ラドン

ラドン 化学特性,用途語,生産方法

• 解説

  ラドン，元素記号 Rn ，原子番号 86。周期表 18族，希ガス元素の1種。天然に存在するのは質量数 222 (半減期 3.8日，狭義のラドン) ，219 (3.9秒，アクチノン) ，220 (51.5秒，トロン) の3核種で，地殻存在量は 4×10-17重量% である。このほか 18種の人工放射性核種が知られている。単体は無色無臭の単原子気体で，沸点は－62℃，密度 9.73g/l (0℃) 。γ線源として癌治療に使われる。ニトン，ラジウムエマネーション，エマネーションの名でも呼ばれる。気体の密度＝9.73g/l(0℃，1気圧)液体の比重＝4.4(－62℃)水に対する溶解度＝50.8ml/100ml(0℃)，24.5ml/100ml(20℃)，8.5ml/100ml(60℃)電子配置＝［Xe］4f145d106s26p6　おもな酸化数＝0周期表0族に属する希ガス元素の一つ。1898年キュリー夫妻はラジウムの化合物の周囲の空気が放射性をもつことを見いだし，これをドルンF.Dornが1900年ラジウムから発生する気体によるものであることを明らかにした。

• 説明

  Radon, Rn, is a gaseous radioactive element from the noble gases in family eight on the periodic table. There are 18 radioactive isotopes of radon, all of which have short half-lives. For example, radon 222 has a half-life of 3.8 days. Radon is a colorless gas that is soluble in water. It can be condensed to a colorless transparent liquid and to an opaque, glowing solid. Radon is the heaviest gas known, with a density of 9.72 g/L at 32°F.

• 化学的特性

  colourless gas

• 物理的性質

  Radon gas fits the criteria to be classed as a noble element located in group 18(VIIIA) orgroup 0. It is the only noble “inert” gas that is naturally radioactive. It is the heaviest of thegases in group 18.
  Radon gas is easily converted to a liquid and will become solid at the relatively hightemperature of –71°C. As a solid, it glows with a yellow light. Its melting point is –71°C, itsboiling point is –62°C, and its density is 0.00973g/cm³.

• 同位体

  There are 37 isotopes of radon. All are radioactive. None are stable. They rangein mass numbers from Rn-196 to Rn-228. Their half-lives range from a few microsecondsto 3.8235 days for Rn-222, which is the most common. It is a gas that is the resultof alpha decay of radium, thorium, or uranium ores and underground rocks.

• 名前の由来

  Originally named “niton” after the Latin word for “shining,” it was given the name “radon” in 1923 because it is the radioactive decay gas of the element radium.

• 天然物の起源

  Radon’s source is a step in the transmutation of several elements: uranium → thorium →radium → radon → polonium → lead. (There are a number of intermediate decay productsand steps involved in this process.) Radon-222 forms and collects just a few inches below thesurface of the ground and is often found in trapped pockets of air. It escapes through poroussoils and crevices.

• 特性

  Radon is the heaviest of the noble gases and is the only one that is radioactive. It is thedecay product of radium, thorium, and uranium ores and rocks found underground. As itdecays, it emits alpha particles (helium nuclei) and is then transmuted to polonium andfinally lead. The Earth’s atmosphere is just 0.0000000000000000001% radon, but becauseradon is 7.5 times heavier than air, it can collect in basements and low places in buildingsand homes.

• 使用

  Radon’s main use is as a short-lived source of radioactivity for medical purposes. It iscollected from the decay of radium as a gas and sealed in small glass capsules that are theninserted at the site of the cancer. It is also used to trace leaks in gas and liquid pipelines and tomeasure their rate of flow. The rate at which radon gas escapes from the Earth is one measurementthat helps scientists predict earthquakes.

• 定義

  A colorless monatomic radioactive element of the rare-gas group, now known to form unstable compounds. It has 19 short-lived radioisotopes; the most stable, radon-222, is a decay product of radium-226 and itself disintegrates into an isotope of polonium with a half-life of 3.82 days. ²²²Rn is sometimes used in radiotherapy. Radon occurs in uranium mines and is also detectable in houses built in certain areas of the country. Symbol: Rn; m.p. –71°C; b.p. –61.8°C; d. 9.73 kg m^–3 (0°C); p.n. 86.

• 用途

  医療用放射線源として用いられる．近年周辺の土壌・岩石あるいはコンクリートなど，建築材料からの建物内のラドンの蓄積が，吸入による内部被ばくの可能性として取り上げられている．旧科学技術庁委託の調査によると，わが国の屋内ラドン濃度平均値は15.5 Bq cm－3 と推定されており，諸外国の数値の数分の一から数千分の一と低いことから，危険性は少ないと判断されている．アメリカの屋内ラドン濃度平均値は48 Bq cm－3 で，タバコにつぐ第二位の肺がん発がん原因とされる．放射性同位元素等による放射線障害防止に関する法律施行令「同位元素の数量等を定める件」(平成17年改正)によると，222Rn の空気中の濃度限界3×10－5 Bq/cm3．

• 危険性

  As for radium.

• 安全性プロファイル

  A carcinogen. A common air contaminant. Radon is a noble gas and thus is relatively unreactive. Radiation Hazard: Natural isotope *zORn (Thoron, Thorium Series), T1/2 = 55 seconds, decays to radoactive 2'6Po by alphas of 6.3 MeV. Natural isotope 222Rn (Uranium Series), T1/2 = 3.8 days, decays to radioactive permissible levels are gven for 222Rn in equilibrium with its daughters. The chief hazard from this isotope is inhalation of the gaseous element and its solid daughters, whch are collected on the normal dust of the air. Ths material is deposited in the lungs and has been considered to be a major causative agent in the hgh incidence of lung cancer found in uranium miners. Radon and its daughters bdd up to an equhbrium value in about a month from radum compounds, whde the bdd-up from uranium compounds is negligble. Good ventilation of areas where radium is handled or stored is recommended to prevent accumulation of hazardous concentration of Rn and its daughters. Accumulation of radon in homes has been implicated in increased incidence of lung cancers. This accumulation is found in wellinsulated buildtngs located over land that has concentrations of uranium

• 発がん性

  Radon and its isotopic forms radon-222 and radon-220 are known to be human carcinogens based on sufficient evidence of carcinogenicity from studies in humans.

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