Radon-222

Last updated
Radon-222, 222Rn
General
Symbol 222Rn
Names radon-222, 222Rn, Rn-222,
Radium emanation
Protons (Z)86
Neutrons (N)136
Nuclide data
Natural abundance Trace
Half-life (t1/2)3.8215 d [1]
Isotope mass 222.0175763 [2] Da
Spin 0
Parent isotopes 226Ra  (α)
Decay products 218Po
Decay modes
Decay mode Decay energy (MeV)
Alpha decay 5.5904 [2]
Isotopes of radon
Complete table of nuclides

Radon-222 (222Rn, Rn-222, historically radium emanation or radon) is the most stable isotope of radon, with a half-life of approximately 3.8 days. It is transient in the decay chain of primordial uranium-238 and is the immediate decay product of radium-226. Radon-222 was first observed in 1899, and was identified as an isotope of a new element several years later. In 1957, the name radon, formerly the name of only radon-222, became the name of the element. Owing to its gaseous nature and high radioactivity, radon-222 is one of the leading causes of lung cancer. [3]

Contents

History

Following the 1898 discovery of radium through chemical analysis of radioactive ore, Marie and Pierre Curie observed a new radioactive substance emanating from radium in 1899 that was strongly radioactive for several days. [4] Around the same time, Ernest Rutherford and Robert B. Owens observed a similar (though shorter-lived) emission from thorium compounds. [5] German physicist Friedrich Ernst Dorn extensively studied these emanations in the early 1900s and attributed them to a new gaseous element, radon. In particular, he studied the product in the uranium series, radon-222, which he called radium emanation. [6]

In the early 20th century, the element radon was known by several different names. Chemist William Ramsay, who extensively studied the element's chemical properties, suggested the name niton, and Rutherford originally suggested emanation. At that time, radon only referred to the isotope 222Rn, whereas the names actinon and thoron denoted 219Rn and 220Rn, respectively. [7] In 1957, the International Union of Pure and Applied Chemistry (IUPAC) promoted the name radon to refer to the element rather than just 222Rn; this was done under a new rule concerning isotope naming conventions. [7] This decision was controversial because it was believed to give undue credit to Dorn's identification of radon-222 over Rutherford's identification of radon-220, and the historical use of the name radon created confusion as to whether the element or the isotope 222Rn was being discussed. [7]

Decay properties

The decay chain of uranium-238, known as the uranium series or radium series, of which radon-222 is a member. Decay chain(4n+2, Uranium series).PNG
The decay chain of uranium-238, known as the uranium series or radium series, of which radon-222 is a member.

Radon-222 is generated in the uranium series from the alpha decay of radium-226, which has a half-life of 1600 years. Radon-222 itself alpha decays to polonium-218 with a half-life of approximately 3.82 days, making it the most stable isotope of radon. [1] Its final decay product is stable lead-206.

In theory, 222Rn is capable of double beta decay to 222Ra, and depending on the mass measurement, single beta decay to 222Fr may also be allowed. [8] [lower-alpha 1] These decay modes have been searched for, yielding lower partial half-life limits of 8 years for both transitions. If the beta decay of 222Rn is possible, it is predicted to have a very low decay energy (24 ± 21 keV) and thus a half-life on the order of 105 years, also resulting in a very low branching probability relative to alpha decay. [8]

Occurrence and hazards

All radon isotopes are hazardous owing to their radioactivity, gaseous nature, chemical inertness, and radioactivity of their decay products (progeny). Radon-222 is especially dangerous because its longer half-life allows it to permeate soil and rocks, where it is produced in trace quantities from decays of uranium-238, and concentrate in buildings and uranium mines. This contrasts with the other natural isotopes that decay far more quickly (half-lives less than 1 minute) and thus do not contribute significantly to radiation exposure. [9] At higher concentrations, gaseous 222Rn may be inhaled and decay before exhalation, which leads to a buildup of its daughters 218Po and 214Po in the lungs, whose high-energy alpha and gamma radiation damages cells. Extended periods of exposure to 222Rn and its progeny ultimately induce lung cancer. [9] Alternatively, radon may enter the body through contaminated drinking water or through the decay of ingested radium [3] – making radon diffusion one of the greatest dangers of radium. [10] Thus, 222Rn is a carcinogen; in fact, it is the second leading cause of lung cancer in the United States after cigarette smoking, [3] with over 20,000 deaths per year attributed to radon-induced lung cancer. [9] [11]

See also

Notes

  1. ↑ AME2016 gives 222Rn a lower mass than 222Fr, [1] which would forbid single beta decay, though it is possible within the given error margin and is explicitly predicted by Belli et al.

Related Research Articles

<span class="mw-page-title-main">Alpha decay</span> Type of radioactive decay

Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms or "decays" into a different atomic nucleus, with a mass number that is reduced by four and an atomic number that is reduced by two. An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons. It has a charge of +2 e and a mass of 4 Da. For example, uranium-238 decays to form thorium-234.

<span class="mw-page-title-main">Radium</span> Chemical element, symbol Ra and atomic number 88

Radium is a chemical element; it has symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rather than oxygen) upon exposure to air, forming a black surface layer of radium nitride (Ra3N2). All isotopes of radium are radioactive, the most stable isotope being radium-226 with a half-life of 1,600 years. When radium decays, it emits ionizing radiation as a by-product, which can excite fluorescent chemicals and cause radioluminescence.

<span class="mw-page-title-main">Radon</span> Chemical element, symbol Rn and atomic number 86

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 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.

<span class="mw-page-title-main">Decay chain</span> Series of radioactive decays

In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". The typical radioisotope does not decay directly to a stable state, but rather it decays to another radioisotope. Thus there is usually a series of decays until the atom has become a stable isotope, meaning that the nucleus of the atom has reached a stable state.

Polonium-210 (210Po, Po-210, historically radium F) is an isotope of polonium. It undergoes alpha decay to stable 206Pb with a half-life of 138.376 days (about 4+1⁄2 months), the longest half-life of all naturally occurring polonium isotopes (210–218Po). First identified in 1898, and also marking the discovery of the element polonium, 210Po is generated in the decay chain of uranium-238 and radium-226. 210Po is a prominent contaminant in the environment, mostly affecting seafood and tobacco. Its extreme toxicity is attributed to intense radioactivity, mostly due to alpha particles, which easily cause radiation damage, including cancer in surrounding tissue. The specific activity of 210
Po is 166 TBq/g, i.e., 1.66 × 1014 Bq/g. At the same time, 210Po is not readily detected by common radiation detectors, because its gamma rays have a very low energy. Therefore, 210
Po can be considered as a quasi-pure alpha emitter.

Thorium-232 is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay chain terminates at stable lead-208.

Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from 214U to 242U. The standard atomic weight of natural uranium is 238.02891(3).

Protactinium (91Pa) has no stable isotopes. The four naturally occurring isotopes allow a standard atomic weight to be given.

Lead (82Pb) has four observationally stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uranium series, the actinium series, and the thorium series, respectively; a fourth decay chain, the neptunium series, terminates with the thallium isotope 205Tl. The three series terminating in lead represent the decay chain products of long-lived primordial 238U, 235U, and 232Th. Each isotope also occurs, to some extent, as primordial isotopes that were made in supernovae, rather than radiogenically as daughter products. The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium.

<span class="mw-page-title-main">Isotopes of thallium</span> Nuclides with atomic number of 81 but with different mass numbers

Thallium (81Tl) has 41 isotopes with atomic masses that range from 176 to 216. 203Tl and 205Tl are the only stable isotopes and 204Tl is the most stable radioisotope with a half-life of 3.78 years. 207Tl, with a half-life of 4.77 minutes, has the longest half-life of naturally occurring Tl radioisotopes. All isotopes of thallium are either radioactive or observationally stable, meaning that they are predicted to be radioactive but no actual decay has been observed.

Potassium has 26 known isotopes from 31
K to 57
K, with the exception of still-unknown 32
K, as well as an unconfirmed report of 59
K. Three of those isotopes occur naturally: the two stable forms 39
K (93.3%) and 41
K (6.7%), and a very long-lived radioisotope 40
K (0.012%)

Although phosphorus (15P) has 22 isotopes from 26P to 47P, only 31P is stable; as such, phosphorus is considered a monoisotopic element. The longest-lived radioactive isotopes are 33P with a half-life of 25.34 days and 32P with a half-life of 14.268 days. All others have half-lives of under 2.5 minutes, most under a second. The least stable known isotope is 47P, with a half-life of 2 milliseconds.

Sulfur (16S) has 23 known isotopes with mass numbers ranging from 27 to 49, four of which are stable: 32S (95.02%), 33S (0.75%), 34S (4.21%), and 36S (0.02%). The preponderance of sulfur-32 is explained by its production from carbon-12 plus successive fusion capture of five helium-4 nuclei, in the so-called alpha process of exploding type II supernovas.

Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was 239Np in 1940, produced by bombarding 238
U
 with neutrons to produce 239
U
, which then underwent beta decay to 239
Np
.

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being plutonium-238 in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years; plutonium-242 with a half-life of 373,300 years; and plutonium-239 with a half-life of 24,110 years; and plutonium-240 with a half-life of 6,560 years. This element also has eight meta states; all have half-lives of less than one second.

<span class="mw-page-title-main">Radium and radon in the environment</span> Significant contributors to environmental radioactivity

Radium and radon are important contributors to environmental radioactivity. Radon occurs naturally as a result of decay of radioactive elements in soil and it can accumulate in houses built on areas where such decay occurs. Radon is a major cause of cancer; it is estimated to contribute to ~2% of all cancer related deaths in Europe.

Iodine-129 (129I) is a long-lived radioisotope of iodine that occurs naturally but is also of special interest in the monitoring and effects of man-made nuclear fission products, where it serves as both a tracer and a potential radiological contaminant.

Radium-226 is the longest-lived isotope of radium, with a half-life of 1600 years. It is an intermediate product in the decay chain of uranium-238; as such, it can be found naturally in uranium-containing minerals.

<span class="mw-page-title-main">Alpha particle</span> Ionizing radiation particle of two protons and two neutrons

Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+
 or 4
2He2+
 indicating a helium ion with a +2 charge. Once the ion gains electrons from its environment, the alpha particle becomes a normal helium atom 4
2He.

<span class="mw-page-title-main">Gold-198</span> Isotope of Gold

Gold-198 (198Au) is a radioactive isotope of gold. It undergoes beta decay to stable 198Hg with a half-life of 2.69464 days.

References

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  2. 1 2 Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003-1–030003-442. doi:10.1088/1674-1137/41/3/030003.
  3. 1 2 3 EPA Facts about Radon (PDF) (Report). United States Environmental Protection Agency. pp. 1–3. Retrieved 22 February 2019.
  4. ↑ Fry, C.; Thoennessen, M. (2013). "Discovery of the astatine, radon, francium, and radium isotopes". Atomic Data and Nuclear Data Tables. 99 (5): 497–519. arXiv: 1205.5841 . Bibcode:2013ADNDT..99..497F. doi:10.1016/j.adt.2012.05.003. S2CID   12590893.
  5. ↑ Thoennessen, M. (2016). The Discovery of Isotopes: A Complete Compilation. Springer. p. 8. doi:10.1007/978-3-319-31763-2. ISBN   978-3-319-31761-8. LCCN   2016935977.
  6. ↑ George, A.C. (2008). "World History of Radon Research and Measurement from the Early 1900s to Today" (PDF). AIP Conference Proceedings. 1034 (1): 20–36. Bibcode:2008AIPC.1034...20G. CiteSeerX   10.1.1.618.9328 . doi:10.1063/1.2991210. Archived from the original (PDF) on 2016-08-24. Retrieved 2019-02-22.
  7. 1 2 3 Thornton, B.F.; Burdette, S.C. (2013). "Recalling radon's recognition". Nature Chemistry. 5 (9): 804. Bibcode:2013NatCh...5..804T. doi: 10.1038/nchem.1731 . PMID   23965684.
  8. 1 2 Belli, P.; Bernabei, R.; Cappella, C.; Caracciolo, V.; Cerulli, R.; Danevich, F.A.; Di Marco, A.; Incicchitti, A.; Poda, D.V.; Polischuk, O.G.; Tretyak, V.I. (2014). "Investigation of rare nuclear decays with BaF2 crystal scintillator contaminated by radium". European Physical Journal A. 50 (9): 134–143. arXiv: 1407.5844 . Bibcode:2014EPJA...50..134B. doi:10.1140/epja/i2014-14134-6. S2CID   118513731.
  9. 1 2 3 EPA assessment of risks from radon in homes (PDF) (Report). Office of Radiation and Indoor Air, United States Environmental Protection Agency. 2003.
  10. ↑ "Radiation protection: Radium". United States Environmental Protection Agency. Archived from the original on 11 February 2015. Retrieved 22 February 2019.
  11. ↑ "Radon Fact Sheet: What it is, how it affects us, why it matters". Air Chek, Inc. Retrieved 22 February 2019.
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