Radon - WikiHQ

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 Rn 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.

The instability of Rn, its most stable isotope, makes 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 chains of U and Th, both of which are abundant radioactive nuclides with half-lives of at least several billion years. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead. Rn occurs in significant quantities as a step in the normal radioactive decay chain of U, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into stable Pb. Rn occurs in minute quantities as an intermediate step in the decay chain of Th, also known as the thorium series, which eventually decays into stable Pb.

Radon was discovered in 1899 by Ernest Rutherford and Robert B. Owens at McGill University in Montreal, and was the fifth radioactive element to be discovered. First known as "emanation", the radioactive gas was identified during experiments with radium, thorium oxide, and actinium by Friedrich Ernst Dorn, Rutherford and Owens, and André-Louis Debierne, respectively, and each element's emanation was considered to be a separate substance: radon, thoron, and actinon. Sir William Ramsay and Robert Whytlaw-Gray considered that the radioactive emanations may contain a new element of the noble gas family, and isolated "radium emanation" in 1909 to determine its properties. In 1911, the element Ramsay and Whytlaw-Gray isolated was accepted by the International Commission for Atomic Weights, and in 1923, the International Committee for Chemical Elements and the International Union of Pure and Applied Chemistry (IUPAC) chose radon as the accepted name for the element's most stable isotope, Rn; thoron and actinon were also recognized by IUPAC as distinct isotopes of the element. Radon trapped in permafrost may be released by climate-change-induced thawing of permafrosts, and radon may also be released into groundwater and the atmosphere following seismic events leading to earthquakes, which has led to its investigation in the field of earthquake prediction.

Epidemiological studies have shown a clear association between breathing high concentrations of radon and incidence of lung cancer. Radon is a contaminant that affects indoor air quality worldwide. Because radon is denser than air it accumulates in basements and crawlspaces under dwellings.

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. Due to local differences in geology, the level of exposure to radon gas differs by location.

Characteristics

Physical properties

thumb|right|[[Emission spectrum of radon-222 (radium emanation), photographed by Ernest Rutherford in 1908. Numbers at the side of the spectrum are wavelengths. The middle spectrum is of radon-222, while the outer two are of helium (added to calibrate the wavelengths).]]

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. Radon is colorless at standard temperature and pressure. When cooled below its boiling point of , concentrated liquid radon emits radioluminescence of varying color; solidified radon emits a blue to yellow to red light when cooled further beyond its freezing point of . Due to the hazards associated with high concentrations of radon, liquid and solid radon is almost never seen.

: <math>\chi = \exp(B/T-A)</math>

where <math>\chi</math> is the molar fraction of radon, <math>T</math> is the absolute temperature, and <math>A</math> and <math>B</math> are solvent constants.

Chemical properties

thumb|Radon in a [[cloud chamber. The ionizing radiation of radon causes condensation to appear as cloud tracks in the chamber.]]

Radon is a member of the zero-valence elements that are called noble gases, and is chemically not very reactive. The inert pair effect stabilizes the 6s shell, making it unavailable for bonding—a consequence only understood within relativistic quantum chemistry. 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 () or sulfur dioxide (), and significantly higher than the stability of the hydrate of hydrogen sulfide ().

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 (). It decomposes back to its elements at a temperature of above , and is reduced by water to radon gas and hydrogen fluoride: it may also be reduced back to its elements by hydrogen gas. The octahedral molecule Radon hexafluoride| 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 (s) → (s) + 2 (g)

For this reason, antimony pentafluoride together with chlorine trifluoride and 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. Radon is also oxidised by dioxygen difluoride to at . only the trioxide () has been confirmed. They may have been observed in experiments where unknown radon-containing products distilled together with xenon hexafluoride: these may have been , , or both.

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 () and the high positive charge on radon in RnF+; spatial separation of molecules may be necessary to clearly identify higher fluorides of radon, of which is expected to be more stable than due to spin–orbit splitting of the 6p shell of radon (RnIV would have a closed-shell 6s6p configuration). Therefore, while should have a similar stability to xenon tetrafluoride (), would likely be much less stable than xenon hexafluoride (): radon hexafluoride would also probably be a regular octahedral molecule, unlike the distorted octahedral structure of , 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. The molecules 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; should be highly unstable chemically although there is no evidence for the formation of stable radon ions or compounds in aqueous solution. Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is Rn (half-life 3.82 days), which is a decay product of Ra, the latter being itself a decay product of U. A trace amount of the (highly unstable) isotope Rn (half-life about 35 milliseconds) is also among the daughters of Rn. The isotope Rn would be produced by the double beta decay of natural Po; while energetically possible, this process has however never been seen.

Three other radon isotopes have a half-life of over an hour: Rn (about 15 hours), Rn (2.4 hours) and Rn (about 1.8 hours). However, none of these three occur naturally. Rn, also called thoron, is a natural decay product of the most stable thorium isotope (Th). It has a half-life of 55.6 seconds and also emits alpha radiation. Similarly, Rn is derived from the most stable isotope of actinium (Ac)—named "actinon"—and is an alpha emitter with a half-life of 3.96 seconds. 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. 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. Several studies on the radioactive equilibrium of elements in the environment find it more useful to use the ratio of other Rn decay products with Pb, such as Po, in measuring overall radiation levels.

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

thumb|upright|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. 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 , to flow out, and , 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 1910, they determined its density (that showed it was the heaviest known gas) and its position in the periodic table. 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 Rn and Rn. 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. 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.]]

Discussions of radon concentrations in the environment refer to 222Rn, the decay product of uranium and radium. 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). Typical domestic exposures average about 48 Bq/m3 indoors, though this varies widely, and 15 Bq/m3 outdoors. Assuming 2000 hours of work per year, this corresponds to a concentration of 1500  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 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

thumb|upright=1.1|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 km to a depth of 15 cm), contains about 1 gram of radium, which releases radon in small amounts to the atmosphere. This is equivalent to some .

Radon concentration can differ widely from place to place. In the open air, it ranges from 1 to 100 Bq/m, even less (0.1 Bq/m) above the ocean. In the United States, the average outdoor radon level is estimated to be 15 Bq/m (0.4 pCi/L). In caves or ventilated mines, or poorly ventilated houses, its concentration climbs to 20–2,000 Bq/m.

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 Rn per liter of air; 20.2 kBq/m, measured from 1976 to 1985).

Radon mostly appears with the radium/uranium series (decay chain) (Rn), and marginally with the thorium series (Rn). 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 Rn), 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/m). The activity of radon mineral water reaches 2 MBq/m in Merano and 4 MBq/m in Lurisia (Italy).

In 1971, Apollo 15 passed above the Aristarchus plateau on the Moon, and detected a significant rise in alpha particles thought to be caused by the decay of Rn. The presence of Rn 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. with the intent of estimating the public exposure to radon and its decay products. From 1975 up until 1984, small studies in Sweden, Austria, the United States and Norway aimed to measure radon indoors and in metropolitan areas. The incident dramatized the fact that radon levels in particular dwellings can occasionally be orders of magnitude higher than typical. Since the incident in Pennsylvania, millions of short-term radon measurements have been taken in homes in the United States. Outside the United States, radon measurements are typically performed over the long term. 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.

thumb|left|A fixed-location device to measure soil concentrations of radon at the [[Warsaw University of Technology]]

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, property owners have an obligation to evaluate potential risks and hazards to health and safety in a residential property. Alpha-radiation monitoring over the long term is a method of testing for radon that is more common in countries outside the United States.

Radon commercialization is regulated, but it is available in small quantities for the calibration of 222Rn measurement systems. In 2008 it was priced at almost per milliliter of radium solution (which only contains about 15 picograms of actual radon at any given moment). Radon is produced commercially 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. Radon sources have also been produced for scientific purposes through the implantation of radium-226 into solid stainless steel.

Concentration scale

{| class="wikitable" style="margin:auto;"

! Bq/m3

! pCi/L

! Occurrence example

|style="color: black; background:silver; text-align:right;"| 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, with changes in radon levels being used to track foreign pollutants.

|style="color: black; background:aqua; text-align:right;"| 10

| 0.27

| Mean continental concentration in the open air: 10 to 30 Bq/m3.

An EPA survey of 11,000 homes across the USA found an average of 46 Bq/m3.

|style="color: black; background:lime; text-align:right;"| 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.

|style="color: black; background:orange; text-align:right;"| 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).

|style="background:maroon; color:white; text-align:right;"| 1,000,000

| 27000

| Concentrations reaching 1,000,000 Bq/m3 can be found in unventilated uranium mines.

|style="background:black; color:white; text-align:right;"|

|style="background:#ddd;"|

|style="background:#ddd;"| 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

Hormesis

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.

Nuclear medicine

thumb|Rn- and [[Iodine-125|I-containing seeds used in brachytherapy]]

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).

211Rn can be used to generate 211At, which has uses in targeted alpha therapy.

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. 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 river may be an indicator that there are local inputs of groundwater.

Radon soil concentration has been used 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 As of 2009, it was under investigation as a possible earthquake precursor by NASA; further research into the subject has suggested that abnormalities in atmospheric radon concentrations can be an indicator of seismic movement.

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. In 1989, a survey of the collective dose received due to radon in geothermal fluids was measured at 2 man-sieverts per gigawatt-year of electricity produced, in comparison to the 2.5 man-sieverts per gigawatt-year produced from C emissions in nuclear power plants.

In the 1940s and 1950s, radon produced from a radium source was used for industrial radiography. Other X-ray sources such as Co and Ir became available after World War II and quickly replaced radium and thus radon for this purpose, being of lower cost and hazard.<!--