Polonium
Polonium is a chemical element; it has symbol Po and atomic number 84. A rare and highly radioactive metal (although sometimes classified as a metalloid) with no stable isotopes, polonium is a chalcogen and 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 longer-lived isotopes exist, such as the 124 years half-life of polonium-209, 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.
Not to be confused with Plutonium.Polonium
α, β
silvery
[209]
84
[Xe] 4f14 5d10 6s2 6p4
2, 8, 18, 32, 18, 6
527 K (254 °C, 489 °F)
1235 K (962 °C, 1764 °F)
α-Po: 9.196 g/cm3
β-Po: 9.398 g/cm3
ca. 13 kJ/mol
102.91 kJ/mol
26.4 J/(mol·K)
−2, +2, +4, +5,[1] +6 (an amphoteric oxide)
Pauling scale: 2.0
- 1st: 812.1 kJ/mol
empirical: 168 pm
140±4 pm
197 pm
23.5 µm/(m⋅K) (at 25 °C)
20 W/(m⋅K) (?)
α-Po: 0.40 µΩ⋅m (at 0 °C)
nonmagnetic
7440-08-6
after Polonia, Latin for Poland, homeland of Marie Curie
Pierre and Marie Curie (1898)
Willy Marckwald (1902)
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Polonium was discovered on July 18, 1898 by Marie Skłodowska-Curie and Pierre Curie, when it was extracted from the uranium ore pitchblende[3] and identified solely by its strong radioactivity: it was the first element to be discovered in this way.[4] 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 e.g. poisoning of Alexander Litvinenko. It is extremely dangerous to humans.
History[edit]
Tentatively called "radium F", polonium was discovered by Marie and Pierre Curie in July 1898,[35][36] and was named after Marie Curie's native land of Poland (Latin: Polonia).[37][38] 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. Polonium may be the first element named to highlight a political controversy.[39]
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.[17][35][40] 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.[41][42]
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.[43] '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.[43]
Much of the basic physics of polonium was classified until after the war. The fact that a polonium-beryllium (Po-Be) initiator was used in the gun-type nuclear weapons was classified until the 1960s.[44]
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.[45][46][47]
Occurrence and production[edit]
Polonium is a very rare element in nature because of the short half-lives of all its isotopes. Nine isotopes, from 210 to 218 inclusive, occur in traces as decay products: 210Po, 214Po, and 218Po occur in the decay chain of 238U; 211Po and 215Po occur in the decay chain of 235U; 212Po and 216Po occur in the decay chain of 232Th; and 213Po and 217Po occur in the decay chain of 237Np. (No primordial 237Np survives, but traces of it are continuously regenerated through (n,2n) knockout reactions in natural 238U.)[48] Of these, 210Po is the only isotope with a half-life longer than 3 minutes.[49]
Polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 1010),[50][51] 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.[52][53][54]
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.[55] Polonium is now usually obtained by irradiating bismuth with high-energy neutrons or protons.[17][56]
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. By irradiating certain bismuth salts containing light element nuclei such as beryllium, a cascading (α,n) reaction can also be induced to produce 210Po in large quantities.[57] The final purification is done pyrochemically followed by liquid-liquid extraction techniques.[58] Polonium may now be made in milligram amounts in this procedure which uses high neutron fluxes found in nuclear reactors.[56] Only about 100 grams are produced each year, practically all of it in Russia, making polonium exceedingly rare.[59][60]
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.[61][62]
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.[63]
Applications[edit]
Polonium-based sources of alpha particles were produced in the former Soviet Union.[64] Such sources were applied for measuring the thickness of industrial coatings via attenuation of alpha radiation.[65]
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.[5][17][66][67] 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).[64][68]
The alpha particles emitted by polonium can be converted to neutrons using beryllium oxide, at a rate of 93 neutrons per million alpha particles.[66] Po-BeO mixtures are used as passive neutron sources with a gamma-ray-to-neutron production ratio of 1.13 ± 0.05, lower than for nuclear fission-based neutron sources.[69] Examples of Po-BeO mixtures or alloys used as neutron sources are a neutron trigger or initiator for nuclear weapons[17][70] and for inspections of oil wells. About 1500 sources of this type, with an individual activity of 1,850 Ci (68 TBq), had been used annually in the Soviet Union.[71]
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.[72] Alpha particles emitted by polonium ionize air molecules that neutralize charges on the nearby surfaces.[73][74] Some anti-static brushes contain up to 500 microcuries (20 MBq) of 210Po as a source of charged particles for neutralizing static electricity.[75] 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",[76] 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.[5]
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.
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.)[77][78]
Biology and toxicity[edit]
Overview[edit]
Polonium can be hazardous and has no biological role.[17] 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[79]). 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 and do not come near the eyes, which are living tissue. 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.[80]
Polonium does not have toxic chemical properties.[81]
It has been reported that some microbes can methylate polonium by the action of methylcobalamin.[82][83] 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.[84]
Acute effects[edit]
The median lethal dose (LD50) for acute radiation exposure is about 4.5 Sv.[85] The committed effective dose equivalent 210Po is 0.51 μSv/Bq if ingested, and 2.5 μSv/Bq if inhaled.[86] 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[87]) 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.[88][89] For comparison, one grain of table salt is about 0.06 mg = 60 μg.[90]
Long term (chronic) effects[edit]
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.[85] 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[91] of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.[92] Tobacco smoking causes additional exposure to polonium.[93]
Regulatory exposure limits and handling[edit]
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).[94] The target organs for polonium in humans are the spleen and liver.[95] 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.[96][97] 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)."[98] 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.
Polonium and its compounds must be handled with caution inside special alpha glove boxes, equipped with HEPA filters and continuously maintained under depression to prevent the radioactive materials from leaking out. Gloves made of natural rubber (latex) do not properly withstand chemical attacks, a.o. by concentrated nitric acid (e.g., 6 M HNO3) commonly used to keep polonium in solution while minimizing its sorption onto glass. They do not provide sufficient protection against the contamination from polonium (diffusion of 210Po solution through the intact latex membrane, or worse, direct contact through tiny holes and cracks produced when the latex begins to suffer degradation by acids or UV from ambient light); additional surgical gloves are necessary (inside the glovebox to protect the main gloves when handling strong acids and bases, and also from outside to protect the operator hands against 210Po contamination from diffusion, or direct contact through glove defects). Chemically more resistant, and also denser, neoprene and butyl gloves shield alpha particles emitted by polonium better than natural rubber.[99] The use of natural rubber gloves is not recommended for handling 210Po solutions.
Cases of poisoning[edit]
Despite the element's highly hazardous properties, circumstances in which polonium poisoning can occur are rare. Its extreme scarcity in nature, the short half-lives of all its isotopes, the specialised facilities and equipment needed to obtain any significant quantity, and safety precautions against laboratory accidents all make harmful exposure events unlikely. As such, only a handful of cases of radiation poisoning specifically attributable to polonium exposure have been confirmed.
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