Polonium

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Polonium,  84Po
Polonium.jpg
Polonium
Pronunciation /pəˈlniəm/ (pə-LOH-nee-əm)
Allotropes α, β
Appearancesilvery
Mass number 209(most stable isotope)
Polonium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Te

Po

Lv
bismuthpoloniumastatine
Atomic number (Z)84
Group group 16 (chalcogens)
Period period 6
Block p-block
Element category   Post-transition metal, but this status is disputed
Electron configuration [ Xe ] 4f14 5d10 6s2 6p4
Electrons per shell
2, 8, 18, 32, 18, 6
Physical properties
Phase at  STP solid
Melting point 527  K (254 °C,489 °F)
Boiling point 1235 K(962 °C,1764 °F)
Density (near r.t.)alpha: 9.196 g/cm3
beta: 9.398 g/cm3
Heat of fusion ca. 13  kJ/mol
Heat of vaporization 102.91 kJ/mol
Molar heat capacity 26.4 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)(846)10031236
Atomic properties
Oxidation states −2 , +2, +4, +5, [1] +6 (an  amphoteric oxide)
Electronegativity Pauling scale: 2.0
Ionization energies
  • 1st: 812.1 kJ/mol
Atomic radius empirical:168  pm
Covalent radius 140±4 pm
Van der Waals radius 197 pm
Color lines in a spectral range Polonium spectrum visible.png
Color lines in a spectral range
Spectral lines of polonium
Other properties
Natural occurrence from decay
Crystal structure cubic
Cubic.svg

α-Po
Crystal structure rhombohedral
Rhombohedral.svg

β-Po
Thermal expansion 23.5 µm/(m·K)(at 25 °C)
Thermal conductivity 20 W/(m·K)(?)
Electrical resistivity α: 0.40 µΩ·m(at 0 °C)
Magnetic ordering nonmagnetic
CAS Number 7440-08-6
History
Namingafter Polonia, Latin for Poland, homeland of Marie Curie
Discovery Pierre and Marie Curie (1898)
First isolation Willy Marckwald (1902)
Main isotopes of polonium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
208Po syn 2.898 y α 204Pb
β+ 208Bi
209Posyn125.2 y [2] α 205Pb
β+ 209Bi
210Po trace 138.376 dα 206Pb
| references

Polonium is a chemical element with the symbol Po and atomic number 84. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character resembles that of its horizontal neighbors in the periodic table: thallium, lead, and bismuth. Due to the short half-life of all its isotopes, its natural occurrence is limited to tiny traces of the fleeting polonium-210 (with a half-life of 138 days) in uranium ores, as it is the penultimate daughter of natural uranium-238. Though slightly longer-lived isotopes exist, they are much more difficult to produce. Today, polonium is usually produced in milligram quantities by the neutron irradiation of bismuth. Due to its intense radioactivity, which results in the radiolysis of chemical bonds and radioactive self-heating, its chemistry has mostly been investigated on the trace scale only.

Chemical element a species of atoms having the same number of protons in the atomic nucleus

A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons.

Symbol (chemistry) an arbitrary or conventional sign used in chemical science to represent a chemical element

In chemistry, a symbol is an abbreviation for a chemical element. Symbols for chemical elements normally consist of one or two letters from the Latin alphabet and are written with the first letter capitalised.

Atomic number number of protons found in the nucleus of an atom

The atomic number or proton number of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element. It is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is also equal to the number of electrons.

Contents

Polonium was discovered in 1898 by Marie and Pierre Curie, when it was extracted from the uranium ore pitchblende and identified solely by its strong radioactivity: it was the first element to be so discovered. Polonium was named after Marie Curie's homeland of Poland. Polonium has few applications, and those are related to its radioactivity: heaters in space probes, antistatic devices, sources of neutrons and alpha particles, and poison. It is a radioactive element, and extremely dangerous to humans.

Marie Curie Polish-French physicist and chemist

Marie Skłodowska Curie was a Polish and naturalized-French physicist and chemist who conducted pioneering research on radioactivity. She was the first woman to win a Nobel Prize, is the only woman to win the Nobel prize twice, and is the only person to win the Nobel Prize in two different scientific fields. She was part of the Curie family legacy of five Nobel Prizes. She was also the first woman to become a professor at the University of Paris, and in 1995 became the first woman to be entombed on her own merits in the Panthéon in Paris.

Pierre Curie French physicist

Pierre Curie was a French physicist, a pioneer in crystallography, magnetism, piezoelectricity, and radioactivity. In 1903, he received the Nobel Prize in Physics with his wife, Marie Skłodowska-Curie, and Henri Becquerel, "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel".

Poland Republic in Central Europe

Poland, officially the Republic of Poland, is a country located in Central Europe. It is divided into 16 administrative subdivisions, covering an area of 312,696 square kilometres (120,733 sq mi), and has a largely temperate seasonal climate. With a population of approximately 38.5 million people, Poland is the sixth most populous member state of the European Union. Poland's capital and largest metropolis is Warsaw. Other major cities include Kraków, Łódź, Wrocław, Poznań, Gdańsk, and Szczecin.

Characteristics

210Po is an alpha emitter that has a half-life of 138.4 days; it decays directly to its stable daughter isotope, 206Pb. A milligram (5  curies) of 210Po emits about as many alpha particles per second as 5 grams of 226Ra. [3] A few curies (1 curie equals 37  gigabecquerels, 1 Ci = 37 GBq) of 210Po emit a blue glow which is caused by ionisation of the surrounding air.

Alpha decay emission of alpha particles by a decaying radioactive atom

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 u. For example, uranium-238 decays to form thorium-234. Alpha particles have a charge +2 e, but as a nuclear equation describes a nuclear reaction without considering the electrons – a convention that does not imply that the nuclei necessarily occur in neutral atoms – the charge is not usually shown.

Decay product

In nuclear physics, a decay product is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps. For example, 238U decays to 234Th which decays to 234mPa which decays, and so on, to 206Pb :

Lead Chemical element with atomic number 82

Lead is a chemical element with the symbol Pb and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, lead is silvery with a hint of blue; it tarnishes to a dull gray color when exposed to air. Lead has the highest atomic number of any stable element and three of its isotopes are endpoints of major nuclear decay chains of heavier elements.

About one in 100,000 alpha emissions causes an excitation in the nucleus which then results in the emission of a gamma ray with a maximum energy of 803 keV. [4] [5]

Solid state form

The alpha form of solid polonium. Alpha po lattice.jpg
The alpha form of solid polonium.

Polonium is a radioactive element that exists in two metallic allotropes. The alpha form is the only known example of a simple cubic crystal structure in a single atom basis at STP, with an edge length of 335.2 picometers; the beta form is rhombohedral. [6] [7] [8] The structure of polonium has been characterized by X-ray diffraction [9] [10] and electron diffraction. [11]

Metal element, compound, or alloy that is a good conductor of both electricity and heat

A metal is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typically malleable or ductile. A metal may be a chemical element such as iron; an alloy such as stainless steel; or a molecular compound such as polymeric sulfur nitride.

Standard conditions for temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

X-ray Röntgen radiation

X-rays make up X-radiation, a form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. In many languages, X-radiation is referred to as Röntgen radiation, after the German scientist Wilhelm Röntgen, who discovered it on November 8, 1895. He named it X-radiation to signify an unknown type of radiation. Spelling of X-ray(s) in the English language includes the variants x-ray(s), xray(s), and X ray(s).

210Po (in common with 238Pu [ citation needed ]) has the ability to become airborne with ease: if a sample is heated in air to 55 °C (131 °F), 50% of it is vaporized in 45 hours to form diatomic Po2 molecules, even though the melting point of polonium is 254 °C (489 °F) and its boiling point is 962 °C (1,764 °F). [12] [13] [1] More than one hypothesis exists for how polonium does this; one suggestion is that small clusters of polonium atoms are spalled off by the alpha decay.

Plutonium-238 isotope of plutonium

Plutonium-238 is a radioactive isotope of plutonium that has a half-life of 87.7 years.

Volatility (chemistry) Tendency of a substance to vaporize

In chemistry, volatility is a material quality which describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapor, while a substance with low volatility is more likely to be a liquid or solid. Volatility can also describe the tendency of a vapor to condense into a liquid or solid; less volatile substances will more readily condense from a vapor than highly volatile ones. Differences in volatility can be observed by comparing how fast a group of substances evaporate when exposed to the atmosphere. A highly volatile substance such as rubbing alcohol will quickly evaporate, while a substance with low volatility such as vegetable oil will remain condensed. In general, solids are much less volatile than liquids, but there are some exceptions. Solids that sublime such as dry ice or iodine can vaporize at a similar rate as some liquids under standard conditions.

Chemistry

The chemistry of polonium is similar to that of tellurium, although it also shows some similarities to its neighbor bismuth due to its metallic character. Polonium dissolves readily in dilute acids but is only slightly soluble in alkalis. Polonium solutions are first colored in pink by the Po2+ ions, but then rapidly become yellow because alpha radiation from polonium ionizes the solvent and converts Po2+ into Po4+. This process is accompanied by bubbling and emission of heat and light by glassware due to the absorbed alpha particles; as a result, polonium solutions are volatile and will evaporate within days unless sealed. [14] [15] At pH about 1, polonium ions are readily hydrolyzed and complexed by acids such as oxalic acid, citric acid, and tartaric acid. [16]

Compounds

Polonium has no common compounds, and almost all of its compounds are synthetically created; more than 50 of those are known. [17] The most stable class of polonium compounds are polonides, which are prepared by direct reaction of two elements. Na2Po has the antifluorite structure, the polonides of Ca, Ba, Hg, Pb and lanthanides form a NaCl lattice, BePo and CdPo have the wurtzite and MgPo the nickel arsenide structure. Most polonides decompose upon heating to about 600 °C, except for HgPo that decomposes at ~300 °C and the lanthanide polonides, which do not decompose but melt at temperatures above 1000 °C. For example, PrPo melts at 1250 °C and TmPo at 2200 °C. [18] PbPo is one of the very few naturally occurring polonium compounds, as polonium alpha decays to form lead. [19]

Polonium hydride (PoH
2
) is a volatile liquid at room temperature prone to dissociation; it is thermally unstable. [18] Water is the only other known hydrogen chalcogenide which is a liquid at room temperature; however, this is due to hydrogen bonding. The two oxides PoO2 and PoO3 are the products of oxidation of polonium. [20]

Halides of the structure PoX2, PoX4 and PoF6 are known. They are soluble in the corresponding hydrogen halides, i.e., PoClX in HCl, PoBrX in HBr and PoI4 in HI. [21] Polonium dihalides are formed by direct reaction of the elements or by reduction of PoCl4 with SO2 and with PoBr4 with H2S at room temperature. Tetrahalides can be obtained by reacting polonium dioxide with HCl, HBr or HI. [22]

Other polonium compounds include potassium polonite as a polonite, polonate, acetate, bromate, carbonate, citrate, chromate, cyanide, formate, (II) and (IV) hydroxides, nitrate, selenate, selenite, monosulfide, sulfate, disulfate and sulfite. [21] [23]

Polonium compounds [22] [24]
FormulaColor m.p. (°C) Sublimation
temp. (°C)
Symmetry Pearson symbol Space group Noa (pm)b(pm)c(pm)Z ρ (g/cm3)ref
PoO2 pale yellow500 (dec.)885 fcc cF12Fm3m225563.7563.7563.748.94 [25]
PoCl2 dark ruby red355130 orthorhombic oP3Pmmm4736743545016.47 [26]
PoBr2 purple-brown270 (dec.) [27]
PoCl4 yellow300200 monoclinic [26]
PoBr4 red330 (dec.) fcc cF100Fm3m2255605605604 [27]
PoI4 black [28]

Isotopes

Polonium has 42 known isotopes, all of which are radioactive. They have atomic masses that range from 186 to 227 u. 210Po (half-life 138.376 days) is the most widely available and is made via neutron capture by natural bismuth. The longer-lived 209Po (half-life 125.2±3.3 years, longest-lived of all polonium isotopes) [2] and 208Po (half-life 2.9 years) can be made through the alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron. [29]

History

Tentatively called "radium F", polonium was discovered by Marie and Pierre Curie in 1898, [30] [31] and was named after Marie Curie's native land of Poland (Latin : Polonia). [32] [33] Poland at the time was under Russian, German, and Austro-Hungarian partition, and did not exist as an independent country. It was Curie's hope that naming the element after her native land would publicize its lack of independence. [34] Polonium may be the first element named to highlight a political controversy. [34]

This element was the first one discovered by the Curies while they were investigating the cause of pitchblende radioactivity. Pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than the uranium and thorium combined. This spurred the Curies to search for additional radioactive elements. They first separated out polonium from pitchblende in July 1898, and five months later, also isolated radium. [14] [30] [35] German scientist Willy Marckwald successfully isolated 3 milligrams of polonium in 1902, though at the time he believed it was a new element, which he dubbed "radio-tellurium", and it was not until 1905 that it was demonstrated to be the same as polonium. [36] [37]

In the United States, polonium was produced as part of the Manhattan Project's Dayton Project during World War II. Polonium and beryllium were the key ingredients of the 'Urchin' initiator at the center of the bomb's spherical pit. [38] 'Urchin' initiated the nuclear chain reaction at the moment of prompt-criticality to ensure that the weapon did not fizzle. 'Urchin' was used in early U.S. weapons; subsequent U.S. weapons utilized a pulse neutron generator for the same purpose. [38]

Much of the basic physics of polonium was classified until after the war. The fact that it was used as an initiator was classified until the 1960s. [39]

The Atomic Energy Commission and the Manhattan Project funded human experiments using polonium on five people at the University of Rochester between 1943 and 1947. The people were administered between 9 and 22 microcuries (330 and 810  kBq ) of polonium to study its excretion. [40] [41] [42]

Occurrence and production

Polonium is a very rare element in nature because of the short half-life of all its isotopes. 210Po, 214Po, and 218Po appear in the decay chain of 238U; thus polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 1010), [43] [44] which is approximately 0.2% of the abundance of radium. The amounts in the Earth's crust are not harmful. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers. [45] [46] [47]

Because it is present in small concentrations, isolation of polonium from natural sources is a tedious process. The largest batch of the element ever extracted, performed in the first half of the 20th century, contained only 40 Ci (1.5 TBq) (9 mg) of polonium-210 and was obtained by processing 37 tonnes of residues from radium production. [48] Polonium is now usually obtained by irradiating bismuth with high-energy neutrons or protons. [14] [49]

In 1934, an experiment showed that when natural 209Bi is bombarded with neutrons, 210Bi is created, which then decays to 210Po via beta-minus decay. The final purification is done pyrochemically followed by liquid-liquid extraction techniques. [50] Polonium may now be made in milligram amounts in this procedure which uses high neutron fluxes found in nuclear reactors. [49] Only about 100 grams are produced each year, practically all of it in Russia, making polonium exceedingly rare. [51] [52]

This process can cause problems in lead-bismuth based liquid metal cooled nuclear reactors such as those used in the Soviet Navy's K-27. Measures must be taken in these reactors to deal with the unwanted possibility of 210Po being released from the coolant. [53] [54]

The longer-lived isotopes of polonium, 208Po and 209Po, can be formed by proton or deuteron bombardment of bismuth using a cyclotron. Other more neutron-deficient and more unstable isotopes can be formed by the irradiation of platinum with carbon nuclei. [55]

Applications

Polonium-based sources of alpha particles were produced in the former Soviet Union. [56] Such sources were applied for measuring the thickness of industrial coatings via attenuation of alpha radiation. [57]

Because of intense alpha radiation, a one-gram sample of 210Po will spontaneously heat up to above 500 °C (932 °F) generating about 140 watts of power. Therefore, 210Po is used as an atomic heat source to power radioisotope thermoelectric generators via thermoelectric materials. [3] [14] [58] [59] For example, 210Po heat sources were used in the Lunokhod 1 (1970) and Lunokhod 2 (1973) Moon rovers to keep their internal components warm during the lunar nights, as well as the Kosmos 84 and 90 satellites (1965). [56] [60]

The alpha particles emitted by polonium can be converted to neutrons using beryllium oxide, at a rate of 93 neutrons per million alpha particles. [58] Thus Po-BeO mixtures or alloys are used as a neutron source, for example, in a neutron trigger or initiator for nuclear weapons [14] [61] and for inspections of oil wells. About 1500 sources of this type, with an individual activity of 1,850 Ci (68 TBq), have been used annually in the Soviet Union. [62]

Polonium was also part of brushes or more complex tools that eliminate static charges in photographic plates, textile mills, paper rolls, sheet plastics, and on substrates (such as automotive) prior to the application of coatings. [63] Alpha particles emitted by polonium ionize air molecules that neutralize charges on the nearby surfaces. [64] [65] Some anti-static brushes contain up to 500 microcuries (20 MBq) of 210Po as a source of charged particles for neutralizing static electricity. [66] In the US, devices with no more than 500 μCi (19 MBq) of (sealed) 210Po per unit can be bought in any amount under a "general license", [67] which means that a buyer need not be registered by any authorities. Polonium needs to be replaced in these devices nearly every year because of its short half-life; it is also highly radioactive and therefore has been mostly replaced by less dangerous beta particle sources. [3]

Tiny amounts of 210Po are sometimes used in the laboratory and for teaching purposes—typically of the order of 4–40 kBq (0.11–1.08 μCi), in the form of sealed sources, with the polonium deposited on a substrate or in a resin or polymer matrix—are often exempt from licensing by the NRC and similar authorities as they are not considered hazardous. Small amounts of 210Po are manufactured for sale to the public in the United States as 'needle sources' for laboratory experimentation, and they are retailed by scientific supply companies. The polonium is a layer of plating which in turn is plated with a material such as gold, which allows the alpha radiation (used in experiments such as cloud chambers) to pass while preventing the polonium from being released and presenting a toxic hazard. According to United Nuclear, they typically sell between four and eight such sources per year. [68] [69]

Polonium spark plugs were marketed by Firestone from 1940 to 1953. While the amount of radiation from the plugs was minuscule and not a threat to the consumer, the benefits of such plugs quickly diminished after approximately a month because of polonium's short half-life and because buildup on the conductors would block the radiation that improved engine performance. (The premise behind the polonium spark plug, as well as Alfred Matthew Hubbard's prototype radium plug that preceded it, was that the radiation would improve ionization of the fuel in the cylinder and thus allow the motor to fire more quickly and efficiently.) [70] [71]

Biology and toxicity

Overview

Polonium is highly dangerous and has no biological role. [14] By mass, polonium-210 is around 250,000 times more toxic than hydrogen cyanide (the LD50 for 210Po is less than 1 microgram for an average adult (see below) compared with about 250 milligrams for hydrogen cyanide [72] ). The main hazard is its intense radioactivity (as an alpha emitter), which makes it difficult to handle safely. Even in microgram amounts, handling 210Po is extremely dangerous, requiring specialized equipment (a negative pressure alpha glove box equipped with high-performance filters), adequate monitoring, and strict handling procedures to avoid any contamination. Alpha particles emitted by polonium will damage organic tissue easily if polonium is ingested, inhaled, or absorbed, although they do not penetrate the epidermis and hence are not hazardous as long as the alpha particles remain outside the body. Wearing chemically resistant and intact gloves is a mandatory precaution to avoid transcutaneous diffusion of polonium directly through the skin. Polonium delivered in concentrated nitric acid can easily diffuse through inadequate gloves (e.g., latex gloves) or the acid may damage the gloves. [73]

Polonium does not have toxic chemical properties. [74]

It has been reported that some microbes can methylate polonium by the action of methylcobalamin. [75] [76] This is similar to the way in which mercury, selenium, and tellurium are methylated in living things to create organometallic compounds. Studies investigating the metabolism of polonium-210 in rats have shown that only 0.002 to 0.009% of polonium-210 ingested is excreted as volatile polonium-210. [77]

Acute effects

The median lethal dose (LD50) for acute radiation exposure is about 4.5  Sv. [78] The committed effective dose equivalent 210Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if inhaled. [79] So a fatal 4.5 Sv dose can be caused by ingesting 8.8 MBq (240 μCi), about 50 nanograms (ng), or inhaling 1.8 MBq (49 μCi), about 10 ng. One gram of 210Po could thus in theory poison 20 million people of whom 10 million would die. The actual toxicity of 210Po is lower than these estimates because radiation exposure that is spread out over several weeks (the biological half-life of polonium in humans is 30 to 50 days [80] ) is somewhat less damaging than an instantaneous dose. It has been estimated that a median lethal dose of 210Po is 15 megabecquerels (0.41 mCi), or 0.089 micrograms (μg), still an extremely small amount. [81] [82] For comparison, one grain of table salt is about 0.06 mg = 60 μg. [83]

Long term (chronic) effects

In addition to the acute effects, radiation exposure (both internal and external) carries a long-term risk of death from cancer of 5–10% per Sv. [78] The general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes 214Po and 218Po are thought to cause the majority [84] of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon. [85] Tobacco smoking causes additional exposure to polonium. [86]

Regulatory exposure limits and handling

The maximum allowable body burden for ingested 210Po is only 1.1 kBq (30 nCi), which is equivalent to a particle massing only 6.8 picograms. The maximum permissible workplace concentration of airborne 210Po is about 10 Bq/m3 (3×10−10 µCi/cm3). [87] The target organs for polonium in humans are the spleen and liver. [88] As the spleen (150 g) and the liver (1.3 to 3 kg) are much smaller than the rest of the body, if the polonium is concentrated in these vital organs, it is a greater threat to life than the dose which would be suffered (on average) by the whole body if it were spread evenly throughout the body, in the same way as caesium or tritium (as T2O).

210Po is widely used in industry, and readily available with little regulation or restriction.[ citation needed ] [89] In the US, a tracking system run by the Nuclear Regulatory Commission was implemented in 2007 to register purchases of more than 16 curies (590 GBq) of polonium-210 (enough to make up 5,000 lethal doses). The IAEA "is said to be considering tighter regulations ... There is talk that it might tighten the polonium reporting requirement by a factor of 10, to 1.6 curies (59 GBq)." [90] As of 2013, this is still the only alpha emitting byproduct material available, as a NRC Exempt Quantity, which may be held without a radioactive material license.[ citation needed ]

Polonium and its compounds must be handled in a glove box, which is further enclosed in another box, maintained at a slightly higher pressure than the glove box to prevent the radioactive materials from leaking out. Gloves made of natural rubber do not provide sufficient protection against the radiation from polonium; surgical gloves are necessary. Neoprene gloves shield radiation from polonium better than natural rubber. [91]

Cases of poisoning

20th century

Polonium was administered to humans for experimental purposes from 1943 to 1947; it was injected into four hospitalised patients, and orally given to a fifth. Studies such as this were funded by the Manhattan Project and the AEC and conducted at the University of Rochester. The objective was to obtain data on human excretion of polonium to correlate with more extensive data from rats. Patients selected as subjects were chosen because experimenters wanted persons who had not been exposed to polonium either through work or accident. All subjects had incurable diseases. Excretion of polonium was followed, and an autopsy was conducted at that time on the deceased patient to determine which organs absorbed the polonium. Patients' ages ranged from "early thirties" to "early forties". The experiments were described in Chapter 3 of Biological Studies with Polonium, Radium, and Plutonium, National Nuclear Energy Series, Volume VI-3, McGraw-Hill, New York, 1950. Not specified is the isotope under study, but at the time polonium-210 was the most readily available polonium isotope. The DoE factsheet submitted for this experiment reported no follow up on these subjects. [92]

It has also been suggested that Irène Joliot-Curie was the first person to die from the radiation effects of polonium. She was accidentally exposed to polonium in 1946 when a sealed capsule of the element exploded on her laboratory bench. In 1956, she died from leukemia. [93]

According to the 2008 book The Bomb in the Basement, several deaths in Israel during 1957–1969 were caused by 210Po. [94] A leak was discovered at a Weizmann Institute laboratory in 1957. Traces of 210Po were found on the hands of Professor Dror Sadeh, a physicist who researched radioactive materials. Medical tests indicated no harm, but the tests did not include bone marrow. Sadeh died from cancer. One of his students died of leukemia, and two colleagues died after a few years, both from cancer. The issue was investigated secretly, and there was never any formal admission that a connection between the leak and the deaths had existed. [95]

21st century

The cause of death in the 2006 homicide of Alexander Litvinenko, a Russian KGB agent who had defected to the British MI6 intelligence agency, was determined to be 210Po poisoning. [96] [97] According to Prof. Nick Priest of Middlesex University, an environmental toxicologist and radiation expert, speaking on Sky News on December 3, 2006, Litvinenko was probably the first person to die of the acute α-radiation effects of 210Po. [98]

Abnormally high concentrations of 210Po were detected in July 2012 in clothes and personal belongings of the Palestinian leader Yasser Arafat, a heavy smoker, who died on 11 November 2004 of uncertain causes. The spokesman for the Institut de Radiophysique in Lausanne, Switzerland, where those items were analyzed, stressed that the "clinical symptoms described in Arafat's medical reports were not consistent with polonium-210 and that conclusions could not be drawn as to whether the Palestinian leader was poisoned or not", and that "the only way to confirm the findings would be to exhume Arafat's body to test it for polonium-210." [99] On 27 November 2012 Arafat's body was exhumed, and samples were taken for separate analysis by experts from France, Switzerland and Russia. [100] On 12 October 2013, The Lancet published the group's finding that high levels of the element were found in Arafat's blood, urine, and in saliva stains on his clothes and toothbrush. [101] The French tests later found some polonium but stated it was from "natural environmental origin". [102] Following later Russian tests, Vladimir Uiba, the head of the Russian Federal Medical and Biological Agency, stated in December 2013 that Arafat died of natural causes, and they had no plans to conduct further tests. [102]

Treatment

It has been suggested that chelation agents, such as British Anti-Lewisite (dimercaprol), can be used to decontaminate humans. [103] In one experiment, rats were given a fatal dose of 1.45 MBq/kg (8.7 ng/kg) of 210Po; all untreated rats were dead after 44 days, but 90% of the rats treated with the chelation agent HOEtTTC remained alive for 5 months. [104]

Detection in biological specimens

Polonium-210 may be quantified in biological specimens by alpha particle spectrometry to confirm a diagnosis of poisoning in hospitalized patients or to provide evidence in a medicolegal death investigation. The baseline urinary excretion of polonium-210 in healthy persons due to routine exposure to environmental sources is normally in a range of 5–15 mBq/day. Levels in excess of 30 mBq/day are suggestive of excessive exposure to the radionuclide. [105]

Occurrence in humans and the biosphere

Polonium-210 is widespread in the biosphere, including in human tissues, because of its position in the uranium-238 decay chain. Natural uranium-238 in the Earth's crust decays through a series of solid radioactive intermediates including radium-226 to the radioactive noble gas radon-222, some of which, during its 3.8-day half-life, diffuses into the atmosphere. There it decays through several more steps to polonium-210, much of which, during its 138-day half-life, is washed back down to the Earth's surface, thus entering the biosphere, before finally decaying to stable lead-206. [106] [107] [108]

As early as the 1920s Antoine Lacassagne, using polonium provided by his colleague Marie Curie, showed that the element has a specific pattern of uptake in rabbit tissues, with high concentrations, particularly in liver, kidney, and testes. [109] More recent evidence suggests that this behavior results from polonium substituting for its congener sulfur, also in group 16 of the periodic table, in sulfur-containing amino-acids or related molecules [110] and that similar patterns of distribution occur in human tissues. [111] Polonium is indeed an element naturally present in all humans, contributing appreciably to natural background dose, with wide geographical and cultural variations, and particularly high levels in arctic residents, for example. [112]

Tobacco

Polonium-210 in tobacco contributes to many of the cases of lung cancer worldwide. Most of this polonium is derived from lead-210 deposited on tobacco leaves from the atmosphere; the lead-210 is a product of radon-222 gas, much of which appears to originate from the decay of radium-226 from fertilizers applied to the tobacco soils. [47] [113] [114] [115] [116]

The presence of polonium in tobacco smoke has been known since the early 1960s. [117] [118] Some of the world's biggest tobacco firms researched ways to remove the substance—to no avail—over a 40-year period. The results were never published. [47]

Food

Polonium is found in the food chain, especially in seafood. [119] [120]

See also

Related Research Articles

Actinium Chemical element with atomic number 89

Actinium is a chemical element with the symbol Ac and atomic number 89. It was first isolated by French chemist André-Louis Debierne in 1899. Friedrich Oskar Giesel later independently isolated it in 1902 and, unaware that it was already known, gave it the name emanium. Actinium gave the name to the actinide series, a group of 15 similar elements between actinium and lawrencium in the periodic table. It is also sometimes considered the first of the 7th-period transition metals, although lawrencium is less commonly given that position. Together with polonium, radium, and radon, actinium was one of the first non-primordial radioactive elements to be isolated.

Astatine Chemical element with atomic number 85

Astatine is a radioactive chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours. A sample of the pure element has never been assembled, because any macroscopic specimen would be immediately vaporized by the heat of its own radioactivity.

Radium Chemical element with atomic number 88

Radium is a chemical element with the 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) on exposure to air, forming a black surface layer of radium nitride (Ra3N2). All isotopes of radium are highly radioactive, with the most stable isotope being radium-226, which has a half-life of 1600 years and decays into radon gas (specifically the isotope radon-222). When radium decays, ionizing radiation is a product, which can excite fluorescent chemicals and cause radioluminescence.

A radionuclide is an atom that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay. These emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single element the decay rate, and thus the half-life (t1/2) for that collection can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms have no known limits and span a time range of over 55 orders of magnitude.

Irène Joliot-Curie French scientist

Irène Joliot-Curie was a French scientist, the daughter of Marie Curie and Pierre Curie and the wife of Frédéric Joliot-Curie. Jointly with her husband, Joliot-Curie was awarded the Nobel Prize in Chemistry in 1935 for their discovery of artificial radioactivity. This made the Curies the family with the most Nobel laureates to date. Both children of the Joliot-Curies, Hélène and Pierre, are also esteemed scientists.

Nuclide atomic species characterized by the specific constitution of its nucleus: mass number, atomic number, and, where long-enough-lived to observe, the nuclear energy state

A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state.

Radioactive decay Process by which an unstable atom emits radiation

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion. A material containing unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, proton emission.

Decay chain series of elements in radioactive decay

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". Most radioisotopes do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached.

The slow neutron-capture process, or s-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly AGB stars. The s-process is responsible for the creation (nucleosynthesis) of approximately half the atomic nuclei heavier than iron.

Polonium-210 is an isotope of polonium. It undergoes alpha decay to stable 206Pb with a half-life of 138.376 days, the longest of all naturally occurring polonium isotopes. 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. It is also extremely toxic to humans as a result of its intense radioactivity.

Radiochemistry is the chemistry of radioactive materials, where radioactive isotopes of elements are used to study the properties and chemical reactions of non-radioactive isotopes. Much of radiochemistry deals with the use of radioactivity to study ordinary chemical reactions. This is very different from radiation chemistry where the radiation levels are kept too low to influence the chemistry.

Induced radioactivity, also called artificial radioactivity or man-made radioactivity, is the process of using radiation to make a previously stable material radioactive. The husband and wife team of Irène Joliot-Curie and Frédéric Joliot-Curie discovered induced radioactivity in 1934, and they shared the 1935 Nobel Prize in Chemistry for this discovery.

Bismuth-209 is the isotope of bismuth with the longest known half-life of any radioisotope that undergoes α-decay. It has 83 protons and a magic number of 126 neutrons, and an atomic mass of 208.9803987 amu. Primordial bismuth consists entirely of this isotope.

The decay scheme of a radioactive substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships. Examples are shown below.

Uranium-236 is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

Long-lived fission products (LLFPs) are radioactive materials with a long half-life produced by nuclear fission of uranium and plutonium.

Radon-222 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.

Alpha particle helium-4 nucleus; a particles consisting of two protons and two neutrons bound together

Alpha particles, also called alpha ray 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
2
He2+
indicating a helium ion with a +2 charge. If the ion gains electrons from its environment, the alpha particle becomes a normal helium atom 4
2
He
.

Nuclear transmutation conversion of an atom from one element to another

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Because any element is defined by its number of protons in its atoms, i.e. in the atomic nucleus, nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus is changed.

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