on the telephone.

Around 2001, Diesel dated his The Fast and the Furious co-star, Michelle Rodriguez.

Diesel has expressed his love for the Dominican Republic, and how he relates to its multicultural facets. He is also acquainted with President Leonel Fernandez, and has since appeared in one of his earlier campaign ads. "Los Bandoleros", a short film directed by Diesel, was also filmed in the Dominican Republic.

Diesel claims that he prefers dating in Europe, where he is less likely to be recognized and where celebrities are not romantically linked to each other. He prefers to maintain his privacy regarding his personal life:

Diesel has played Dungeons & Dragons for over twenty years and wrote the foreword for the commemorative book 30 Years of Adventure: A Celebration of Dungeons & Dragons. In the 30th Anniversary of Dungeons & Dragons issue of Dragon Magazine, they examine the fact that Diesel played Dungeons & Dragons, and reveal that he had a fake tattoo of his character's name, "Melkor," on his stomach while filming xXx.

Diesel has a daughter, Hania Riley, born April 2, 2008, with his girlfriend, model Paloma Jimenez. Speaking to An tEolas, an Irish newspaper, Diesel stated he has been seen as a hard man, but is in touch with his soft side as a father.

Filmography

Actor

Multi-FacialStrays (film)Saving Private Ryan The Iron Giant Boiler Room (film)The Fast and the Furious (2001 film)xXx A Man Apart The Chronicles of Riddick The Pacifier Find Me Guilty Babylon A.D. (film)Fast & Furious ''Fast Five (film)

Year !! Movie !! Role !! Other notes
1990	Awakenings	Orderly
1994	|	Mike
1997	Strays ||	Rick
1998	|	Private Adrian Caparzo
1999	|	The Iron Giant (voice)	Animated
rowspan="2"	2000	Boiler Room ||	Chris Varick
Pitch Black (film)	Pitch Black	Richard B. Riddick
rowspan="2"	2001	The Fast and the Furious ||	Dominic Toretto
Knockaround Guys	Taylor Reese
2002	|	Xander Cage
2003	|	Sean Vetter
rowspan="2"	2004	|	Richard B. Riddick
The Chronicles of Riddick: Dark Fury	Richard B. Riddick (voice)
2005	|	Lieutenant Shane Wolfe
rowspan="2"	2006	|	Jack DiNorscio
The Fast and the Furious: Tokyo Drift	Dominic Toretto
2008	Babylon A.D. ||	Hugo Cornelius Toorop
2009	|	Dominic "Dom" Toretto
2011	Fast Five ||	Dominic "Dom" Toretto

Producer

! Film Title	! Year	! Description
Multi-Facial	1994	Producer
	1997	Executive Producer, Producer
xXx	2002	Executive Producer
A Man Apart	2003	Producer
Chronicles of Riddick	2004	Executive Producer
Life is a Dream	2004	Documentary, Executive Producer
Find Me Guilty	2006	Producer
	2007	Executive Producer
Fast & Furious	2009	Producer
	2011	Producer

Director & Writer

! Film	! Year
Multi-Facial	1994
	1997

Games

# The Chronicles of Riddick: Escape from Butcher Bay (2004) # The Chronicles of Riddick: Assault on Dark Athena (2009) # Wheelman (2009)

References

External links

Category:1967 births Category:Living people Category:Actors from New York City Category:American film actors Category:American film directors Category:American writers Category:American film actors Category:American film directors Category:American people of Italian descent Category:American film directors of Italian descent Category:American film producers Category:American screenwriters Category:American stage actors Category:American voice actors Category:American video game actors Category:American writers of Italian descent Category:Hunter College alumni Category:Dungeons & Dragons writers Category:Twin people from the United States Category:Writers from New York City

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