Radon

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Radon

Radon, ₈₆Rn

Radon
Pronunciation	/ˈreɪdɒn/ ​(RAY-don)
Appearance	colorless gas
Mass number	[222]
Radon in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Xe ↑ Rn ↓ Og astatine ← radon → francium
Atomic number (Z)	86
Group	group 18 (noble gases)
Period	period 6
Block	p-block
Electron configuration	[Xe] 4f14 5d10 6s2 6p6
Electrons per shell	2, 8, 18, 32, 18, 8
Physical properties
Phase at STP	gas
Melting point	202 K ​(−71 °C, ​−96 °F)
Boiling point	211.5 K ​(−61.7 °C, ​−79.1 °F)
Density (at STP)	9.73 g/L
when liquid (at b.p.)	4.4 g/cm3
Critical point	377 K, 6.28 MPa
Heat of fusion	3.247 kJ/mol
Heat of vaporization	18.10 kJ/mol
Molar heat capacity	5R/2 = 20.786 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 110 121 134 152 176 211
Atomic properties
Oxidation states	0, +2, +6
Electronegativity	Pauling scale: 2.2
Ionization energies	1st: 1037 kJ/mol
Covalent radius	150 pm
Van der Waals radius	220 pm
Spectral lines of radon
Other properties
Natural occurrence	from decay
Crystal structure	​face-centered cubic (fcc) (predicted)
Thermal conductivity	3.61×10−3  W/(m⋅K)
Magnetic ordering	non-magnetic
CAS Number	10043-92-2
History
Discovery	Ernest Rutherford and Robert B. Owens (1899)
First isolation	William Ramsay and Robert Whytlaw-Gray (1910)
Isotopes of radon
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 210Rn synth 2.4 h α 206Po 211Rn synth 14.6 h ε 211At α 207Po 222Rn trace 3.8235 d α 218Po 224Rn synth 1.8 h β− 224Fr

Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only radon-222 has a sufficiently long half-life (3.825 days) for it to be released from the soil and rock, where it is generated. Radon isotopes are the immediate decay products of radium isotopes. Radon's most stable isotope, radon-222, has a half-life of only 3.8 days, making radon one of the rarest elements. Radon will be present on Earth for several billion more years, despite its short half-life, because it is constantly being produced as a step in the decay chain of uranium-238, and that of thorium-232, each of which is an extremely abundant radioactive nuclide with a half-life of several billion years. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead. Radon-222 occurs in significant quantities as a step in the normal radioactive decay chain of uranium-238, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into lead-206, which is stable. Radon-220 occurs in minute quantities as an intermediate step in the decay chain of thorium-232, also known as the thorium series, which eventually decays into lead-208, which is stable.

Under standard conditions, radon is gaseous and can be easily inhaled, posing a health hazard. However, the primary danger comes not from radon itself, but from its decay products, known as radon daughters. These decay products, often existing as single atoms or ions, can attach themselves to airborne dust particles. Although radon is a noble gas and does not adhere to lung tissue, meaning it is often exhaled before decaying, the radon daughters attached to dust are more likely to stick to the lungs. This increases the risk of harm, as the radon daughters can cause damage to lung tissue. Radon and its daughters are, taken together, often the single largest contributor to an individual's background radiation dose, but due to local differences in geology, the level of exposure to radon gas differs from place to place. A common source is uranium-containing minerals in the ground, and therefore it accumulates in subterranean areas such as basements. Radon can also occur in some ground water like spring waters and hot springs. Radon trapped in permafrost may be released by climate change induced thawing of permafrosts. It is possible to test for radon in buildings, and to use techniques such as sub-slab depressurization for mitigation.

Epidemiological studies have shown a clear link between breathing high concentrations of radon and incidence of lung cancer. Radon is a contaminant that affects indoor air quality worldwide. According to the United States Environmental Protection Agency (EPA), radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the United States. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA policy-oriented estimates. Significant uncertainties exist for the health effects of low-dose exposures.

Characteristics

Emission spectrum of radon, photographed by Ernest Rutherford in 1908. Numbers at the side of the spectrum are wavelengths. The middle spectrum is of radium emanation (radon), while the outer two are of helium (added to calibrate the wavelengths).

Physical properties

Radon is a colorless, odorless, and tasteless gas and therefore is not detectable by human senses alone. At standard temperature and pressure, it forms a monatomic gas with a density of 9.73 kg/m³, about 8 times the density of the Earth's atmosphere at sea level, 1.217 kg/m³. It is one of the densest gases at room temperature (a few are denser, e.g. CF₃(CF₂)₂CF₃ and WF₆) and is the densest of the noble gases. Although colorless at standard temperature and pressure, when cooled below its freezing point of 202 K (−71 °C; −96 °F), it emits a brilliant radioluminescence that turns from yellow to orange-red as the temperature lowers. Upon condensation, it glows because of the intense radiation it produces. It is sparingly soluble in water, but more soluble than lighter noble gases. It is appreciably more soluble in organic liquids than in water. Its solubility equation is as follows,

${\displaystyle \chi =\exp (B/T-A),}$

where ${\displaystyle \chi }$ is the molar fraction of radon, ${\displaystyle T}$ is the absolute temperature, and ${\displaystyle A}$ and ${\displaystyle B}$ are solvent constants.

Chemical properties

Radon is a member of the zero-valence elements that are called noble gases, and is chemically not very reactive. The 3.8-day half-life of radon-222 makes it useful in physical sciences as a natural tracer. Because radon is a gas at standard conditions, unlike its decay-chain parents, it can readily be extracted from them for research.

It is inert to most common chemical reactions, such as combustion, because the outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound. Its first ionization energy—the minimum energy required to extract one electron from it—is 1037 kJ/mol. In accordance with periodic trends, radon has a lower electronegativity than the element one period before it, xenon, and is therefore more reactive. Early studies concluded that the stability of radon hydrate should be of the same order as that of the hydrates of chlorine (Cl
₂) or sulfur dioxide (SO
₂), and significantly higher than the stability of the hydrate of hydrogen sulfide (H
₂S).

Because of its cost and radioactivity, experimental chemical research is seldom performed with radon, and as a result there are very few reported compounds of radon, all either fluorides or oxides. Radon can be oxidized by powerful oxidizing agents such as fluorine, thus forming radon difluoride (RnF
₂). It decomposes back to its elements at a temperature of above 523 K (250 °C; 482 °F), and is reduced by water to radon gas and hydrogen fluoride: it may also be reduced back to its elements by hydrogen gas. It has a low volatility and was thought to be RnF
₂. Because of the short half-life of radon and the radioactivity of its compounds, it has not been possible to study the compound in any detail. Theoretical studies on this molecule predict that it should have a Rn–F bond distance of 2.08 ångströms (Å), and that the compound is thermodynamically more stable and less volatile than its lighter counterpart xenon difluoride (XeF
₂). The octahedral molecule RnF
₆ was predicted to have an even lower enthalpy of formation than the difluoride. The [RnF]⁺ ion is believed to form by the following reaction:

Rn (g) + 2 [O
₂]⁺
[SbF
₆]⁻
(s) → [RnF]⁺
[Sb
₂F
₁₁]⁻
(s) + 2 O
₂ (g)

For this reason, antimony pentafluoride together with chlorine trifluoride and N
₂F
₂Sb
₂F
₁₁ have been considered for radon gas removal in uranium mines due to the formation of radon–fluorine compounds. Radon compounds can be formed by the decay of radium in radium halides, a reaction that has been used to reduce the amount of radon that escapes from targets during irradiation. Additionally, salts of the [RnF]⁺ cation with the anions SbF⁻
₆, TaF⁻
₆, and BiF⁻
₆ are known. Radon is also oxidised by dioxygen difluoride to RnF
₂ at 173 K (−100 °C; −148 °F).

Radon oxides are among the few other reported compounds of radon; only the trioxide (RnO
₃) has been confirmed. The higher fluorides RnF
₄ and RnF
₆ have been claimed and are calculated to be stable, but their identification is unclear. They may have been observed in experiments where unknown radon-containing products distilled together with xenon hexafluoride: these may have been RnF
₄, RnF
₆, or both. Trace-scale heating of radon with xenon, fluorine, bromine pentafluoride, and either sodium fluoride or nickel fluoride was claimed to produce a higher fluoride as well which hydrolysed to form RnO
₃. While it has been suggested that these claims were really due to radon precipitating out as the solid complex [RnF]⁺
₂[NiF₆]²⁻, the fact that radon coprecipitates from aqueous solution with CsXeO
₃F has been taken as confirmation that RnO
₃ was formed, which has been supported by further studies of the hydrolysed solution. That [RnO₃F]⁻ did not form in other experiments may have been due to the high concentration of fluoride used. Electromigration studies also suggest the presence of cationic [HRnO₃]⁺ and anionic [HRnO₄]⁻ forms of radon in weakly acidic aqueous solution (pH > 5), the procedure having previously been validated by examination of the homologous xenon trioxide.

The decay technique has also been used. Avrorin et al. reported in 1982 that ²¹²Fr compounds cocrystallised with their caesium analogues appeared to retain chemically bound radon after electron capture; analogies with xenon suggested the formation of RnO₃, but this could not be confirmed.

It is likely that the difficulty in identifying higher fluorides of radon stems from radon being kinetically hindered from being oxidised beyond the divalent state because of the strong ionicity of radon difluoride (RnF
₂) and the high positive charge on radon in RnF⁺; spatial separation of RnF
₂ molecules may be necessary to clearly identify higher fluorides of radon, of which RnF
₄ is expected to be more stable than RnF
₆ due to spin–orbit splitting of the 6p shell of radon (Rn^IV would have a closed-shell 6s²
6p²
_1/2 configuration). Therefore, while RnF
₄ should have a similar stability to xenon tetrafluoride (XeF
₄), RnF
₆ would likely be much less stable than xenon hexafluoride (XeF
₆): radon hexafluoride would also probably be a regular octahedral molecule, unlike the distorted octahedral structure of XeF
₆, because of the inert pair effect. Because radon is quite electropositive for a noble gas, it is possible that radon fluorides actually take on highly fluorine-bridged structures and are not volatile. Extrapolation down the noble gas group would suggest also the possible existence of RnO, RnO₂, and RnOF₄, as well as the first chemically stable noble gas chlorides RnCl₂ and RnCl₄, but none of these have yet been found.

Radon carbonyl (RnCO) has been predicted to be stable and to have a linear molecular geometry. The molecules Rn
₂ and RnXe were found to be significantly stabilized by spin-orbit coupling. Radon caged inside a fullerene has been proposed as a drug for tumors. Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist; RnF
₈ should be highly unstable chemically (XeF₈ is thermodynamically unstable). It is predicted that the most stable Rn(VIII) compound would be barium perradonate (Ba₂RnO₆), analogous to barium perxenate. The instability of Rn(VIII) is due to the relativistic stabilization of the 6s shell, also known as the inert pair effect.

Radon reacts with the liquid halogen fluorides ClF, ClF
₃, ClF
₅, BrF
₃, BrF
₅, and IF
₇ to form RnF
₂. In halogen fluoride solution, radon is nonvolatile and exists as the RnF⁺ and Rn²⁺ cations; addition of fluoride anions results in the formation of the complexes RnF⁻
₃ and RnF²⁻
₄, paralleling the chemistry of beryllium(II) and aluminium(III). The standard electrode potential of the Rn²⁺/Rn couple has been estimated as +2.0 V, although there is no evidence for the formation of stable radon ions or compounds in aqueous solution.

Isotopes

Main article: Isotopes of radon

Radon has no stable isotopes. Thirty-nine radioactive isotopes have been characterized, with mass numbers ranging from 193 to 231. Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is ²²²Rn (half-life 3.82 days), which is a decay product of ²²⁶Ra, the latter being itself a decay product of ²³⁸U. A trace amount of the (highly unstable) isotope ²¹⁸Rn (half-life about 35 milliseconds) is also among the daughters of ²²²Rn. The isotope ²¹⁶Rn would be produced by the double beta decay of natural ²¹⁶Po; while energetically possible, this process has however never been seen.

Three other radon isotopes have a half-life of over an hour: ²¹¹Rn (about 15 hours), ²¹⁰Rn (2.4 hours) and ²²⁴Rn (about 1.8 hours). However, none of these three occur naturally. The ²²⁰Rn isotope is a natural decay product of the most stable thorium isotope (²³²Th), and is commonly referred to as thoron. It has a half-life of 55.6 seconds and also emits alpha radiation. Similarly, ²¹⁹Rn is derived from the most stable isotope of actinium (²²⁷Ac)—named "actinon"—and is an alpha emitter with a half-life of 3.96 seconds. No radon isotopes occur significantly in the neptunium (²³⁷Np) decay series, though trace amounts of the isotopes ²²¹Rn (26 minutes) and ²¹⁷Rn (0.5 milliseconds) are produced in minor branches.

The radium or uranium series

Daughters

Main article: Decay chain § Uranium series

²²²Rn belongs to the radium and uranium-238 decay chain, and has a half-life of 3.8235 days. Its first four products (excluding marginal decay schemes) are very short-lived, meaning that the corresponding disintegrations are indicative of the initial radon distribution. Its decay goes through the following sequence:

• ²²²Rn, 3.82 days, alpha decaying to...
• ²¹⁸Po, 3.10 minutes, alpha decaying to...
• ²¹⁴Pb, 26.8 minutes, beta decaying to...
• ²¹⁴Bi, 19.9 minutes, beta decaying to...
• ²¹⁴Po, 0.1643 ms, alpha decaying to...
• ²¹⁰Pb, which has a much longer half-life of 22.3 years, beta decaying to...
• ²¹⁰Bi, 5.013 days, beta decaying to...
• ²¹⁰Po, 138.376 days, alpha decaying to...
• ²⁰⁶Pb, stable.

The radon equilibrium factor is the ratio between the activity of all short-period radon progenies (which are responsible for most of radon's biological effects), and the activity that would be at equilibrium with the radon parent.

If a closed volume is constantly supplied with radon, the concentration of short-lived isotopes will increase until an equilibrium is reached where the overall decay rate of the decay products equals that of the radon itself. The equilibrium factor is 1 when both activities are equal, meaning that the decay products have stayed close to the radon parent long enough for the equilibrium to be reached, within a couple of hours. Under these conditions, each additional pCi/L of radon will increase exposure by 0.01 working level (WL, a measure of radioactivity commonly used in mining). These conditions are not always met; in many homes, the equilibrium factor is typically 40%; that is, there will be 0.004 WL of daughters for each pCi/L of radon in the air. ²¹⁰Pb takes much longer (decades) to come in equilibrium with radon, but, if the environment permits accumulation of dust over extended periods of time, ²¹⁰Pb and its decay products may contribute to overall radiation levels as well.

Because of their electrostatic charge, radon progenies adhere to surfaces or dust particles, whereas gaseous radon does not. Attachment removes them from the air, usually causing the equilibrium factor in the atmosphere to be less than 1. The equilibrium factor is also lowered by air circulation or air filtration devices, and is increased by airborne dust particles, including cigarette smoke. The equilibrium factor found in epidemiological studies is 0.4.

History and etymology

Apparatus used by Ramsay and Whytlaw-Gray to isolate radon. M is a [[Capillary tube]], where approximately 0.1 mm³ were isolated. Radon mixed with hydrogen entered the evacuated system through siphon A; mercury is shown in black.

Radon was discovered in 1899 by Ernest Rutherford and Robert B. Owens at McGill University in Montreal. It was the fifth radioactive element to be discovered, after uranium, thorium, radium, and polonium. In 1899, Pierre and Marie Curie observed that the gas emitted by radium remained radioactive for a month. Later that year, Rutherford and Owens noticed variations when trying to measure radiation from thorium oxide. Rutherford noticed that the compounds of thorium continuously emit a radioactive gas that remains radioactive for several minutes, and called this gas "emanation" (from Latin: emanare, to flow out, and emanatio, expiration), and later "thorium emanation" ("Th Em"). In 1900, Friedrich Ernst Dorn reported some experiments in which he noticed that radium compounds emanate a radioactive gas he named "radium emanation" ("Ra Em"). In 1901, Rutherford and Harriet Brooks demonstrated that the emanations are radioactive, but credited the Curies for the discovery of the element. In 1903, similar emanations were observed from actinium by André-Louis Debierne, and were called "actinium emanation" ("Ac Em").

Several shortened names were soon suggested for the three emanations: exradio, exthorio, and exactinio in 1904; radon (Ro), thoron (To), and akton or acton (Ao) in 1918; radeon, thoreon, and actineon in 1919, and eventually radon, thoron, and actinon in 1920. (The name radon is not related to that of the Austrian mathematician Johann Radon.) The likeness of the spectra of these three gases with those of argon, krypton, and xenon, and their observed chemical inertia led Sir William Ramsay to suggest in 1904 that the "emanations" might contain a new element of the noble-gas family.

In 1909, Ramsay and Robert Whytlaw-Gray isolated radon and determined its melting temperature and approximate density. In 1910, they determined that it was the heaviest known gas. They wrote that "L'expression l'émanation du radium est fort incommode" ("the expression 'radium emanation' is very awkward") and suggested the new name niton (Nt) (from Latin: nitens, shining) to emphasize the radioluminescence property, and in 1912 it was accepted by the International Commission for Atomic Weights. In 1923, the International Committee for Chemical Elements and International Union of Pure and Applied Chemistry (IUPAC) chose the name of the most stable isotope, radon, as the name of the element. The isotopes thoron and actinon were later renamed ²²⁰Rn and ²¹⁹Rn. This has caused some confusion in the literature regarding the element's discovery as while Dorn had discovered radon the isotope, he was not the first to discover radon the element.

As late as the 1960s, the element was also referred to simply as emanation. The first synthesized compound of radon, radon fluoride, was obtained in 1962. Even today, the word radon may refer to either the element or its isotope ²²²Rn, with thoron remaining in use as a short name for ²²⁰Rn to stem this ambiguity. The name actinon for ²¹⁹Rn is rarely encountered today, probably due to the short half-life of that isotope.

The danger of high exposure to radon in mines, where exposures can reach 1,000,000 Bq/m³, has long been known. In 1530, Paracelsus described a wasting disease of miners, the mala metallorum, and Georg Agricola recommended ventilation in mines to avoid this mountain sickness (Bergsucht). In 1879, this condition was identified as lung cancer by Harting and Hesse in their investigation of miners from Schneeberg, Germany. The first major studies with radon and health occurred in the context of uranium mining in the Joachimsthal region of Bohemia. In the US, studies and mitigation only followed decades of health effects on uranium miners of the Southwestern US employed during the early Cold War; standards were not implemented until 1971.

In the early 20th century in the US, gold contaminated with the radon daughter ²¹⁰Pb entered the jewelry industry. This was from gold brachytherapy seeds that had held ²²²Rn, which were melted down after the radon had decayed.

The presence of radon in indoor air was documented as early as 1950. Beginning in the 1970s, research was initiated to address sources of indoor radon, determinants of concentration, health effects, and mitigation approaches. In the US, the problem of indoor radon received widespread publicity and intensified investigation after a widely publicized incident in 1984. During routine monitoring at a Pennsylvania nuclear power plant, a worker was found to be contaminated with radioactivity. A high concentration of radon in his home was subsequently identified as responsible.

Occurrence

See also: Radium and radon in the environment

Concentration units

²¹⁰Pb is formed from the decay of ²²²Rn. Here is a typical deposition rate of ²¹⁰Pb as observed in Japan as a function of time, due to variations in radon concentration.

All discussions of radon concentrations in the environment refer to ²²²Rn. While the average rate of production of ²²⁰Rn (from the thorium decay series) is about the same as that of ²²²Rn, the amount of ²²⁰Rn in the environment is much less than that of ²²²Rn because of the short half-life of ²²⁰Rn (55 seconds, versus 3.8 days respectively).

Radon concentration in the atmosphere is usually measured in becquerel per cubic meter (Bq/m³), the SI derived unit. Another unit of measurement common in the US is picocuries per liter (pCi/L); 1 pCi/L = 37 Bq/m³. Typical domestic exposures average about 48 Bq/m³ indoors, though this varies widely, and 15 Bq/m³ outdoors.

In the mining industry, the exposure is traditionally measured in working level (WL), and the cumulative exposure in working level month (WLM); 1 WL equals any combination of short-lived ²²²Rn daughters (²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po) in 1 liter of air that releases 1.3 × 10⁵ MeV of potential alpha energy; 1 WL is equivalent to 2.08 × 10⁻⁵ joules per cubic meter of air (J/m³). The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (J·h/m³). One WLM is equivalent to 3.6 × 10⁻³ J·h/m³. An exposure to 1 WL for 1 working-month (170 hours) equals 1 WLM cumulative exposure. A cumulative exposure of 1 WLM is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m³.

²²²Rn decays to ²¹⁰Pb and other radioisotopes. The levels of ²¹⁰Pb can be measured. The rate of deposition of this radioisotope is weather-dependent.

Radon concentrations found in natural environments are much too low to be detected by chemical means. A 1,000 Bq/m³ (relatively high) concentration corresponds to 0.17 picogram per cubic meter (pg/m³). The average concentration of radon in the atmosphere is about 6×10⁻¹⁸ molar percent, or about 150 atoms in each milliliter of air. The radon activity of the entire Earth's atmosphere originates from only a few tens of grams of radon, consistently replaced by decay of larger amounts of radium, thorium, and uranium.

Natural

Radon concentration next to a uranium mine

Radon is produced by the radioactive decay of radium-226, which is found in uranium ores, phosphate rock, shales, igneous and metamorphic rocks such as granite, gneiss, and schist, and to a lesser degree, in common rocks such as limestone. Every square mile of surface soil, to a depth of 6 inches (2.6 km² to a depth of 15 cm), contains approximately 1 gram of radium, which releases radon in small amounts to the atmosphere. On a global scale, it is estimated that 2.4 billion curies (90 EBq) of radon are released from soil annually. This is equivalent to some 15.3 kilograms (34 lb).

Radon concentration can differ widely from place to place. In the open air, it ranges from 1 to 100 Bq/m³, even less (0.1 Bq/m³) above the ocean. In caves or ventilated mines, or poorly ventilated houses, its concentration climbs to 20–2,000 Bq/m³.

Radon concentration can be much higher in mining contexts. Ventilation regulations instruct to maintain radon concentration in uranium mines under the "working level", with 95th percentile levels ranging up to nearly 3 WL (546 pCi ²²²Rn per liter of air; 20.2 kBq/m³, measured from 1976 to 1985). The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m³ (1.2 nCi/L) with maximal value of 160 kBq/m³ (4.3 nCi/L).

Radon mostly appears with the decay chain of the radium and uranium series (²²²Rn), and marginally with the thorium series (²²⁰Rn). The element emanates naturally from the ground, and some building materials, all over the world, wherever traces of uranium or thorium are found, and particularly in regions with soils containing granite or shale, which have a higher concentration of uranium. Not all granitic regions are prone to high emissions of radon. Being a rare gas, it usually migrates freely through faults and fragmented soils, and may accumulate in caves or water. Owing to its very short half-life (four days for ²²²Rn), radon concentration decreases very quickly when the distance from the production area increases. Radon concentration varies greatly with season and atmospheric conditions. For instance, it has been shown to accumulate in the air if there is a meteorological inversion and little wind.

High concentrations of radon can be found in some spring waters and hot springs. The towns of Boulder, Montana; Misasa; Bad Kreuznach, Germany; and the country of Japan have radium-rich springs that emit radon. To be classified as a radon mineral water, radon concentration must be above 2 nCi/L (74 kBq/m³). The activity of radon mineral water reaches 2,000 kBq/m³ in Merano and 4,000 kBq/m³ in Lurisia (Italy).

Natural radon concentrations in the Earth's atmosphere are so low that radon-rich water in contact with the atmosphere will continually lose radon by volatilization. Hence, ground water has a higher concentration of ²²²Rn than surface water, because radon is continuously produced by radioactive decay of ²²⁶Ra present in rocks. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone because of diffusional losses to the atmosphere.

In 1971, Apollo 15 passed 110 km (68 mi) above the Aristarchus plateau on the Moon, and detected a significant rise in alpha particles thought to be caused by the decay of ²²²Rn. The presence of ²²²Rn has been inferred later from data obtained from the Lunar Prospector alpha particle spectrometer.

Radon is found in some petroleum. Because radon has a similar pressure and temperature curve to propane, and oil refineries separate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can become contaminated because of decaying radon and its products.

Residues from the petroleum and natural gas industry often contain radium and its daughters. The sulfate scale from an oil well can be radium rich, while the water, oil, and gas from a well often contains radon. Radon decays to form solid radioisotopes that form coatings on the inside of pipework.

Accumulation in buildings

Typical log-normal radon distribution in dwellings

 

Predicted fraction of U.S. homes having concentrations of radon exceeding the EPA's recommended action level of 4 pCi/L

High concentrations of radon in homes were discovered by chance in 1985 after the stringent radiation testing conducted at the new Limerick Generating Station nuclear power plant revealed that Stanley Watras, a construction engineer at the plant, was contaminated by radioactive substances even though the reactor had never been fueled. Typical domestic exposures are of approximately 100 Bq/m³ (2.7 pCi/L) indoors. Some level of radon will be found in all buildings. Radon mostly enters a building directly from the soil through the lowest level in the building that is in contact with the ground. High levels of radon in the water supply can also increase indoor radon air levels. Typical entry points of radon into buildings are cracks in solid foundations and walls, construction joints, gaps in suspended floors and around service pipes, cavities inside walls, and the water supply. Radon concentrations in the same place may differ by double/half over one hour. Also, the concentration in one room of a building may be significantly different from the concentration in an adjoining room. The soil characteristics of the dwellings are the most important source of radon for the ground floor and higher concentration of indoor radon observed on lower floors. Most of the high radon concentrations have been reported from places near fault zones; hence the existence of a relation between the exhalation rate from faults and indoor radon concentrations is obvious.

The distribution of radon concentrations will generally differ from room to room, and the readings are averaged according to regulatory protocols. Indoor radon concentration is usually assumed to follow a log-normal distribution on a given territory. Thus, the geometric mean is generally used for estimating the "average" radon concentration in an area.

The mean concentration ranges from less than 10 Bq/m³ to over 100 Bq/m³ in some European countries.

Some of the highest radon hazard in the US is found in Iowa and in the Appalachian Mountain areas in southeastern Pennsylvania. Iowa has the highest average radon concentrations in the US due to significant glaciation that ground the granitic rocks from the Canadian Shield and deposited it as soils making up the rich Iowa farmland. Many cities within the state, such as Iowa City, have passed requirements for radon-resistant construction in new homes. The second highest readings in Ireland were found in office buildings in the Irish town of Mallow, County Cork, prompting local fears regarding lung cancer.

In a few places, uranium tailings have been used for landfills and were subsequently built upon, resulting in possible increased exposure to radon.

Since radon is a colorless, odorless gas, the only way to know how much is present in the air or water is to perform tests. In the US, radon test kits are available to the public at retail stores, such as hardware stores, for home use, and testing is available through licensed professionals, who are often home inspectors. Efforts to reduce indoor radon levels are called radon mitigation. In the US, the EPA recommends all houses be tested for radon. In the UK under the Housing Health & Safety Rating System (HHSRS) property owners have an obligation to evaluate potential risks and hazards to health and safety in a residential property.

Industrial production

Radon is obtained as a by-product of uraniferous ores processing after transferring into 1% solutions of hydrochloric or hydrobromic acids. The gas mixture extracted from the solutions contains H
₂, O
₂, He, Rn, CO
₂, H
₂O and hydrocarbons. The mixture is purified by passing it over copper at 993 K (720 °C; 1,328 °F) to remove the H
₂ and the O
₂, and then KOH and P
₂O
₅ are used to remove the acids and moisture by sorption. Radon is condensed by liquid nitrogen and purified from residue gases by sublimation.

Radon commercialization is regulated, but it is available in small quantities for the calibration of ²²²Rn measurement systems, at a price, in 2008, of almost US$6,000 (equivalent to $8,491 in 2023) per milliliter of radium solution (which only contains about 15 picograms of actual radon at any given moment). Radon is produced by a solution of radium-226 (half-life of 1,600 years). Radium-226 decays by alpha-particle emission, producing radon that collects over samples of radium-226 at a rate of about 1 mm³/day per gram of radium; equilibrium is quickly achieved and radon is produced in a steady flow, with an activity equal to that of the radium (50 Bq). Gaseous ²²²Rn (half-life of about four days) escapes from the capsule through diffusion.

Concentration scale

Bq/m3	pCi/L	Occurrence example
1	~0.027	Radon concentration at the shores of large oceans is typically 1 Bq/m3. Radon trace concentration above oceans or in Antarctica can be lower than 0.1 Bq/m3.
10	0.27	Mean continental concentration in the open air: 10 to 30 Bq/m3. Based on a series of surveys, the global mean indoor radon concentration is estimated to be 39 Bq/m3.
100	2.7	Typical indoor domestic exposure. Most countries have adopted a radon concentration of 200–400 Bq/m3 for indoor air as an Action or Reference Level. If testing shows levels less than 4 picocuries radon per liter of air (150 Bq/m3), then no action is necessary. A cumulated exposure of 230 Bq/m3 of radon gas concentration during a period of 1 year corresponds to 1 WLM.
1,000	27	Very high radon concentrations (>1000 Bq/m3) have been found in houses built on soils with a high uranium content and/or high permeability of the ground. If levels are 20 picocuries radon per liter of air (800 Bq/m3) or higher, the home owner should consider some type of procedure to decrease indoor radon levels. Allowable concentrations in uranium mines are approximately 1,220 Bq/m3 (33 pCi/L)
10,000	270	The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m3 (about 1.2 nCi/L) with maximal value of 160 kBq/m3 (about 4.3 nCi/L).
100,000	~2700	About 100,000 Bq/m3 (2.7 nCi/L) was measured in Stanley Watras's basement.
1,000,000	27000	Concentrations reaching 1,000,000 Bq/m3 can be found in unventilated uranium mines.
~5.54 × 1019	~1.5 × 1018	Theoretical upper limit: Radon gas (222Rn) at 100% concentration (1 atmosphere, 0 °C); 1.538×105 curies/gram; 5.54×1019 Bq/m3.

Applications

Medical

Main article: Radioactive quackery

An early-20th-century form of quackery was the treatment of maladies in a radiotorium. It was a small, sealed room for patients to be exposed to radon for its "medicinal effects". The carcinogenic nature of radon due to its ionizing radiation became apparent later. Radon's molecule-damaging radioactivity has been used to kill cancerous cells, but it does not increase the health of healthy cells. The ionizing radiation causes the formation of free radicals, which results in cell damage, causing increased rates of illness, including cancer.

Exposure to radon has been suggested to mitigate autoimmune diseases such as arthritis in a process known as radiation hormesis. As a result, in the late 20th century and early 21st century, "health mines" established in Basin, Montana, attracted people seeking relief from health problems such as arthritis through limited exposure to radioactive mine water and radon. The practice is discouraged because of the well-documented ill effects of high doses of radiation on the body.

Radioactive water baths have been applied since 1906 in Jáchymov, Czech Republic, but even before radon discovery they were used in Bad Gastein, Austria. Radium-rich springs are also used in traditional Japanese onsen in Misasa, Tottori Prefecture. Drinking therapy is applied in Bad Brambach, Germany, and during the early 20th century, water from springs with radon in them was bottled and sold (this water had little to no radon in it by the time it got to consumers due to radon's short half-life). Inhalation therapy is carried out in Gasteiner-Heilstollen, Austria; Świeradów-Zdrój, Czerniawa-Zdrój, Kowary, Lądek-Zdrój, Poland; Harghita Băi, Romania; and Boulder, Montana. In the US and Europe, there are several "radon spas", where people sit for minutes or hours in a high-radon atmosphere, such as at Bad Schmiedeberg, Germany.

Radon has been produced commercially for use in radiation therapy, but for the most part has been replaced by radionuclides made in particle accelerators and nuclear reactors. Radon has been used in implantable seeds, made of gold or glass, primarily used to treat cancers, known as brachytherapy. The gold seeds were produced by filling a long tube with radon pumped from a radium source, the tube being then divided into short sections by crimping and cutting. The gold layer keeps the radon within, and filters out the alpha and beta radiations, while allowing the gamma rays to escape (which kill the diseased tissue). The activities might range from 0.05 to 5 millicuries per seed (2 to 200 MBq). The gamma rays are produced by radon and the first short-lived elements of its decay chain (²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, ²¹⁴Po).

After 11 half-lives (42 days), radon radioactivity is at 1/2,048 of its original level. At this stage, the predominant residual activity of the seed originates from the radon decay product ²¹⁰Pb, whose half-life (22.3 years) is 2,000 times that of radon and its descendants ²¹⁰Bi and ²¹⁰Po.

Scientific

Radon emanation from the soil varies with soil type and with surface uranium content, so outdoor radon concentrations can be used to track air masses to a limited degree. This fact has been put to use by some atmospheric scientists (Radon storm). Because of radon's rapid loss to air and comparatively rapid decay, radon is used in hydrologic research that studies the interaction between groundwater and streams. Any significant concentration of radon in a stream is a good indicator that there are local inputs of groundwater.

Radon soil-concentration has been used in an experimental way to map buried close-subsurface geological faults because concentrations are generally higher over the faults. Similarly, it has found some limited use in prospecting for geothermal gradients.

Some researchers have investigated changes in groundwater radon concentrations for earthquake prediction. Increases in radon were noted before the 1966 Tashkent and 1994 Mindoro earthquakes. Radon has a half-life of approximately 3.8 days, which means that it can be found only shortly after it has been produced in the radioactive decay chain. For this reason, it has been hypothesized that increases in radon concentration is due to the generation of new cracks underground, which would allow increased groundwater circulation, flushing out radon. The generation of new cracks might not unreasonably be assumed to precede major earthquakes. In the 1970s and 1980s, scientific measurements of radon emissions near faults found that earthquakes often occurred with no radon signal, and radon was often detected with no earthquake to follow. It was then dismissed by many as an unreliable indicator. As of 2009, it was under investigation as a possible precursor by NASA.

Radon is a known pollutant emitted from geothermal power stations because it is present in the material pumped from deep underground. It disperses rapidly, and no radiological hazard has been demonstrated in various investigations. In addition, typical systems re-inject the material deep underground rather than releasing it at the surface, so its environmental impact is minimal. However, similar things can be said about trivial releases from operating nuclear power plants.

In the 1940s and 1950s, radon was used for industrial radiography. Other X-ray sources, which became available after World War II, quickly replaced radon for this application, as they were lower in cost and had less hazard of alpha radiation.

Health risks

Main article: Health effects of radon

In mines

Radon-222 decay products have been classified by the International Agency for Research on Cancer as being carcinogenic to humans, and as a gas that can be inhaled, lung cancer is a particular concern for people exposed to elevated levels of radon for sustained periods. During the 1940s and 1950s, when safety standards requiring expensive ventilation in mines were not widely implemented, radon exposure was linked to lung cancer among non-smoking miners of uranium and other hard rock materials in what is now the Czech Republic, and later among miners from the Southwestern US and South Australia. Despite these hazards being known in the early 1950s, this occupational hazard remained poorly managed in many mines until the 1970s. During this period, several entrepreneurs opened former uranium mines in the US to the general public and advertised alleged health benefits from breathing radon gas underground. Health benefits claimed included relief from pain, sinus problems, asthma, and arthritis, but these were proven to be false and the government banned such advertisements in 1975.

Since that time, ventilation and other measures have been used to reduce radon levels in most affected mines that continue to operate. In recent years, the average annual exposure of uranium miners has fallen to levels similar to the concentrations inhaled in some homes. This has reduced the risk of occupationally-induced cancer from radon, although health issues may persist for those who are currently employed in affected mines and for those who have been employed in them in the past. As the relative risk for miners has decreased, so has the ability to detect excess risks among that population.

Residues from processing of uranium ore can also be a source of radon. Radon resulting from the high radium content in uncovered dumps and tailing ponds can be easily released into the atmosphere and affect people living in the vicinity.

In addition to lung cancer, researchers have theorized a possible increased risk of leukemia due to radon exposure. Empirical support from studies of the general population is inconsistent, and a study of uranium miners found a correlation between radon exposure and chronic lymphocytic leukemia.

Miners (as well as milling and ore transportation workers) who worked in the uranium industry in the US between the 1940s and 1971 may be eligible for compensation under the Radiation Exposure Compensation Act (RECA). Surviving relatives may also apply in cases where the formerly employed person is deceased.

Not only uranium mines are affected by elevated levels of radon. Coal mines in particular are affected as well since coal may contain more uranium and thorium than commercially operational uranium mines.

Domestic-level exposure

Prolonged exposure to higher concentrations of radon has been linked to an increase in lung cancer. Since 1999, there have been investigations worldwide on how radon concentrations are estimated. In the United States alone averages have been recorded to be at least 40 Bq/m³. Steck et al. did a study on the variation between indoor and outdoor radon in Iowa and Minnesota. Higher radiation was found in a populated region rather than in unpopulated regions in Central America as a whole. In some northwestern Iowa and southwestern Minnesota counties, the outdoor radon concentrations exceed the national average indoor radon concentrations. Despite the above average, both Minnesota and Iowa's numbers were exceptionally close, regardless of the distance. Accurate studies of radon exposure are needed to further understand the problems radon exposure can have on a community. Current research supports a link between radon exposure and poor health outcomes (i.e., an increased risk of lung cancer), but further research could support stricter radon limits both inside and outside of housing units.

Radon exposure (mostly radon daughters) has been linked to lung cancer in numerous case-control studies performed in the US, Europe and China. There are approximately 21,000 deaths per year in the US (0.0063% of a population of 333 million) due to radon-induced lung cancers. In Slovenia, a country with a high concentration of radon, about 120 people (0.0057% of a population of 2.11 million) yearly die because of radon. One of the most comprehensive radon studies performed in the US by epidemiologist R. William Field and colleagues found a 50% increased lung cancer risk even at the protracted exposures at the EPA's action level of 4 pCi/L. North American and European pooled analyses further support these findings. However, the discussion about the opposite results is still continuing, especially a 2008 retrospective case-control study of lung cancer risk which showed substantial cancer rate reduction for radon concentrations between 50 and 123 Bq/m³.

Most models of residential radon exposure are based on studies of miners, and direct estimates of the risks posed to homeowners would be more desirable. Because of the difficulties of measuring the risk of radon relative to smoking, models of their effect have often made use of them.

Radon has been considered the second leading cause of lung cancer and leading environmental cause of cancer mortality by the EPA, with the first one being smoking. Others have reached similar conclusions for the United Kingdom and France. Radon exposure in homes and offices may arise from certain subsurface rock formations, and also from certain building materials (e.g., some granites). The greatest risk of radon exposure arises in buildings that are airtight, insufficiently ventilated, and have foundation leaks that allow air from the soil into basements and dwelling rooms.

Thoron (²²⁰Rn) was measured at comparatively high concentrations in buildings with earthen architecture, such as traditional half-timbered houses and modern houses with clay wall finishes. Because of its short half-life, thoron occurs only close to the earthen surfaces as its sources whereas its progeny can be found throughout the indoor air of such buildings. Therefore, radiation exposure occurs at any location within such houses. In different dwellings with earthen architecture in Germany, a study found annual internal radiation doses due to the inhalation of thoron and its progeny of up to several milli-Sieverts.

Action and reference level

WHO presented in 2009 a recommended reference level (the national reference level), 100 Bq/m³, for radon in dwellings. The recommendation also says that where this is not possible, 300 Bq/m³ should be selected as the highest level. A national reference level should not be a limit, but should represent the maximum acceptable annual average radon concentration in a dwelling.

The actionable concentration of radon in a home varies depending on the organization doing the recommendation, for example, the EPA encourages that action be taken at concentrations as low as 74 Bq/m³ (2 pCi/L), and the European Union recommends action be taken when concentrations reach 400 Bq/m³ (11 pCi/L) for old houses and 200 Bq/m³ (5 pCi/L) for new ones. On 8 July 2010, the UK's Health Protection Agency issued new advice setting a "Target Level" of 100 Bq/m³ whilst retaining an "Action Level" of 200 Bq/m³. Similar levels (as in UK) are published by Norwegian Radiation and Nuclear Safety Authority (DSA) with the maximum limit for schools, kindergartens, and new dwellings set at 200 Bq/m³, where 100 Bq/m³ is set as the action level. In all new housings preventative measures should be taken against radon accumulation.

Inhalation and smoking

Results from epidemiological studies indicate that the risk of lung cancer increases with exposure to residential radon. A well known example of source of error is smoking, the main risk factor for lung cancer. In the US, cigarette smoking is estimated to cause 80% to 90% of all lung cancers.

According to the EPA, the risk of lung cancer for smokers is significant due to synergistic effects of radon and smoking. For this population about 62 people in a total of 1,000 will die of lung cancer compared to 7 people in a total of 1,000 for people who have never smoked. It cannot be excluded that the risk of non-smokers should be primarily explained by an effect of radon.

Radon, like other known or suspected external risk factors for lung cancer, is a threat for smokers and former smokers. This was demonstrated by the European pooling study. A commentary to the pooling study stated: "it is not appropriate to talk simply of a risk from radon in homes. The risk is from smoking, compounded by a synergistic effect of radon for smokers. Without smoking, the effect seems to be so small as to be insignificant."

According to the European pooling study, there is a difference in risk for the histological subtypes of lung cancer and radon exposure. Small-cell lung carcinoma, which has a high correlation with smoking, have a higher risk after radon exposure. For other histological subtypes such as adenocarcinoma, the type that primarily affects non-smokers, the risk from radon appears to be lower.

A study of radiation from post-mastectomy radiotherapy shows that the simple models previously used to assess the combined and separate risks from radiation and smoking need to be developed. This is also supported by new discussion about the calculation method, the linear no-threshold model, which routinely has been used.

A study from 2001, which included 436 non-smokers with lung cancer and a control group of 1649 non-smokers without lung cancer, showed that exposure to radon increased the risk of lung cancer in non-smokers. The group that had been exposed to tobacco smoke in the home appeared to have a much higher risk, while those who were not exposed to passive smoking did not show any increased risk with increasing radon exposure.

Ingestion

The effects of radon if ingested are unknown, although studies have found that its biological half-life ranges from 30 to 70 minutes, with 90% removal at 100 minutes. In 1999, the US National Research Council investigated the issue of radon in drinking water. The risk associated with ingestion was considered almost negligible. Water from underground sources may contain significant amounts of radon depending on the surrounding rock and soil conditions, whereas surface sources generally do not.

Ocean effects of radon

The major importance of understanding ²²²Rn flux from the ocean, is to know that the increase use of radon is also circulating and increasing in the atmosphere. Ocean surface concentrations have an exchange within the atmosphere, causing ²²²Rn to increase through the air-sea interface. Although areas tested were very shallow, additional measurements in a wide variety of coastal regimes should help define the nature of ²²²Rn observed. As well as being ingested through drinking water, radon is also released from water when temperature is increased, pressure is decreased and when water is aerated. Optimum conditions for radon release and exposure occurred during showering. Water with a radon concentration of 10⁴ pCi/L can increase the indoor airborne radon concentration by 1 pCi/L under normal conditions.

Testing and mitigation

Main article: Radon mitigation

A digital radon detector

A radon test kit

There are relatively simple tests for radon gas. In some countries these tests are methodically done in areas of known systematic hazards. Radon detection devices are commercially available. Digital radon detectors provide ongoing measurements giving both daily, weekly, short-term and long-term average readouts via a digital display. Short-term radon test devices used for initial screening purposes are inexpensive, in some cases free. There are important protocols for taking short-term radon tests and it is imperative that they be strictly followed. The kit includes a collector that the user hangs in the lowest habitable floor of the house for two to seven days. The user then sends the collector to a laboratory for analysis. Long term kits, taking collections for up to one year or more, are also available. An open-land test kit can test radon emissions from the land before construction begins. Radon concentrations can vary daily, and accurate radon exposure estimates require long-term average radon measurements in the spaces where an individual spends a significant amount of time.

Radon levels fluctuate naturally, due to factors like transient weather conditions, so an initial test might not be an accurate assessment of a home's average radon level. Radon levels are at a maximum during the coolest part of the day when pressure differentials are greatest. Therefore, a high result (over 4 pCi/L) justifies repeating the test before undertaking more expensive abatement projects. Measurements between 4 and 10 pCi/L warrant a long-term radon test. Measurements over 10 pCi/L warrant only another short-term test so that abatement measures are not unduly delayed. Purchasers of real estate are advised to delay or decline a purchase if the seller has not successfully abated radon to 4 pCi/L or less.

Because the half-life of radon is only 3.8 days, removing or isolating the source will greatly reduce the hazard within a few weeks. Another method of reducing radon levels is to modify the building's ventilation. Generally, the indoor radon concentrations increase as ventilation rates decrease. In a well-ventilated place, the radon concentration tends to align with outdoor values (typically 10 Bq/m³, ranging from 1 to 100 Bq/m³).

The four principal ways of reducing the amount of radon accumulating in a house are:

• Sub-slab depressurization (soil suction) by increasing under-floor ventilation;
• Improving the ventilation of the house and avoiding the transport of radon from the basement into living rooms;
• Installing a radon sump system in the basement;
• Installing a positive pressurization or positive supply ventilation system.

According to the EPA, the method to reduce radon "...primarily used is a vent pipe system and fan, which pulls radon from beneath the house and vents it to the outside", which is also called sub-slab depressurization, active soil depressurization, or soil suction. Generally indoor radon can be mitigated by sub-slab depressurization and exhausting such radon-laden air to the outdoors, away from windows and other building openings. "[The] EPA generally recommends methods which prevent the entry of radon. Soil suction, for example, prevents radon from entering your home by drawing the radon from below the home and venting it through a pipe, or pipes, to the air above the home where it is quickly diluted" and the "EPA does not recommend the use of sealing alone to reduce radon because, by itself, sealing has not been shown to lower radon levels significantly or consistently".

Positive-pressure ventilation systems can be combined with a heat exchanger to recover energy in the process of exchanging air with the outside, and simply exhausting basement air to the outside is not necessarily a viable solution as this can actually draw radon gas into a dwelling. Homes built on a crawl space may benefit from a radon collector installed under a "radon barrier" (a sheet of plastic that covers the crawl space). For crawl spaces, the EPA states "An effective method to reduce radon levels in crawl space homes involves covering the earth floor with a high-density plastic sheet. A vent pipe and fan are used to draw the radon from under the sheet and vent it to the outdoors. This form of soil suction is called submembrane suction, and when properly applied is the most effective way to reduce radon levels in crawl space homes."

Acoustic levitation

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Acoustic_levitation

Acoustic levitation is a method for suspending matter in air against gravity using acoustic radiation pressure from high intensity sound waves.

It works on the same principles as acoustic tweezers by harnessing acoustic radiation forces. However acoustic tweezers are generally small scale devices which operate in a fluid medium and are less affected by gravity, whereas acoustic levitation is primarily concerned with overcoming gravity. Technically dynamic acoustic levitation is a form of acoustophoresis, though this term is more commonly associated with small scale acoustic tweezers.

Typically sound waves at ultrasonic frequencies are used thus creating no sound audible to humans. This is primarily due to the high intensity of sound required to counteract gravity. However, there have been cases of audible frequencies being used.

There are various techniques for generating the sound, but the most common is the use of piezoelectric transducers which can efficiently generate high amplitude outputs at the desired frequencies.

Levitation is a promising method for containerless processing of microchips and other small, delicate objects in industry. Containerless processing may also be used for applications requiring very-high-purity materials or chemical reactions too rigorous to happen in a container. This method is harder to control than others such as electromagnetic levitation but has the advantage of being able to levitate nonconducting materials.

Although originally static, acoustic levitation has progressed from motionless levitation to dynamic control of hovering objects, an ability useful in the pharmaceutical and electronics industries. This dynamic control was first realised with a prototype with a chessboard-like array of square acoustic emitters that move an object from one square to another by slowly lowering the sound intensity emitted from one square while increasing the sound intensity from the other, allowing the object to travel virtually "downhill". More recently the development of phased array transducer boards have allowed more arbitrary dynamic control of multiple particles and droplets at once.

Recent advancements have also seen the price of the technology decrease significantly. The "TinyLev" is an acoustic levitator which can be constructed with widely available, low-cost off-the-shelf components, and a single 3D printed frame.

History

Experimental

A drawing of the Kundt's tube experiment. The movement of the particles due to the acoustic radiation forces were the first demonstration of the possibility of acoustic levitation.

The first demonstration of the possibility of acoustic levitation was made in Kundt's Tube experiments in 1866. The experiment in a resonant chamber demonstrated that the particles could be gathered at the nodes of a standing wave by the acoustic radiation forces. However, the original experiment was conducted with the intention of calculating the wavelengths and therefore the speed of sound within a gas.

The first levitation was demonstrated by Bücks and Muller in 1933 who levitated alcohol droplets between a quartz crystal and a reflector. The next advance came from Hilary St Clair, who was interested in acoustic radiation forces primarily for their applications on the agglomeration of dust particles for use in mining applications. He created the first electromagnetic device for creating the excitation amplitudes necessary for levitation, then went on to levitate larger and heavier objects, including a coin.

Taylor Wang was the leader of a team which made significant use of acoustic radiation forces as a containment mechanism in zero gravity, taking a device up on the Space Shuttle Challenger mission STS-51-B to investigate the behaviour of levitated droplets in micro-gravity. Further experiments were conducted in 1992 aboard United States Microgravity Laboratory 1 (USML-1), and in 1995 aboard USML-2.

The most common levitator from at least the 1970s until 2017 was the Langevin Horn, consisting of a piezo-electric actuator, a metal transmitter and a reflector. However, this required precise tuning of the distance between the transmitter and the reflector as the distance between source and reflector needed to be an exact multiple of the wavelength. This is more difficult than it sounds as the wavelength varies with the speed of sound, which varies with environmental factors such as temperature and altitude. Significant studies have been made with such devices including into contactless chemistry and the levitation of small animals. A number of these were also combined to create continuous planar motion by reducing the sound intensity from one source whilst increasing that of the adjacent source, allowing the particle to travel "downhill" in the acoustic potential field.

A TinyLev acoustic levitator including the electronics and a diagram of the peak pressure field.

A new generation of acoustic levitators employing a large number of small individual piezoelectric-transducers have recently become more common. The first of these levitators was a single-axis standing wave levitator called the TinyLev. The key differences from the Langevin Horn were the use of sources from both top and bottom (rather than a source and a reflector) and the use of a large number of small transducers with parallel excitation, rather than a single piezoelectric element. The use of two opposing travelling waves, as opposed to a single source and a reflector, meant that levitation was still possible even when the distance between the top and bottom was not a precise multiple of the wavelength. This led to a more robust system which does not require any tuning before operation. The use of multiple small sources was initially designed as a cost saving measure, but also opened the door for phased array levitation, discussed below. The use of 3D printed components for the frame which positions and focuses the transducers and Arduinos as signal generators also significantly reduced the cost whilst increasing the accessibility, The reduction in cost was particularly important as a principal aim of this device was the democratisation of the technology.

This new approach also led to significant developments using Phased Array Ultrasonic Transducers (often referred to as PATs) for levitation. Phased Array Ultrasonic Transducers are a collection of ultrasonic speakers which are controlled to create a single desired sound field. This is achieved by controlling the relative phase (i.e. the delay time) between each output, and sometimes the relative output magnitudes. Unlike their counterparts in the non-destructive testing or imaging fields, these arrays will use a continuous output, as opposed to short bursts of energy. This has enabled single sided levitation as well as manipulation of large numbers of particles simultaneously.

Another approach which is growing in popularity is the use of 3D-printed components to apply the phase delays necessary for levitation, creating a similar effect to the PATs but with the advantage that they can have a higher spatial resolution than the phased array, allowing more complex fields to be formed. These are sometimes referred to as Acoustic Holograms, Metasurfaces, Delay lines or Metamaterials. The differences in terms are primarily based on the area from which the design technique originated, but the basic idea behind all techniques is essentially the same. They can also be used in conjunction with PATs to obtain dynamic reconfigurability and higher sound field resolution. Another advantage is the reduction in cost, with a prominent example being the low cost ultrasonic tractor beam for which an instructables was created.

Although many new techniques for manipulation have been developed, Langevin Horns are still used in research. They are often favoured for research into the dynamics of levitated objects due to the simplicity of their geometry and subsequent ease of simulation and control of experimental factors.

Theoretical

Lord Rayleigh developed theories about the pressure force associated with sound waves in the early 1900s, however this work was primarily based around the theoretical forces and energy contained within a sound wave. The first analysis of particles was conducted by L.V. King in 1934, who calculated the force on incompressible particles in an acoustic field. This was followed by Yosioka and Kawisama, who calculated the forces on compressible particles in plane acoustic waves. This was followed by Lev P. Gor'kov's work which generalised the field into the Gor'kov potential, the mathematical foundation for acoustic levitation which is still in widespread use today.

The Gor'kov potential is limited by its assumptions to spheres with a radius significantly less than the wavelength, the typical limit is considered to be one-tenth of the wavelength. Further analytical solutions are available for simple geometries however, to extend to larger or non-spherical objects it is common to use numerical methods, particularly the finite element method or the boundary element method. Radiation pressure of sound can also be controlled through sub-wavelength patterning of the surface of the object.

Types of Levitation

Acoustic levitation can broadly be divided into five different categories:

1. Standing Wave Levitation: Particles are trapped at the nodes of a standing wave, formed by either a sound source and reflector (in the case of the Langevin Horn) or two sets of sources (in the case of the TinyLev). This relies upon the particles being small relative to the wavelength, typically in the region of 10% or less, and the maximum levitated weight is usually in the order of a few milligrams. It is also worth noting that if the particle is too small relative to the wavelength then it will behave differently and travel to the anti-nodes. Typically these levitators are single-axis, meaning that all particles are trapped along a single, central axis of the levitator. However, with the use of PATs they can also be dynamic. This is the strongest technique for levitation at a distance greater than a wavelength due to the constructive interference from the two travelling waves which form it. The forces from single beam levitation at a distance are 30 times weaker than a simple standing wave.

   

   A single beam acoustic levitator using a vortex trap to levitate an expanded polystyrene particle approximately twice the size of the wavelength. The vortices are alternated rapidly in direction to avoid spinning the particle to the point of instability. Here 450 transducers at 40kHz are used.

2. Far Field Acoustic Levitation: Larger than wavelength objects are levitated by generating a field which is tailored to the levitated object's size and shape. This allows objects larger than the wavelength to be levitated at distances greater than the wavelength from the source. However, the object must not be high density. In early approaches this was a simple vertical standing wave for disks or a three transducer arrangement to stabilise a sphere. However, recent developments have used a PAT and the boundary element method to levitate much larger objects at much longer distances. The heaviest object lifted by this technique is a 30mm diameter expanded polystyrene sphere of mass 0.6g. An expanded polystyrene octahedron with a diagonal length of 50mm and mass 0.5g is the largest object ever acoustically levitated by this technique using PATs above and below the object.
3. Single Beam Levitation: Levitation of objects at a distance greater than a single wavelength from the sources with access only from a single side. In this case the trap must be specially designed, and usually takes the form of a twin trap or a vortex trap, although a third trap type called a bottle trap is also possible. The twin trap is the simplest of these possibilities which forms two high pressure "tweezers" on either side of the particle. If geometric focusing is used then this can be used to build a tractor beam with commonly available parts. The vortex trap creates a "hole" of low pressure in the centre. It requires a more complex phase field but, unlike the twin trap, can be used to lift larger than wavelength objects. In 2019 the largest object ever lifted by a tractor beam was done so at the University of Bristol and shown on "The Edge of Science," a BBC Earth production for YouTube Originals by presenter Rick Edwards. It was a 19.53mm diameter expanded polystyrene ball.
4. Near Field Levitation: A large, planar object is placed very close to the transducer surface and acts as a reflector, allowing it to levitate on a very thin film of air. This technique is capable of lifting several kilograms, but cannot go higher than hundreds of micrometers above the surface. As such on a human scale it appears more as a huge reduction in friction, rather than as levitation.
5. Inverted Near Field Acoustic Levitation: Under certain conditions the repulsive force which produces near field levitation inverts and become an attractive force. In this case the transducer can be pointed downwards and the set up will levitate the object will be levitated below it. The object will be levitated at a distance of tens of micrometers and objects in the milligram scale have been levitated. Current research suggests it occurs where the equivalent radius of the disk is less than 38% of the wavelength 

These broad classifications are a single way of sorting the types of levitation, but they are not definitive. Further work is being conducted on combining techniques to obtain greater abilities, such as the stable levitation of non axis-symmetric objects by combining standing wave levitation with a twin-trap (typically a single beam levitation technique). There is also a significant amount of work into combining these techniques with 3D printed phase shifting components for advantages such as passive field forming or higher spatial resolution. There is also significant variation in the control techniques. Whilst PATs are common it has also been shown that Chladni Plates can be used as a single standing wave source to manipulate levitated objects by changing the frequency.

Applications

The main applications of acoustic levitation are likely to be scientific and industrial.

A selection of acoustically Levitated Objects in a TinyLev including solids, liquids, an ant and an electrical component. All in the size range of 2 mm-6 mm.

(Left) Images of acoustically levitated droplets during liquid evaporation and particle formation. (Right) X-ray microtomography gives insights into the final particle 3D structure.

Acoustic levitation provides a container-less environment for droplet drying experiments to study liquid evaporation and particle formation. The contactless manipulation of droplets has also gained significant interest as it promises small scale contactless chemistry.  There is particular interest in the mixing of multiple droplets using PATs so that chemical reactions can be studied in isolation from containers. There is also interest in using a small levitated droplet as container of protein crystals for X-ray diffraction experiments to determine the crystal structure at atomic resolution at room temperature at high thoughput.

The levitation of small living animals has also been studied, and the vitality of animals which typically exist in air was not affected. In the future it could be used as a tool to study the animals themselves.

There is active research in the field of contactless assembly. The levitation of surface mount electrical components has been demonstrated as has micro-assembly with a combination of acoustic and magnetic fields. There is also commercial interest in 3D printing whilst levitated, with Boeing filing a patent on the concept.

Acoustic levitation has also been proposed as a technique for creating a volumetric display, with light projected onto a particle, which moves along the path to create the image faster than the eye can process. This has already proved possible and has been brought together with audio and haptic feedback from the same PAT.