2011年9月1日木曜日

環境中の放射性セシウム

環境中の放射性セシウム [編集]

セシウム137のγスペクトル：660 keV γ- と 30 keV Ba Kα線
ネバダ核実験場でのアメリカの核実験後での堆積物のうち、最もセシウム137が含まれていた10回の爆発。SimonとHarryという爆発実験は1953年のアップショット・ノットホール作戦のもので、GeorgeとHowは1952年のタンブラー・スナッパー作戦のものである。
現在環境中に存在しているセシウム137などの多くは、1940年代〜1960年代の核実験や核事故で放出された。ただしセシウム134は、半減期の関係で数十年前に放出されたものは殆ど残っていない。1960年代前半に日本人は1日に1 Bq以上を摂取していたと推定されている。チェルノブイリ原子力発電所事故も例外ではなく、2005年現在、セシウム137はチェルノブイリ原子力発電所周辺の放射線汚染地域での、主な放射線源である。原子炉がメルトダウンしたとき、セシウム134、ヨウ素131、ストロンチウム90、プルトニウムとともに、セシウム137は健康への影響が最も大きい同位体の1つである。
チェルノブイリ事故以後のドイツ全土について平均してみると、セシウム137による汚染は、平均で2000-4000 Bq/m2となっている。これは1 mg/km2に相当し、500 gのセシウム137がドイツ全土にばらまかれたことになる。
セシウム137は人為的に生成された核種である。極めて低い確率で起こる天然ウランの自発核分裂で痕跡量が生成する程度である。他の大半の放射性同位体は安定同位体から生成され得るのに対し，セシウム137はウランから生成される。そのため核実験が 始まる以前には自然環境中に殆ど存在していなかった。この同位体から放出される特性γ線を観測することにより、密封容器の内容物が核実験の幕開け以前に製 造されたものかどうか判別することができる。この手順は、高級ワインが偽物であるかどうかの鑑定のため、特に「ジェファーソン・ボトル」事件などで用いら れた。
ウラン235の熱中性子核分裂において直接生成する場合の核分裂収率は、 セシウム137は0.06 %に過ぎないが、ヨウ素137（半減期24.5秒）は2.6 %、キセノン137（半減期3.82分）は3.2 %生成し、そ の他テルル137（半減期2.5分、0.39 %）なども含めて、これら短寿命核種がベータ崩壊してセシウム137となるため、累積の核分裂収率は 6.2 %となる[4]。

海洋中の分布 [編集]

海洋中では水深約200 m付近にある水温躍層(温度変化の急激な変化点)より浅い海域に多く存在し、濃度は比較的均一である。つまり、水温躍層が一種のバリヤ的機能を果たしている。生物濃縮により魚食性の高い魚種での高い濃縮度を示すデータが得られている[5]。

放射性セシウムの危険性 [編集]

アクチノイドと、その核分裂生成物
崩壊系列 半減期(年) 核分裂収率
4n 4n+1 4n+2 4n+3 >7% >5% >1% >0.1%
244Cm 241Pu 250Cf 243Cm 10～30 137Cs 90Sr 85Kr
232U
238Pu
60～90

151Sm

249Cf 242Am
100～400

241Am
251Cf 400～900
240Pu 229Th 246Cm 243Am 5～7千

245Cm 250Cm 239Pu 8千～3万

233U 230Th 231Pa 3万～16万


234U
20～30万 99Tc
126Sn 79Se
248Cm
242Pu
30～40万 この7核種が長寿命核分裂生成物

237Np

1～2百万 93Zr 135Cs

236U

247Cm 6百万～3千万
107Pd 129I
244Pu


8千万
232Th
238U 235U 7～140億
太字の核種は核分裂性
太字の核種は中性子毒
セシウム137に限らず、セシウムの化合物は多くが水溶性であり、生体内での振る舞いはカリウムやルビジウムに似ている。体内に入るとセシウムは体中に分配され、ベータ線による内部被ばくを起こす。濃度は骨組織で低く、筋組織で高い。生体内での半減期は70日以下であるという報告もあるが[6]、100-200日と言われることが一般的である。犬を使った実験では、3800 µCi/kg（ベクレルに換算すると 1.4×108 Bq/kg、約44 µg/kgのセシウム137、人間の実効線量で換算すれば1.82 Sv/kg）を服用したものは3週間以内に死亡した[7]。
事故でセシウム137を摂取してしまった場合、プルシアンブルーで治療される。これはセシウムやカリウムに結合し、体外への排出を促進する[8]。

生体に対する影響 [編集]

経口で10000 Bqを摂取した時の実効線量は0.13 mSvとされ、1 mの距離に1.00 MBqの線源があった場合、ガンマ線によって1日に1.9 µSvの外部被曝を受ける[9]。
世界保健機関 (WHO) の飲料水中の放射性核種のガイダンスレベルは、平常時の値は10 Bq/Lで原子力危機時の誘導介入レベル(介入レベルを超えないように環境汚染物質や汚染食品の摂取、流通を制限するため、二次的に設定される制限レベル、「暫定規制値」とも言う)であり、国際原子力機関は介入レベル（敷地外の一般公衆が過度の被ばくを生ずる恐れのある場合は、実行可能な限り被ばく低減のための対策をとることが必要となるため、その判断の基礎となる線量）を3,000 Bq/Lとしているが平常時の値や誘導介入レベルは定めていない[10]。日本では一定の基準は無く WHO の基準相当[11]を守っていた。しかし2011年東北地方太平洋沖地震における福島第一原子力発電所事故の影響から、放射性セシウムの飲料水中及び牛乳・乳製品中の暫定規制値を WHO 基準の20倍以上に相当する200 Bq/kgと定めた[12][13]。

生物濃縮 [編集]

詳細は「セシウム#生物濃縮」を参照

事故例 [編集]

一般に、放射性物質を不適切に管理すると、外部へ持ち出される恐れがある。その際に一般市民が被曝した事故としてゴイアニア被曝事故がある。
また、原料に混入すると、放射性物質を含む製品を生産することになる。アセリノックス事故（英語）では、スペインのアセリノックスというリサイクル会社が、γ線発生器のセシウム137を溶融してしまった[14][15]。 2009年には、中国陝西省のセメント会社が、古い使われていないセメント工場を取り壊し、放射性物質取り扱いの基準に則らなかった。それにより測定機器 に使われていたセシウム137が、トラック8台分の金属片とともに製鉄所に送られた。そうして、放射性セシウムが鋼鉄に混入することになった[16]。
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2011年4月1日金曜日

Caesium


Caesium

From Wikipedia, the free encyclopedia
xenon ← caesium → barium
Rb
↑
Cs
↓
Fr
Appearance
silvery gold
Some silvery-gold metal, with a liquid-like texture and lustre, sealed in a glass ampoule
General properties
Name, symbol, numbercaesium, Cs, 55
Pronunciation/ˈsiːziəm/ see-zee-əm
Element categoryalkali metal
Group, period, block1, 6, s
Standard atomic weight132.9054519(2)
Electron configuration[Xe] 6s1
Electrons per shell2, 8, 18, 18, 8, 1 (Image)
Physical properties
Phasesolid
Density (near r.t.)1.93 g·cm−3
Liquid density at m.p.1.843 g·cm−3
Melting point301.59 K, 28.44 °C, 83.19 °F
Boiling point944 K, 671 °C, 1240 °F
Critical point1938 K, 9.4 MPa
Heat of fusion2.09 kJ·mol−1
Heat of vaporization63.9 kJ·mol−1
Molar heat capacity32.210 J·mol−1·K−1
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)418469534623750940
Atomic properties
Oxidation states1
(strongly basic oxide)
Electronegativity0.79 (Pauling scale)
Ionization energies1st: 375.7 kJ·mol−1
2nd: 2234.3 kJ·mol−1
3rd: 3400 kJ·mol−1
Atomic radius265 pm
Covalent radius244±11 pm
Van der Waals radius343 pm
Miscellanea
Crystal structurebody-centered cubic
Magnetic orderingparamagnetic[1]
Electrical resistivity(20 °C) 205 nΩ·m
Thermal conductivity35.9 W·m−1·K−1
Thermal expansion(25 °C) 97 µm·m−1·K−1
Young's modulus1.7 GPa
Bulk modulus1.6 GPa
Mohs hardness0.2
Brinell hardness0.14 MPa
CAS registry number7440-46-2
Most stable isotopes
Main article: Isotopes of caesium
isoNAhalf-lifeDMDE (MeV)DP
133Cs100%133Cs is stable with 78 neutrons
134Cssyn2.0648 yε1.229134Xe
β−2.059134Ba
135Cstrace2.3×106 yβ−0.269135Ba
137Cstrace30.17 y[2]β−1.174137Ba
  • v
    •  
  • t
    •  
  • e
    •  
· r
Caesium or cesium[note 1] (play /ˈsiːziəm/ see-zee-əm) is the chemical element with the symbol Csand atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at (or near) room temperature.[note 2] Caesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium. The metal is extremely reactive and pyrophoric, reacting with water even at −116 °C (−177 °F). It is the least electronegative element having a stable isotope, caesium-133. Caesium is mined mostly from pollucite, while the radioisotopes, especiallycaesium-137, a fission product, are extracted from waste produced by nuclear reactors.
Two German chemists, Robert Bunsen and Gustav Kirchhoff, discovered caesium in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for caesium were as a "getter" in vacuum tubes and in photoelectric cells. In 1967, a specific frequency from the emission spectrum of caesium-133 was chosen to be used in the definition of the second by the International System of Units. Since then, caesium has been widely used in atomic clocks.
Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids. It has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radiation leaks.

Contents

  [hide] 

[edit]Characteristics

[edit]Physical properties

Y-shaped yellowish crystal in glass ampoule, looking like the branch of a pine tree
High-purity caesium-133 preserved under argon
Caesium is a very soft (it has the lowest hardness of all elements, 0.2 Mohs), very ductile, silvery-white metal, which develops a silvery-gold hue in the presence of trace amounts of oxygen.[7][8] It has amelting point of 28.4 °C (83.1 °F), making it one of the few elemental metals which are liquid near room temperature. Mercury is the only elemental metal with a known melting point lower than caesium.[note 3][10] In addition, the metal has a rather low boiling point, 641 °C (1,186 °F), the lowest of all metals other than mercury.[11] Its compounds burn with a blue[12][13] or violet[13] color.
Secure caesium sample for teaching
Caesium forms alloys with the other alkali metals, as well as with gold, and amalgams with mercury. At temperatures below 650 °C (1,202 °F), it alloys withcobalt, iron, molybdenum, nickel, platinum, tantalumor tungsten. It forms well-defined intermetallic compounds with antimony, gallium, indium andthorium, which are photosensitive.[7] It mixes with the other alkali metals (except with lithium), and the alloy with a molar distribution of 41% caesium, 47%potassium, and 12% sodium has the lowest melting point of any known metal alloy, at −78 °C(−108 °F).[10][14] A few amalgams have been studied:CsHg2 is black with a purple metallic lustre, while CsHg is golden-coloured, also with a metallic lustre.[15]

[edit]Chemical properties

A person adds a small amount of metal to a petri dish with cold water which produces a small explosion.
Addition of a small amount of caesium to cold water is explosive.
Caesium metal is highly reactive and very pyrophoric. In addition to igniting spontaneously in air, it reacts explosively with water even at low temperatures, more so than other members of the first group of the periodic table.[7] The reaction with solid water occurs at temperatures as low as −116 °C(−177 °F).[10] Because of its high reactivity, the metal is classified as a hazardous material. It is stored and shipped in dry saturated hydrocarbons such as mineral oil. Similarly, it must be handled under inert atmosphere such as argon. It can be stored in vacuum-sealed borosilicate glass ampoules. In quantities of more than about 100 grams (3.5 oz), caesium is shipped in hermetically sealed, stainless steel containers.[7]
The chemistry of caesium is similar to that of other alkali metals, but is more closely similar to that ofrubidium, the element above caesium in the periodic table.[16] Some small differences arise from the fact that it has a higher atomic mass and is more electropositive than other (nonradioactive) alkali metals.[17] Caesium is the most electropositive stable chemical element.[note 4][10] The caesium ion is also larger and less "hard" than those of the lighter alkali metals.

[edit]Compounds

27 small grey spheres in 3 evenly spaced layers of nine. 8 spheres form a regular cube and 8 of those cubes form a larger cube. The grey spheres represent the caesium atoms. The center of each small cube is occupied by a small green sphere representing a chlorine atom. Thus, every chlorine is in the middle of a cube formed by caesium atoms and every caesium is in the middle of a cube formed by chlorine.
Ball-and-stick model of the cubic coordination of Cs and Cl in CsCl
The vast majority of caesium compounds contain the element as the cation Cs+, which binds ionically to a wide variety of anions. One noteworthy exception is provided by the caeside anion (Cs−).[19] Other exceptions include the several suboxides (see section on oxides below).
Returning to more normal compounds, salts of Cs+ are almost invariably colorless unless the anion itself is colored. Many of the simple salts are hygroscopic, but less so than the corresponding salts of the lighter alkali metals. Thephosphate,[20] acetate, carbonate, halides, oxide, nitrate, and sulfate salts are water-soluble. Double salts are often less soluble, and the low solubility of caesium aluminium sulfate is exploited in the purification of Cs from its ores. The double salt with antimony (such as CsSbCl4), bismuth, cadmium, copper, iron, and lead are also poorlysoluble.[7]
Caesium hydroxide (CsOH) is hygroscopic and a very strong base.[16] It rapidly etches the surface of semiconductorssuch as silicon.[21] CsOH has been previously regarded by chemists as the "strongest base", reflecting the relatively weak attraction between the large Cs+ ion and OH–.;[12] it is indeed the strongest Arrhenius base, but a number of compounds that cannot exist in aqueous solution, such as n-butyllithium and sodium amide,[16] are more basic.
A stoichiometric mixture of caesium and gold will react to form caesium auride.

[edit]Complexes

Like all metal cations, Cs+ forms complexes with Lewis bases in solution. Because of its large size, Cs+ usually adopts coordination numbers greater than six-coordination, which is typical for the lighter alkali metal cations. This trend is already apparent by the 8-coordination in CsCl, vs the halite motif adopted by the other alkali metal chlorides. Its high coordination number and softness (tendency to form covalent bonds) are the basis of the separation of Cs+ from other cations, as is practiced in the remediation of nuclear wastes, where 137Cs+ is separated from large amounts of nonradioactive K+.[22]

[edit]Halides

Caesium chloride (CsCl) crystallizes in the simple cubic crystal system. Also called the "caesium chloride structure",[17] this structural motif is composed of a primitive cubic lattice with a two-atom basis, each with an eightfold coordination; the chloride atoms lie upon the lattice points at the edges of the cube, while the caesium atoms lie in the holes in the center of the cubes. This structure is shared with CsBr and CsI, and many other compounds that do not contain Cs. In contrast, most other alkaline halides adopt the sodium chloride (NaCl) structure.[17] The CsCl structure is preferred because Cs+ has an ionic radius of 174 pm and Cl− 181 pm.[23]

[edit]Oxides

The stick and ball diagram shows three regular octahedra which are connected to the next one by one surface and the last one shares one surface with the first. All three have one edge in common. All eleven vertices are purple spheres representing caesium, and at the center of each octahedron is a small red sphere representing oxygen.
Cs11O3 cluster
More so than the other alkali metals, caesium forms numerous binary compounds with oxygen. When caesium burns in air, the superoxide CsO2 is the main product.[24] The "normal" caesium oxide (Cs2O) forms yellow-orange hexagonal crystals,[25]and is the only oxide of the anti-CdCl2 type.[26] It vaporizes at 250 °C (482 °F), and decomposes to caesium metal and theperoxide Cs2O2 at temperatures above 400 °C (752 °F).[27] Aside from the superoxide and the ozonide CsO3,[28][29] several brightly colored suboxides have also been studied.[30] These include Cs7O, Cs4O, Cs11O3, Cs3O (dark-green[31]), CsO,Cs3O2,[32] as well as Cs7O2.[33][34] The latter may be heated under vacuum to generate Cs2O.[26] Binary compounds withsulfur, selenium, and tellurium also exist.[7]

[edit]Isotopes

Main article: Isotopes of caesium
Caesium has a total of 39 known isotopes that range in their mass number (i.e. number of nucleons in its nucleus) from 112 to 151. Several of these are synthesized from lighter elements by the slow neutron capture process (S-process) inside old stars,[35] as well as inside supernova explosions (R-process).[36] However, the only stable isotope is 133Cs, which has 78 neutrons. Although it has a large nuclear spin (7/2+), nuclear magnetic resonance studies can be done with this isotope at a resonating frequency of 11.7 MHz.[37]
A graph showing the energetics of caesium-137 (nuclear spin: I=7/2+, half-life of about 30 years) decay. With a 94.6% probability, it decays by a 512 keV beta emission into barium-137m (I=11/2-, t=2.55min); this further decays by a 662 keV gamma emission with an 85.1% probability into barium-137 (I=3/2+). Alternatively, caesium-137 may decay directly into barium-137 by a 0.4% probability beta emission.
Decay of caesium-137
The radioactive 135Cs has a very long half-life of about 2.3 million years, while 137Cs and 134Cshave half-lives of 30 and two years, respectively. 137Cs decomposes to a short-lived 137mBa bybeta decay, and then to nonradioactive barium, while 134Cs transforms into 134Ba directly. The isotopes with mass numbers of 129, 131, 132 and 136, have half-times between a day and two weeks, while most of the other isotopes have half-lives from a few seconds to fractions of a second. There are at least 21 metastable nuclear isomers. Other than 134mCs (with a half-life of just under 3 hours), all are very unstable and decay with half-lives of a few minutes or less.[38][39]
The isotope 135Cs is one of the long-lived fission products of uranium which form in nuclear reactors.[40] However, its fission product yield is reduced in most reactors because its predecessor, 135Xe, is an extremely potent neutron poison and transmutes frequently to stable136Xe before it can decay to 135Cs.[41][42]
Because of its beta decay (to 137mBa), 137Cs is a strong emitter of gamma radiation.[43] Its half-life makes it the principal medium-lived fission product along with 90Sr—both are responsible for radioactivity of spent nuclear fuel after several years of cooling up to several hundred years after use.[44] For example 137Cs together with 90Sr currently generate the largest source of radioactivity generated in the area around the Chernobyl disaster.[45] It is not feasible to dispose of 137Cs through neutron capture (due to the low capture rate) and as a result it must be allowed to decay.[46]
Almost all caesium produced from nuclear fission comes from beta decay of originally more neutron-rich fission products, passing through variousisotopes of iodine and of xenon.[47] Because iodine and xenon are volatile and can diffuse through nuclear fuel or air, radioactive caesium is often created far from the original site of fission.[48] With the commencement of nuclear weapons testing around 1945, 137Cs was released into theatmosphere and then returned to the surface of the earth as a component of radioactive fallout.[7]

[edit]Occurrence

A white mineral, from which white and pale pink crystals protrude
Pollucite, a caesium mineral
See also: Caesium minerals
Caesium is a relatively rare element as it is estimated to average approximately 3 parts per million in the Earth's crust.[49] This makes it the 45th most abundant of all elements and the 36th of all the metals. Nevertheless, it is more abundant than such elements as antimony, cadmium, tin and tungsten, and two orders of magnitude more abundant than mercury or silver, but 3.3%as abundant asrubidium—with which it is so closely chemically associated.[7]
Due to its large ionic radius, caesium is one of the "incompatible elements".[50] During magma crystallization, caesium is concentrated in the liquid phase and crystallizes last. Therefore, the largest deposits of caesium are zone pegmatite ore bodies formed by this enrichment process. Because caesium does not substitute for potassium as readily as does rubidium, the alkali evaporite mineralssylvite (KCl) and carnallite (KMgCl3·6H2O) may contain only 0.002% caesium. Consequently, Cs is found in few minerals. Percentage amounts of caesium may be found in beryl (Be3Al2(SiO3)6) and avogadrite ((K,Cs)BF4), up to 15 wt% Cs2O in the closely related mineral pezzottaite(Cs(Be2Li)Al2Si6O18), up to 8.4 wt% Cs2O in the rare mineral londonite ((Cs,K)Al4Be4(B,Be)12O28), and less in the more widespread rhodizite.[7] The only economically important source mineral for caesium is pollucite Cs(AlSi2O6), which is found in a few places around the world in zoned pegmatites, and is associated with the more commercially important lithium minerals lepidolite and petalite. Within the pegmatites, the large grain size and the strong separation of the minerals create high-grade ore for mining.[51]
One of the world's most significant and richest sources of the metal is the Tanco mine at Bernic Lake in Manitoba, Canada. The deposits there are estimated to contain 350,000 metric tons of pollucite ore, which represent more than two-thirds of the world's reserve base.[51][52] Although the stoichiometric content of caesium in pollucite is 42.6%, pure pollucite samples from this deposit contain only about 34% caesium, while the average content is 24 wt%.[52] Commercial pollucite contains over 19% caesium.[53] The Bikita pegmatite deposit in Zimbabwe is mined for its petalite, but it also contains a significant amount of pollucite. Notable amounts of pollucite are also mined in the Karibib Desert, Namibia.[52] At the present rate of world mine production of 5 to 10 metric tons per year, reserves will last for thousands of years.[7]

[edit]Production

The mining of pollucite ore is a selective process and is conducted on a small scale in comparison with most metal mining operations. The ore is crushed, hand-sorted, but not usually concentrated, and then ground. Caesium is then extracted from pollucite mainly by three methods: acid digestion, alkaline decomposition, and direct reduction.[7][54]
In the acid digestion, the silicate pollucite rock is dissolved with strong acids such as hydrochloric (HCl), sulfuric (H2SO4), hydrobromic (HBr), orhydrofluoric (HF) acids. With hydrochloric acid, a mixture of soluble chlorides is produced, and the insoluble chloride double salts of caesium are precipitated as caesium antimony chloride (Cs4SbCl7), caesium iodine chloride (Cs2ICl), or caesium hexachlorocerate (Cs2(CeCl6)). After separation, the pure precipitated double salt is decomposed, and pure CsCl is obtained after evaporating the water. The method using sulfuric acid yields the insoluble double salt directly as caesium alum (CsAl(SO4)2·12H2O). The aluminium sulfate in it is converted to the insoluble aluminium oxide by roasting the alum with carbon, and the resulting product is leached with water to yield a Cs2SO4 solution.[7]
The roasting of pollucite with calcium carbonate and calcium chloride yields insoluble calcium silicates and soluble caesium chloride. Leaching with water or dilute ammonia (NH4OH) yields then a dilute chloride (CsCl) solution. This solution can be evaporated to produce caesium chloride or transformed into caesium alum or caesium carbonate. Albeit not commercially feasible, direct reduction of the ore with potassium, sodium or calcium in vacuum can produce caesium metal directly.[7]
Most of the mined caesium (as salts) is directly converted into caesium formate (HCOO−Cs+) for applications such as oil drilling. To supply the developing market, Cabot Corporation built a production plant in 1997 at the Tanco mine near Bernic Lake in Manitoba, with a capacity of 12,000 barrels (1,900 m3) per year of caesium formate solution.[55] The primary smaller-scale commercial compounds of caesium are caesium chloride and its nitrate.[56]
Alternatively, caesium metal may be obtained from the purified compounds derived from the ore. Caesium chloride, and the other caesium halides, as well, can be reduced at 700 to 800 °C (1,292 to 1,472 °F) with calcium or barium, followed by distillation of the caesium metal. In the same way, the aluminate, carbonate, or hydroxide may be reduced by magnesium.[7] The metal can also be isolated by electrolysis of fused caesium cyanide(CsCN). Exceptionally pure and gas-free caesium can be made by the thermal decomposition at 390 °C (734 °F) of caesium azide CsN3, which is produced from aqueous caesium sulfate and barium azide.[54] In vacuum applications, caesium dichromate can be reacted with zirconium forming pure caesium metal without other gaseous products.[56]
Cs2Cr2O7 + 2 Zr → 2 Cs + 2 ZrO2+ Cr2O3
The price of 99.8% pure caesium (metal basis) in 2009 was about US$10 per gram ($280 per ounce), but its compounds are significantly cheaper.[52]

[edit]History

Three middle-aged men, with the one in the middle sitting down. All wear long jackets, and the shorter man on the left has a beard.
Gustav Kirchhoff (left) and Robert Bunsen(center) discovered caesium spectroscopically.
In 1860, Robert Bunsen and Gustav Kirchhoff discovered caesium in the mineral water from Dürkheim, Germany. Due to the bright blue lines in its emission spectrum, they chose a name derived from the Latin word caesius, meaning sky-blue.[note 5][57][58][59] Caesium was the first element to be discoveredspectroscopically, only one year after the invention of the spectroscope by Bunsen and Kirchhoff.[10]
To obtain a pure sample of caesium, 44,000 litres (9,700 imp gal; 12,000 US gal) of mineral water had to be evaporated to yield 240 kilograms (530 lb) of concentrated salt solution. The alkaline earth metals were precipitated either as sulfates or oxalates, leaving the alkali metal in the solution. After conversion to the nitrates and extraction with ethanol, a sodium-free mixture was obtained. From this mixture, the lithium was precipitated by ammonium carbonate. Potassium, rubidium and caesium form insoluble salts with chloroplatinic acid, but these salts show a slight difference in solubility in hot water. Therefore, the less-soluble caesium and rubidium hexachloroplatinate ((Cs,Rb)2PtCl6) could be obtained by fractional crystallization. After reduction of the hexachloroplatinate with hydrogen, caesium and rubidium could be separated by the difference in solubility of their carbonates in alcohol. The process yielded 9.2 grams (0.32 oz) of rubidium chloride and 7.3 grams (0.26 oz) of caesium chloride from the initial 44,000 liters of mineral water.[58]
The two scientists used the caesium chloride thus obtained to estimate the atomic weight of the new element at 123.35 (compared to the currently accepted one of 132.9).[58] They tried to generate elemental caesium by electrolysis of molten caesium chloride, but instead of a metal, they obtained a blue homogenous substance which "neither under the naked eye nor under the microscope" showed the slightest trace of metallic substance"; as a result, they assigned it as a subchloride (Cs2Cl). In reality, the product was probably a colloidal mixture of the metal and caesium chloride.[60] The electrolysis of the aqueous solution of chloride with a mercury anode produced a caesium amalgam which readily decomposed under the aqueous conditions.[58] The pure metal was eventually isolated by the German chemist Carl Setterberg while working on his doctorate with Kekulé and Bunsen.[59] In 1882, he produced caesium metal by electrolyzing caesium cyanide, and thus avoiding the problems with the chloride.[61]
Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical fields. Very few applications existed for caesium until the 1920s, when it came to be used in radio vacuum tubes. It had two functions; as a getter, it removed excess oxygen after manufacture, and as a coating on the heated cathode, it increased its electrical conductivity. Caesium did not become recognized as a high-performance industrial metal until the 1950s.[62] Applications of nonradioactive caesium included photoelectric cells, photomultiplier tubes, optical components of infrared spectrophotometers, catalysts for several organic reactions, crystals for scintillation counters, and inmagnetohydrodynamic power generators.[7]
Since 1967, the International System of Measurements has based its unit of time, the second, on the properties of caesium. The International System of Units (SI) defines the second as 9,192,631,770 cycles of the radiation, which corresponds to the transition between two hyperfine energy levels of the ground state of the caesium-133 atom.[63] The 13th General Conference on Weights and Measures of 1967 defined a second as: "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of caesium-133 atoms in their ground state undisturbed by external fields".

[edit]Applications

[edit]Petroleum exploration

The largest current end-use of nonradioactive caesium is in caesium formate-based drilling fluids for the extractive oil industry.[7] Aqueous solutions of caesium formate (HCOO–Cs+)—made by reacting caesium hydroxide with formic acid—were developed in the mid-1990s for use as oil well drilling and completion fluids. The function of caesium formate as a drilling fluid is to lubricate drill bits, to bring rock cuttings to the surface, and to maintain pressure on the formation during drilling of the well. As completion fluid, which assists the emplacement of control hardware after drilling but prior to production, the function of caesium formate is to maintain the pressure.[7]
The high density of the caesium formate brine (up to 2.3 g·cm−3, or 19.2 pounds per gallon),[64] coupled with the relatively benign nature of most caesium compounds, reduces the requirement for toxic high-density suspended solids in the drilling fluid—a significant technological, engineering and environmental advantage. Unlike the components of many other heavy liquids, caesium formate is relatively environment-friendly.[64] The caesium formate brine can be blended with potassium and sodium formates to decrease the density of the fluids down to that of water (1.0 g·cm−3, or 8.3 pounds per gallon). Furthermore, it is biodegradable and reclaimable, and may be recycled, which is important in view of its high cost (about $4,000 per barrel in 2001).[65] Alkali formates are safe to handle and do not damage the producing formation or downhole metals as their corrosive alternative, high-density brines (such as zinc bromide ZnBr2 solutions),sometimes do; they also require less cleanup and disposal costs.[7]

[edit]Atomic clocks

A room with a black box in the foreground and six control cabinets with space for five to six racks each. Most, but not all, of the cabinets are filled with white boxes.
Atomic clock ensemble at the U.S. Naval Observatory
A laboratory table with some optical devices on it.
FOCS-1, a continuous cold caesium fountain atomic clock in Switzerland, started operating in 2004 at an uncertainty of one second in 30 million years
Caesium-based atomic clocks observe electromagnetic transitions in the hyperfine structure of caesium-133 atoms and use it as a reference point. The first accurate caesium clock was built byLouis Essen in 1955 at the National Physical Laboratory in the UK.[66] Since then, they have been improved repeatedly over the past half-century, and form the basis for standards-compliant time and frequency measurements. These clocks measure frequency with an error of 2 to 3 parts in 1014, which would correspond to a time measurement accuracy of 2 nanoseconds per day, or one second in 1.4 million years. The latest versions are accurate to better than 1 part in 1015, which means they would be off by about 2 seconds since the extinction of the dinosaurs65 million years ago,[7] and has been regarded as "the most accurate realization of a unit that mankind has yet achieved."[63]
Caesium clocks are also used in networks that oversee the timing of cell phone transmissions and the information flow on the Internet.[67]

[edit]Electric power and electronics

Caesium vapor thermionic generators are low-power devices that convert heat energy to electrical energy. In the two-electrode vacuum tube converter, it neutralizes the space charge that builds up near the cathode, and in doing so, it enhances the current flow.[68]
Caesium is also important for its photoemissive properties by which light energy is converted to electron flow. It is used in photoelectric cells because caesium-based cathodes such as the intermetallic compound K2CsSb have low threshold voltage for emission of electrons.[69] The range of photoemissive devices using caesium include optical character recognition devices, photomultiplier tubes, and video camera tubes.[70][71] Nevertheless, germanium, rubidium, selenium, silicon, tellurium, and several other elements can substitute caesium in photosensitive materials.[7]
Caesium iodide (CsI), bromide (CsBr) and caesium fluoride (CsF) crystals are employed for scintillatorsin scintillation counters widely used in mineral exploration and particle physics research, as they are well-suited for the detection of gamma and X-ray radiation. Caesium, being a heavy element, provides good stopping power, contributing to better detectivity. Caesium compounds may also provide a faster response (CsF) and be less hygroscopic (CsI).
Caesium vapor is used in many common magnetometers.[72] The element is also used as an internal standard in spectrophotometry.[73] Like otheralkali metals, caesium has a great affinity for oxygen and is used as a "getter" in vacuum tubes.[74] Other uses of the metal include high-energylasers, vapor glow lamps, and vapor rectifiers.[7]

[edit]Centrifugation fluids

Because of their high density, solutions of caesium chloride, caesium sulfate, and caesium trifluoroacetate (Cs(O2CCF3)) are commonly used in molecular biology for density gradient ultracentrifugation.[75] This technology is primarily applied to the isolation of viral particles, subcellularorganelles and fractions, and nucleic acids from biological samples.[76]

[edit]Chemical and medical use

Some fine white powder on a laboratory watch glass
A sample of caesium chloride
Relatively few chemical applications exist for caesium.[77] Doping with caesium compounds is used to enhance the effectiveness of several metal-ion catalysts used in the production of chemicals, such asacrylic acid, anthraquinone, ethylene oxide, methanol, phthalic anhydride, styrene, methyl methacrylate monomers, and various olefins. It is also used in the catalytic conversion of sulfur dioxideinto sulfur trioxide in the production of sulfuric acid.[7]
Caesium fluoride enjoys niche use in organic chemistry as a base,[16] or as an anhydrous source offluoride ion.[78] Caesium salts sometimes replace potassium or sodium salts in organic synthesis, such as cyclization, esterification, and polymerization.

[edit]Nuclear and isotope applications

Caesium-137 is a very common radioisotope used as a gamma-emitter in industrial applications. Its advantages include a half-life of roughly 30 years, its availability from the nuclear fuel cycle, and having 137Ba as stable end product. The high water solubility is a disadvantage which makes it incompatible with irradiation of food and medical supplies.[79] It has been used in agriculture, cancer treatment, and the sterilization of food, sewage sludge, and surgical equipment.[7][80] Radioactive isotopes of caesium in radiation devices were used in the medical field to treat certain types of cancer,[81] but emergence of better alternatives and the use of water-soluble caesium chloride in the sources, which could create wide-ranging contamination, gradually put some of these caesium sources out of use.[82][83] Caesium-137 has been employed in a variety of industrial measurement gauges, including moisture, density, leveling, and thickness gauges.[84] It has also been used inwell logging devices for measuring the electron density of the rock formations, which is analogous to the bulk density of the formations.[85]
Isotope 137 has also been used in hydrologic studies analogous to those using tritium. It is a daughter product of nuclear fission reactions. With the commencement of nuclear testing around 1945, and continuing through the mid-1980s, caesium-137 was released into the atmosphere, where it is absorbed readily into solution. Known year-to-year variation within that period allows correlation with soil and sediment layers. Caesium-134, and to a lesser extent caesium-135, have also been used in hydrology as a measure of caesium output by the nuclear power industry. While they are less prevalent than either caesium-133 or caesium-137, these isotopes have the advantage of being produced solely from anthropogenic sources.[86]

[edit]Other uses

Electrons shooting out of an electron gun hit neutral fuel nuclei which leads to their ionization; in a chamber surrounded by magnets, the positive ions are directed towards a negative grid which accelerates them; once out of this chamber, the positive ions are neutralized from another electron gun leaving the chamber behind with a significant momentum thus propelling the previous chamber in the opposite direction.
Schematics of an electrostatic ion thruster which was initially developed for use with caesium or mercury
Caesium and mercury were used as a propellant in early ion engines designed forspacecraft propulsion on very long interplanetary or extraplanetary missions. The ionization method was to strip the outer electron from the propellant upon contact with a tungsten electrode that had voltage applied. Concerns about the corrosive action of caesium on spacecraft components have pushed development in the direction of use of inert gas propellants such as xenon; this is easier to handle in ground-based tests and has less potential to interfere with the spacecraft.[7] Eventually, xenon was used in the experimental spacecraft Deep Space 1 launched in 1998.[87][88] Nevertheless, field emission electric propulsion thrusters which use a simple system of accelerating liquid metal ions such as of caesium to create thrust have been built.[89]
Caesium nitrate is used as an oxidizer and pyrotechnic colorant to burn silicon in infraredflares[90] such as the LUU-19 flare,[91] because it emits much of its light in the near infrared spectrum.[92] Caesium has been used to reduce the radar signature of exhaust plumes in the SR-71 Blackbird military aircraft.[93] Caesium, along with rubidium, has been added as a carbonate to glass because it reduces electrical conductivity and improves stability and durability of fiber optics and night vision devices. Caesium fluoride or caesium aluminium fluoride are used in fluxes formulated for the brazing of aluminium alloys that contain magnesium.[7]

[edit]Prognostications

Magnetohydrodynamic (MHD) power-generating systems were researched, but failed to gain widespread acceptance.[94] Caesium metal has also been considered as the working fluid in high-temperature Rankine cycle turboelectric generators.[95] Caesium salts have been evaluated as antishock reagents to be used following the administration of arsenical drugs. Because of their effect on heart rhythms, however, they are less likely to be used than potassium or rubidium salts. They have also been used to treat epilepsy.[7]

[edit]Health and safety hazards

Graph of percentage of the radioactive output by each nuclide that form after a nuclear fallout vs logarithm of time after the incident. In curves of various colors, the predominant source of radiation are depicted in order: Te-132/I-132 for the first five or so days; I-131 for the next five; Ba-140/La-140 briefly; Zr-95/Nb-95 from day 10 until about day 200; and finally Cs-137. Other nuclides producing radioactivity, but not peaking as a major component are Ru, peaking at about 50 days, and Cs-134 at around 600 days.
The portion of the total radiation dose (in air) contributed by each isotope versus time after the Chernobyl disasterdepicting caesium-137 becoming the largest source of radiation about 200 days after the accident.[96]
Caesium compounds are rarely encountered by most people, but most are mildly toxic because of chemical similarity of caesium to potassium. Exposure to large amounts of caesium compounds can cause hyperirritability and spasms, but as such amounts would not ordinarily be encountered in natural sources, caesium is not a major chemical environmental pollutant.[97] The median lethal dose (LD50) value for caesium chloridein mice is 2.3 g per kilogram, which is comparable to the LD50 values of potassium chloride and sodium chloride.[98]
NFPA 704
NFPA 704.svg
3
3
2
W
The fire diamondhazard sign for caesium metal
Caesium metal is one of the most reactive elements and is highlyexplosive when it comes in contact with water. The hydrogen gas produced by the reaction is heated by the thermal energy released at the same time, causing ignition and a violent explosion. This can occur with other alkali metals, but caesium is so potent that this explosive reaction can even be triggered by cold water.[7] The metal is highly pyrophoric, and ignites spontaneously in air to form caesium hydroxide and various oxides. Caesium hydroxide is a very strong base, and can rapidly corrode glass.[11]
The isotopes 134 and 137 (present in the biosphere in small amounts from radiation leaks) represent a radioactivity burden which varies depending on location. Radiocaesium does not accumulate in the body as effectively as many other fission products (such as radioiodine and radiostrontium). About 10% of absorbed radiocaesium washes out of the body relatively quickly in sweat and urine. The remaining 90% has a half-life between 50 and 150 days.[99] Radiocaesium follows potassium and tends to accumulate in plant tissues, including fruits and vegetables.[100][101][102] It is also well-documented that mushrooms from contaminated forests accumulate radiocaesium (caesium-137) in their fungal sporocarps.[103] Accumulation of caesium-137 in lakes has been a high concern after the Chernobyl disaster.[104][105] Experiments with dogs showed that a single dose of 3.8 millicuries (140 MBq, 4.1 μg of caesium-137) per kilogram is lethal within three weeks;[106] smaller amounts may cause infertility and cancer.[107] The International Atomic Energy Agency and other sources have warned that radioactive materials, such as caesium-137, could be used in radiological dispersion devices, or "dirty bombs".[108]

[edit]See also

  • Goiânia accident, a major radioactive contamination incident involving a rod of caesium chloride
  • Acerinox accident, a caesium-137 contamination accident
  • Caesium standard, primary frequency standard, which allows to produce one of the most accurate types of clock
  • Fluorine, the most electronegative element, oppositely to caesium
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[edit]Notes

  1. ^ Caesium is the spelling recommended by the International Union of Pure and Applied Chemistry (IUPAC).[3] The American Chemical Society (ACS) has used the spelling cesium since 1921,[4][5] following Webster's New International Dictionary. The element was named after the Latin word caesius, meaning "bluish gray". More spelling explanation at ae/oe vs e.
  2. ^ Along with rubidium (39 °C [102 °F]), francium (estimated at 27 °C [81 °F]), mercury (−39 °C [−38 °F]), and gallium (30 °C [86 °F]); bromine is also liquid at room temperature (melting at −7.2 °C, 19 °F) but it is a halogen, not a metal.[6]
  3. ^ The radioactive element francium may also have a lower melting point, but its radioactivity prevents enough of it from being isolated for direct testing.[9]
  4. ^ Francium may be more electropositive, but this has not been experimentally measured due to its high radioactivity. Measurements of the firstionization energy of francium suggest its relativistic effects may lower its reactivity and raise its electronegativity above that expected from periodic trends.[18]
  5. ^ Bunsen quotes Aulus Gellius Noctes Atticae II, 26 by Nigidius Figulus: Nostris autem veteribus caesia dicts est quae Graecis, ut Nigidus ait, de colore coeli quasi coelia.

[edit]References

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