ネプツニウム

ネプツニウム 化学特性,用途語,生産方法

• 解説

  ネプツニウム,Np．原子番号93の元素．電子配置[Rn]5f 46d7s2の周期表3族超ウラン元素の一つ．アクチノイド元素の一つ．質量数237，半減期2.14×106 y でα崩壊するもっとも長寿命の核種のほか，質量数225～244の放射性核種が知られている．1940年，E. McMillan(マクミラン)とP. Abelsonにより，カリフォルニア大学バークレー校のサイクロトロンでBeを重陽子で照射して発生させた中性子による天然ウランの衝撃で質量数239のβ崩壊核種(半減期2.4 d)が合成された．ウランの次の元素であるところから，太陽系の天王星uranusの次の惑星・海王星neptuneにちなみ，ネプツニウムと命名された．

• 性質

  237Npは原子炉中で238Uの(n,2n)反応あるいは235Uの2段階中性子捕獲によりつくられる．人工放射性元素であるが，ウラン鉱石中にごく微量存在する．三フッ化物を金属バリウムで還元すると，銀白色のネプツニウム金属が得られる．α，β，γの3変態があり，α相(斜方晶系)は，278 ℃ でβ相(正方晶系)に，577 ℃ でγ相(立方晶系)に転移する．融点640 ℃，沸点3900 ℃．密度(g cm－3)：α 20.25(20 ℃)，β 19.36(313 ℃)，γ 18.0(600 ℃)．第一イオン化エネルギー6.266 eV．常温では空気中で安定である．かなり反応性に富み，水素と反応し水素化物NpH3をつくる．酸化数は3～7で，もっとも安定な酸化数は5(ウランは6)．超ウラン元素のなかでは化学的性質は一番ウランに似ているが，ウランに比べて低酸化状態が安定である．塩酸に可溶，硫酸，硝酸，酢酸には難溶．酸化物NpⅣ O2(若緑色)は，金属の酸素雰囲気中での燃焼で得られる．

• 化学的特性

  silvery metal; α: ortho-rhomb, a=0.4721 nm, b=0.4888 nm, c=0.6661nm, stable from room temp to 280°C; β: tetra, a=0.4895 nm, c=0.3386nm, stable from 280–577°C; γ: a=0.3518 nm; bcc, stable from 577–637°C; enthalpy of vaporization 418 kJ/mol; enthalpy of fusion 3.20 kJ/mol; 237Np, t1/2=2.14×10+6 years, t1/2 of 236Np 1.29×10+6 years; discovered in 1940; produced in kg amounts as a by-product of plutonium production [MER06] [KIR78] [CRC10]

• 物理的性質

  The chemistry of neptunium (₉₃Np) is somewhat similar to that of uranium (₉₂U) and plutonium(₉₄Pu), which immediately precede and follow it in the actinide series on the periodictable. The discovery of neptunium provided a solution to a puzzle as to the missing decayproducts of the thorium decay series, in which all the elements have mass numbers evenlydivisible by four; the elements in the uranium series have mass numbers divisible by fourwith a remainder of two. The actinium series elements have mass numbers divisible by fourwith a remainder of three. It was not until the neptunium series was discovered that a decayseries with a mass number divisible by four and a remainder of one was found. The neptuniumdecay series proceeds as follows, starting with the isotope plutonium-241: Pu-241→Am-241→Np-237→Pa-233→U-233→Th-229→Ra-225→Ac-225→Fr-221→At-217→Bi-213→Ti-209→Pb-209→Bi-209.
  Neptunium is a silvery-white radioactive, heavy metal. Its melting point is 644°C, its boilingpoint is 3,902°C, and its density is 20.25g/cm³.

• 同位体

  There are a total of 23 isotopes of neptunium. None are stable. All are radioactivewith half-lives ranging from two microseconds to 2.144×10⁺⁶years for the isotopeNp-237, which spontaneously fissions through alpha decay.

• 名前の由来

  Named for the planet Neptune.

• 天然物の起源

  At one time, neptunium’s entire existence was synthesized by man. Sometime later, in themid-twentieth century, it was discovered that a very small amount is naturally produced inuranium ore through the actions of neutrons produced by the decay of uranium in the orepitchblende. Even so, a great deal more neptunium is artificially produced every year than everdid or does exist in nature. Neptunium is recovered as a by-product of the commercial productionof plutonium in nuclear reactors. It can also be synthesized by bombarding uranium-238with neutrons, resulting in the production of neptunium-239, an isotope of neptunium witha half-life of 2.3565 days.

• 特性

  Neptunium is the first of the subseries of the actinide series known as the transuranic elements—those heavy, synthetic (man-made) radioactive elements that have an atomic numbergreater than uranium in the actinide series of the periodic table. An interesting fact is thatneptunium was artificially synthesized before small traces of it were discovered in nature. Moreis produced by scientists every year than exists in nature.
  Neptunium has an affinity for combining with nonmetals (as do all transuranic elements)such as oxygen, the halogens, sulfur, and carbon.

• 来歴

  Neptunium was the first synthetic transuranium element of the actinide series discovered; the isotope ²³⁹Np was produced by McMillan and Abelson in 1940 at Berkeley, California, as the result of bombarding uranium with cyclotron-produced neutrons. The isotope ²³⁷Np (half-life of 2.14 × 106 years) is currently obtained in gram quantities as a by-product from nuclear reactors in the production of plutonium. Twenty-three isotopes and isomers of neptunium are now recognized. Trace quantities of the element are actually found in nature due to transmutation reactions in uranium ores produced by the neutrons which are present. Neptunium is prepared by the reduction of NpF3 with barium or lithium vapor at about 1200°C. Neptunium metal has a silvery appearance, is chemically reactive, and exists in at least three structural modifications: α-neptunium, orthorhombic, density 20.25 g/cm3, β-neptunium (above 280°C), tetragonal, density (313°C) 19.36 g/cm3; γ-neptunium (above 577°C), cubic, density (600°C) 18.0 g/cm3. Neptunium has four ionic oxidation states in solution: Np+3 (pale purple), analogous to the rare earth ion Pm+3, Np+4 (yellow green); NpO+ (green blue); and NpO++ (pale pink). These latter oxygenated species are in contrast to the rare earths that exhibit only simple ions of the (II), (III), and (IV) oxidation states in aqueous solution. The element forms triand tetrahalides such as NpF3, NpF4, NpCl4, NpBr3, NpI3, and oxides of various compositions such as are found in the uranium-oxygen system, including Np₃O₈8 and NpO2.

• 使用

  The most important radioactive isotope of neptunium is Neptunium-237, with a half-lifeof 2.144×10⁺⁶years, or about 2.1 million years, and decays into protactinium-233 throughalpha decay. Neptunium’s most important use is in nuclear research and for instrumentsdesigned to detect neutrons.

• 定義

  A radioactive transuranic element having atomic number 93, first formed by bombarding uranium with high-speed deuterons aw 237.0482, valences of 3, 4, 5, 6; d 20.45. Neptunium-237, the longest-lived of the 11 isotopes, has been found naturally in extremely small amounts in uranium ores. It is produced in weighable amounts as a by-product in the production of plutonium.

• 調製方法

  Neptunium-237 is obtained as a by-product of making plutonium from uranium isotopes in nuclear reactors. Significant amounts of this element may be recovered from plutonium plant nuclear wastes. Both the recovery and purification of neptunium can be carried out by various chemical processes, including precipitation, solvent extraction and ion exchange.
  Neptunium-237 may be synthesized by bombarding uranium-235 or uranium-238 with neutrons:
  
  Neptunium-239 may be obtained from uranium-238 by neutron bombardment as it was first produced:
  
  Neptunium may be prepared in the metallic state by the reduction of its trifluoride with barium vapor at 1,200°C followed by rapid cooling. Its tetrafluoride may be reduced with excess calcium metal at about 750°C under argon atmosphere.

• 危険性

  All isotopes of neptunium are highly radioactive and are hazardous and thus need to becarefully used in controlled laboratory settings. These isotopes as well as neptunium’s compoundsare radioactive poisons.

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