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- This page is about the chemical element helium. For the American indie rock band Helium see Helium (band).
Helium (He) is a colorless, odorless, tasteless, non-toxic, nearly inert monatomic chemical element that heads the noble gas series in the periodic table and whose atomic number is 2. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure. Its most abundant stable isotope is helium-4 and its rare stable isotope is helium-3. The behavior of liquid helium-4's two varieties—helium I and helium II—is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and those looking at the effects that near absolute zero temperatures have on matter (such as superconductivity).
Helium is the second most abundant and second lightest element in the periodic table. In the modern Universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth it is created by the radioactive decay of much heavier elements (alpha particles are helium nuclei produced by the decay of uranium). After its creation, part of it is trapped with natural gas in concentrations up to 7% by volume. It is extracted from the natural gas by a low temperature separation process called fractional distillation.
In 1868 the French astronomer Pierre Janssen first detected helium as an unknown yellow spectral line signature in light from a solar eclipse. Since then large reserves of helium have been found in the natural gas fields of the United States, which is by far the largest supplier of the gas. Helium is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships and as a protective gas for many industrial uses (such as arc welding and growing silicon wafers). Inhaling a small volume of the gas temporarily changes the quality of one's voice.
- 1 Notable characteristics
- 2 Electron energy levels
- 3 Applications
- 4 History
- 5 Occurrence
- 6 Isotopes
- 7 Vocal effect and health precautions
- 8 References
- 9 External links
Gas and plasma phases
Helium is a colorless, odorless, and non-toxic gas. It is the least reactive member of group 18 (the noble gases) of the periodic table and therefore virtually inert. Under standard temperature and pressure helium behaves very much like an ideal gas. Under virtually all conditions helium is monatomic. It has a thermal conductivity that is greater than any gas except hydrogen and its specific heat is unusually high. Helium is also less water soluble than any other gas known and its diffusion rate through solids is three times that of air and around 65% that of hydrogen. Helium's index of refraction is closer to unity than any other gas. This gas 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 40 K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion cooling.
Helium is chemically unreactive under all normal conditions due to its valence of zero. 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 with tungsten, iodine, fluorine, sulfur and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10, WHe2 and the molecular ions He2+, He2++, HeH+, and HeD+ 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 only held together by polarization forces . Theoretically, other compounds, like helium fluorohydride (HHeF), may also be possible.
Throughout the Universe, helium is mostly found in the plasma state whose properties are quite different to molecular helium. As a plasma, helium's electrons and protons are not bound together, 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, they interact with the Earth's magnetosphere giving rising to Birkeland currents and the aurora.
Solid and liquid phases
Helium solidifies only under great pressure. The resulting colorless almost invisible solid is highly compressible; applying pressure in the laboratory can decrease its volume by more than 30%. With a bulk modulus on the order of 5×107 Pa  it is 50 times more compressible than water. Unlike any other element, helium will fail to solidify and remain a liquid down to absolute zero at normal pressures. Solid helium requires a temperature of 1–1.5 K and about 26 standard atmospheres (2.6 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.
Helium I state
Below its boiling point of 4.21 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 heat is added to it. It also contracts when its temperature is lowered until it reaches the lambda point, when it stops boiling and suddenly expands. The rate of expansion decreases below the lambda point until about 1 K is reached; at which point expansion completely stops and helium I starts to contract again.
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 1/8th that of water, which is only 1/4th 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 is probably due to its boiling point being so close to absolute zero, which prevents random molecular motion (heat) 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. The isotope 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-3.
Helium II is a superfluid, a quantum-mechanical state of matter with strange properties. For example, when it flows through even capillaries of 10-7 to 10-8 m width 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.
Helium II also exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly 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, B. 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.
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-mechanical 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. So when heat is introduced, it will move at 20 meters per second at 1.8 K through helium II as waves in a phenomenon called second sound.
Electron energy levels
Depending on the spin orientation of the two electrons in the Helium atom, one speaks of parahelium for two anti-parallel spins (S=0) and of orthohelium for two parallel spins (S=1). For the orthohelium one of the electrons does not sit in the ground orbital (1s). 
Pressurized helium is commercially available. Helium is used for many purposes that require one or more of its unique properties; low boiling point, low density, low solubility, high thermal conductivity, or its inertness.
Airships and balloons (toy, weather, and research) are inflated with helium because it is lighter than air (1 m³ of helium will lift 1 kg). Helium is currently preferred to hydrogen in airships because, while it is more expensive, it is not flammable and has 92.64% of the lifting power of hydrogen.
Trimix, a mixture of helium, oxygen, and nitrogen, is used in deep-sea breathing gas systems to reduce the risk of nitrogen narcosis (high pressure nitrogen having a narcotic effect on the brain) and oxygen toxicity at high pressures. Higher pressures require a greater proportion of helium and reduced amounts of nitrogen and oxygen (every ten-meter increase in depth yields a one atmosphere increase of pressure). Heliox, a mixture of helium and oxygen, and heliair, a mixture of air and helium, is also used in this way. Below 600 meters (2000 ft) a mixture of hydrogen, helium, and oxygen called hydreliox is used to help prevent high pressure nervous syndrome. All these uses rely on helium's very low solubility in water (the major component of blood).
The extremely low boiling point makes helium useful as a coolant in magnetic resonance imaging, superconducting magnets, cryogenics, and to remove thermal noise from detectors used in astronomy. The extreme coldness of liquid helium is also used to produce superconductivity in some ordinary metals such as lead (lead becomes superconductive at 7.3 K), allowing for a completely free flow of electrons in the metal.
- Because of its high thermal conductivity and inertness, helium is used as a coolant in some nuclear reactors (for example, pebble-bed reactors) and in arc welding air-sensitive metals that require heavy welds.
- The high thermal conductivity and sound velocity of helium is also desirable in thermoacoustic refrigeration. The inertness of helium adds to the environmental advantage of this technology over conventional refrigeration systems which may contribute to ozone depleting and global warming effects.
- Its inertness makes it useful as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, protecting important historical documents, and in gas chromatography. This property also makes it useful in pressurizing liquid fuel rockets (see below) and in supersonic wind tunnels.
- The gain medium of the helium-neon laser (the first gas laser) most commonly used to scan bar codes is a mixture of helium and neon.
- This gas' rate of diffusion through solids is three times that of normal air, making it an excellent component in leak detection in high-vacuum equipment and high pressure containers.
- In rocketry 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 precool liquid hydrogen in space vehicles. For example, the Saturn 5 booster used in the Apollo program needed about 13 million ft³ (370,000 m³) of helium to launch.
- Physics researchers use alpha particles (helium nuclei) in particle accelerators and nuclear reaction experiments.
- Helium gas is used to fill the space between lenses in some solar telescopes because its extremely low index of refraction reduces the distorting effect of temperature variations in the gas filling the telescope (some telescopes are filled with vacuum instead).
- Radioactive decay of uranium and thorium produces alpha particles that quickly become helium. This happens at a known constant rate so if the containing rock or mineral can retain its helium then the ratio of helium to its radioactive parent atoms indicates its age. Alternatively, if the helium is not well-retained, the ratio of helium-3 to helium-4 contains some of the same information, since only helium-4 is produced by radioactive decay. Use of helium in this way is called helium dating.
Helium was first detected on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nm in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a total solar eclipse in India. Janssen was at first ridiculed since no element had ever been detected in space before being found on Earth. October 20th the same year, English astronomer Norman Lockyer also observed the same yellow line in the solar spectrum and concluded that it was caused by an unknown element after unsuccessfully testing to see if it were some new type of hydrogen. Since it was near the Fraunhofer D line he later named the new line D3, distinguishing it from the nearby D1 and D2 doublet lines of sodium. He and English chemist Edward Frankland named the element after the Greek word for the Sun god, Helios, and, assuming it was a metal, gave it an -ium ending (a mistake that was never corrected).
British chemist William Ramsay isolated helium on March 26, 1895 by treating cleveite (now known to be uraninite) with mineral acids. Ramsay was looking for argon but noticed the yellow D3 line after he removed nitrogen and oxygen from the gas liberated by the sulfuric acid he put on the cleveite sample. These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite the same year by Swedish chemists Per Teodor Cleve and Abraham Langlet in Uppsala in Sweden. They collected enough of the gas to accurately determine its atomic weight.
An oil drilling operation in Dexter, Kansas created a gas geyser in 1903 that contained 12% by volume of an unidentified gas. American chemists Hamilton Cady and David McFarland of the University of Kansas discovered it was helium and published a paper in 1907 saying that helium could be extracted from natural gas. Also in 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle is a helium nucleus.
Helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes in 1908 in Leiden by cooling the gas to less than one kelvin. He tried to solidify it by reducing the temperature to 0.8 K but failed because helium does not have a triple point temperature where the solid, liquid and gas phases are at equilibrium. It was first solidified in 1926 by his student Willem Hendrik Keesom who subjected helium to a similar amount of cooling as Kamerlingh Onnes but at 25 standard atmospheres of pressure.
In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that liquid helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. In 1972, the same phenomenon was observed in liquid helium-3 by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.
Production and use
Great quantities of helium were found in the natural gas fields of the American Great Plains, putting the United States in a very good position to become the leading world supplier. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium production plants during World War I. The goal was to supply barrage balloons with the non-flammable lifting gas. A total of 200,000 ft³ (5700 m³) of 92% helium was produced in the program even though only a few cubic feet (less than 100 liters) 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 7, 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. Helium 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. Helium use following World War II was depressed but the reserve was expanded in the 1950s to ensure a supply 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 425-mile pipeline from Bushton, Kansas to connect those plants with the government's Cliffside partially depleted gasfield, near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gasfield until needed, when it then was further purified.
By 1995 32 billion ft³ (1 billion m³) of the gas had been collected and the reserve was US$ 1.4 billion in debt, prompting the Congress of the United States to phase out the reserve starting the next year. The resulting "Helium Privatization Act of 1996" (P.L. 104-273) directed the United States Department of the Interior to start liquidating the reserve by 2005.
Helium produced before 1945 was about 98% 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 99.995% helium were available.
For many years the United States produced over 90% of commercially usable helium in the world. Extraction plants created in Canada, Poland, Russia, and other nations produced the remaining helium. In the early 2000s, Algeria and Qatar were added as well. Algeria quickly became the second leading producer of helium (16% of total in 2002). Through this time helium consumption has increased, as well as costs.
Helium is the second most abundant element in the known Universe after hydrogen and constitutes 23% of all elemental matter measured by mass even though there are 8 times as many hydrogen atoms as helium ('elemental matter' does not include dark matter or dark energy, which together may account for 96% of the Universe). It is concentrated in stars, where it is formed from hydrogen by the nuclear fusion of the proton-proton chain reaction and CNO cycle. This so-called 'hydrogen burning' process provides the energy stars need to shine. According to the Big Bang model of the early development of the Universe, the vast majority of helium was formed in the first three minutes after the Big Bang. Its widespread abundance is seen as part of the evidence that supports this theory.
However, in the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million at sea level and up to 15 miles (24 km), largely because most helium in the Earth's atmosphere escapes into space due to its inertness and low mass. There is a layer in the heterosphere (a part of the Earth's upper atmosphere) at 600 miles (about 1000 km) where helium is the dominant gas (although the total pressure is very low). Helium is the 71st most abundant element in the Earth's crust where it is found in 8 parts per billion (109). Helium only makes up 4 parts per trillion (1012) in seawater.
Essentially all helium on Earth is a result of radioactive decay of elements such as uranium and radon. A type of radiation called alpha particles are made of two protons and two neutrons, which also makes them helium-4 nuclei. These +2 positive ions easily gain the two electrons needed to make complete helium atoms. In this way an estimated 0.5 ft³ of helium is produced from every cubic mile of the Earth's crust (3.4 L/km3) per year . This decay product is found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite, monazite and beryl. There are also small amounts in mineral springs, volcanic gas and meteoric iron.
Helium in the crust is produced by the radioactive decay of uranium and thorium which are present in varying concentrations throughout the crust, but helium migrates and can collect in certain areas when conditions are right. Thus the greatest concentrations (trace amounts up to 7% by volume) of helium on the planet are in natural gas fields, from which most commercial helium is derived. As of 2002 over 100 million m³ (3.5 billion ft³) were produced annually with 80% of production from the United States, 16% from Algeria, and most of the rest from Russia. The principal source for U.S. production is the natural gas wells of the U.S. states of Texas, Oklahoma, Arizona and Kansas. Helium is also produced in Canada, Poland, the People's Republic of China, and Qatar.
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 hydrocarbons such as methane) from natural gas in order to extract gaseous helium (the general process is called fractional distillation). The resulting crude helium gas is subjected to a process of purification in which almost all of the remaining nitrogen and other gases are precipitated out of the mixture through successive exposures to lowering temperatures. 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.
Diffusion of crude natural gas through special semi-permeable membranes and other barriers is another method to recover and/or purify helium. Helium can also be synthesized by bombardment of lithium-6 or boron with high-velocity neutrons in a nuclear reactor to produce He-4 and tritium. The tritium decays with a half life of 12.5 years to produce He-3. This method of production, however, is not economically viable—at least for making normal commercial-grade helium. Fusion in exploding hydrogen bombs creates helium as well.
Although there are eight known isotopes of helium, only helium-3 and helium-4 are stable. In the Earth's atmosphere, there is one He-3 atom for every million He-4. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of He-3 is around a hundred times higher. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to study the origin of such rocks.
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, and its abundance serves as a test of cosmological models.
Equal mixtures of liquid He-3 and He-4 below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: He-4 atoms are bosons while He-3 atoms are fermions). There is only a trace amount of helium-3 on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. In stars, however, helium-3 is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds.
The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. These differing isotope abundances can be used to investigate the origin of rocks and the composition of the Earth's mantle.
It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived isotope is helium-5 with a half-life of 7.6×10−22 second. 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 hyperfragments that are created in certain nuclear reactions.
Vocal effect and health precautions
The voice of a person who has inhaled helium temporarily sounds high-pitched, resembling those of the cartoon characters Alvin and the Chipmunks (although their voices were produced by shifting the pitch of normal voices). This is because the speed of sound in helium is nearly three times that in air. As a result, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. The higher perceived pitch is only due to a different frequency shaping of the voice, the fundamental frequency of the vocal cords remains more or less the same.
Although the vocal effect of inhaling helium may be amusing, it can be dangerous if done to excess. The reason is not due to toxicity or any property of helium but simply due to it displacing oxygen needed for normal respiration. One must be aware that in mammals (with the notable exception of seals) the breathing reflex is not triggered by insufficient oxygen but rather excess of carbon dioxide. Unconsciousness, brain damage and even asphyxiation followed by death may result in extreme cases. Also, if helium is inhaled directly from pressurized cylinders the high flow rate can fatally rupture lung tissue.
Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. At high pressures, a mixture of helium and oxygen (heliox) can lead to high pressure nervous syndrome; a small proportion of nitrogen can alleviate the problem.
Containers of helium gas at 5 to 10 K should be treated as if they have liquid inside. This is due to the rapid and large increases in pressure and, if allowed, volume that occur when helium gas at that temperature is warmed to room temperature.
Sulfur hexafluoride has the opposite effect on the speed of sound as helium. It slows down the speed of sound to about one third of the speed of sound in air. It also non-toxic, like helium.
Specific references are indicated by comments in the article source
- The Encyclopedia of the Chemical Elements, edited by Cifford A. Hampel, "Helium" entry by L. W. Brandt (New York; Reinhold Book Corporation; 1968; pages 256-267) Library of Congress Catalog Card Number: 68-29938
- Nature's Building Blocks: An A-Z Guide to the Elements, by John Emsley (New York; Oxford University Press; 2001; pages 175-179) ISBN 0-19-850340-7
- Los Alamos National Laboratory (LANL.gov): Periodic Table, "Helium" (viewed 10 October 2002 and 25 March 2005)
- Guide to the Elements: Revised Edition, by Albert Stwertka (New York; Oxford University Press; 1998; pages 22-24) ISBN 0-19-512708-0
- The Elements: Third Edition, by John Emsley (New York; Oxford University Press; 1998; pages 94-95) ISBN 0-19-855818-X
- United States Geological Survey (usgs.gov): Mineral Information for Helium (PDF) (viewed 31 March 2005)
- The thermosphere: a part of the heterosphere, by J. Vercheval (viewed 1 Apr 2005)
- Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements, Zastenker G.N. et al., , published in Astrophysics, April 2002, vol. 45, no. 2, pp. 131-142(12)
- Dynamic and thermodynamic properties of solid helium in the reduced all-neighbours approximation of the self-consistent phonon theory, C. Malinowska-Adamska, P. Sŀoma, J. Tomaszewski, physica status solidi (b), Volume 240, Issue 1 , Pages 55 - 67; Published Online: 19 Sep 2003
- The Two Fluid Model of Superfluid Helium, S. Yuan, (viewed 4 Apr 2005)
- Rollin Film Rates in Liquid Helium, Henry A. Fairbank and C. T. Lane, Phys. Rev. 76, 1209–1211 (1949), from the online archive
- Introduction to Liquid Helium, at the NASA Goddard Space Flight Center (viewed 4 Apr 2005)
- Tests of vacuum VS helium in a solar telescope, Engvold, O.; Dunn, R. B.; Smartt, R. N.; Livingston, W. C.. Applied Optics, vol. 22, Jan. 1, 1983, p. 10-12.
- Bureau of Mines (1967). Minerals yearbook mineral fuels Year 1965, Volume II (1967), U. S. Government Printing Office.
- Helium: Fundamental models, Don L. Anderson, G. R. Foulger & Anders Meibom (viewed 5 Apr 2005)
- High Pressure Nervous Syndrome, Diving Medicine Online (viewed 5 Apr 2005)
- Nuclides and Isotopes Fourteenth Edition: Chart of the Nuclides, General Electric Company, 1989
- WebElements.com and EnvironmentalChemistry.com per the guidelines at Wikipedia's WikiProject Elements (viewed 10 October 2002)
- More detail
- Helium at the Helsinki University of Technology; includes pressure-temperature phase diagrams for helium-3 and helium-4.
- Helium Safety regarding inhalation
- Physics in Speech with audio samples that demonstrate the unchanged voice pitch
- Article about Helium and other noble gases
- this article contains phase diagram for helium
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