Polonium

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Polonium, 84Po
Polonium
Pronunciation/pəˈlniəm/ (pə-LOH-nee-əm)
Allotropesα, β
Appearancesilvery
Mass number[209]
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
kJ/mol
Heat of vaporization102.91 kJ/mol
Molar heat capacity26.4 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) (846) 1003 1236
Atomic properties
Discovery
Pierre and Marie Curie (1898)
First isolationWilly Marckwald (1902)
Isotopes of polonium
Main isotopes[2] Decay
abun­dance half-life (t1/2) mode pro­duct
208Po synth 2.898 y α
204Pb
β+
208Bi
209Po synth 124 y α
205Pb
β+ 209Bi
210Po trace 138.376 d α
206Pb
 Category: Polonium
| references

Polonium is a

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.

Polonium was discovered on July 18, 1898 by

. It is extremely dangerous to humans.

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] which means it is 5,000 times more radioactive than radium. 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.

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

Polonium is a radioactive element that exists in two

rhombohedral.[6][7][8] The structure of polonium has been characterized by X-ray diffraction[9][10] and electron diffraction.[11]

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.[14]

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+. As polonium also emits alpha-particles after disintegration so 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.[15][16] At pH about 1, polonium ions are readily hydrolyzed and complexed by acids such as oxalic acid, citric acid, and tartaric acid.[17]

Compounds

Polonium has no common compounds, and almost all of its compounds are synthetically created; more than 50 of those are known.

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, the polonide of praseodymium (PrPo) melts at 1250 °C, and that of thulium (TmPo) melts at 2200 °C.[19] PbPo is one of the very few naturally occurring polonium compounds, as polonium alpha decays to form lead.[20]

Polonium hydride (PoH
2
) is a volatile liquid at room temperature prone to dissociation; it is thermally unstable.[19] Water is the only other known hydrogen chalcogenide which is a liquid at room temperature; however, this is due to hydrogen bonding. The three oxides, PoO, PoO2 and PoO3, are the products of oxidation of polonium.[21]

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.[22] 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.[23]

Other polonium compounds include the

disulfate or sulfite salts.[22][24]

A limited

Polonium compounds[23][27]
Formula Color m.p. (°C) Sublimation
temp. (°C)
Symmetry Pearson symbol Space group No a (pm) b(pm) c(pm) Z ρ (g/cm3) ref
PoO black
PoO2 pale yellow 500 (dec.) 885
fcc
cF12 Fm3m 225 563.7 563.7 563.7 4 8.94 [28]
PoH2 -35.5
PoCl2 dark ruby red 355 130
orthorhombic
oP3 Pmmm 47 367 435 450 1 6.47 [29]
PoBr2 purple-brown 270 (dec.) [30]
PoCl4 yellow 300 200
monoclinic
[29]
PoBr4 red 330 (dec.)
fcc
cF100 Fm3m 225 560 560 560 4 [30]
PoI4 black [31]

Isotopes

Polonium has 42 known isotopes, all of which are

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 124 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.[32]

History

Tentatively called "

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.[37]

This element was the first one discovered by the Curies while they were investigating the cause of

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.[15][33][38] 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.[39][40]

In the United States, polonium was produced as part of the

fizzle. 'Urchin' was used in early U.S. weapons; subsequent U.S. weapons utilized a pulse neutron generator for the same purpose.[41]

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.[42]

The

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.[43][44][45]

Occurrence and production

Polonium is a very rare element in nature because of the short

237Np. (No primordial 237Np survives, but traces of it are continuously regenerated through (n,2n) knockout reactions in natural 238U.)[46] Of these, 210Po is the only isotope with a half-life longer than 3 minutes.[47]

Polonium can be found in

metric ton (1 part in 1010),[48][49] 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.[50][51][52]

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.[53] Polonium is now usually obtained by irradiating bismuth with high-energy neutrons or protons.[15][54]

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.[55] The final purification is done pyrochemically followed by liquid-liquid extraction techniques.[56] Polonium may now be made in milligram amounts in this procedure which uses high neutron fluxes found in nuclear reactors.[54] Only about 100 grams are produced each year, practically all of it in Russia, making polonium exceedingly rare.[57][58]

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.[59][60]

The longer-lived isotopes of polonium, 208Po and 209Po, can be formed by

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.[61]

Applications

Polonium-based sources of alpha particles were produced in the former Soviet Union.[62] Such sources were applied for measuring the thickness of industrial coatings via attenuation of alpha radiation.[63]

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

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).[62][66]

The alpha particles emitted by polonium can be converted to neutrons using beryllium oxide, at a rate of 93 neutrons per million alpha particles.

neutron trigger or initiator for nuclear weapons[15][68] 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.[69]

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.[70] Alpha particles emitted by polonium ionize air molecules that neutralize charges on the nearby surfaces.[71][72] Some anti-static brushes contain up to 500 microcuries (20 MBq) of 210Po as a source of charged particles for neutralizing static electricity.[73] 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",[74] 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.[citation needed
]

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.)[75][76]

Biology and toxicity

Overview

Polonium can be hazardous and has no biological role.

latex gloves) or the acid may damage the gloves.[78]

Polonium does not have toxic chemical properties.[79]

It has been reported that some

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.[82]

Acute effects

The

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[85]) 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.[86][87] For comparison, one grain of table salt is about 0.06 mg = 60 μg.[88]

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.

Tobacco smoking causes additional exposure to polonium.[91]

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).[92] The target organs for polonium in humans are the spleen and liver.[93] 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).[citation needed]

210Po is widely used in industry, and readily available with little regulation or restriction.[94]}[95] 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)."[96] 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 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.[97]
The use of natural rubber gloves is not recommended for handling 210Po solutions.

Cases of poisoning

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.[citation needed]

20th century

In response to concerns about the risks of occupational polonium exposure, quantities of 210Po were administered to five human volunteers at the University of Rochester from 1944 to 1947, in order to study its biological behaviour. These studies were funded by the Manhattan Project and the AEC. Four men and a woman participated, all suffering from terminal cancers, and ranged in age from their early thirties to early forties; all were chosen because experimenters wanted subjects who had not been exposed to polonium either through work or accident.[98] 210Po was injected into four hospitalised patients, and orally given to a fifth. None of the administered doses (all ranging from 0.17 to 0.30 μCi kg−1) approached fatal quantities.[99][98]

The first documented death directly resulting from polonium poisoning occurred in the Soviet Union, on 10 July 1954.[100][101] An unidentified 41-year-old man presented for medical treatment on 29 June, with severe vomiting and fever; the previous day, he had been working for five hours in an area in which, unknown to him, a capsule containing 210Po had depressurised and begun to disperse in aerosol form. Over this period, his total intake of airborne 210Po was estimated at 0.11 GBq (almost 25 times the estimated LD50 by inhalation of 4.5 MBq). Despite treatment, his condition continued to worsen and he died 13 days after the exposure event.[100]

From 1955 to 1957 the Windscale Piles had been releasing Polonium-210. The Windscale fire brought the need for testing of the land downwind for radioactive material contamination, and this is how it was found. An estimate of 8.8 terabecquerels (240 Ci) of polonium-210 has been made.

It has also been suggested that Irène Joliot-Curie's 1956 death from leukaemia was owed to the radiation effects of polonium. She was accidentally exposed in 1946 when a sealed capsule of the element exploded on her laboratory bench.[102]

As well, several deaths in Israel during 1957–1969 have been alleged to have resulted from 210Po exposure.

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, one of his students, and two colleagues died from various cancers over the subsequent few years. The issue was investigated secretly, but there was never any formal admission of a connection between the leak and the deaths.[104]

The Church Rock uranium mill spill July 16, 1979 is reported to have released polonium-210. The report states animals had higher concentrations of lead-210, polonium-210 and radium-226 than the tissues from control animals.[105]

21st century

The cause of the 2006 death of Alexander Litvinenko, a former Russian FSB agent who had defected to the United Kingdom in 2001, was identified to be poisoning with a lethal dose of 210Po;[106][107] it was subsequently determined that the 210Po had probably been deliberately administered to him by two Russian ex-security agents, Andrey Lugovoy and Dmitry Kovtun.[108][109] As such, Litvinenko's death was the first (and, to date, only) confirmed instance in which polonium's extreme toxicity has been used with malicious intent.[110][111][112]

In 2011, an allegation surfaced that the death of Palestinian leader Yasser Arafat, who died on 11 November 2004 of uncertain causes, also resulted from deliberate polonium poisoning,[113][114] and in July 2012, concentrations of 210Po many times more than normal were detected in Arafat's clothes and personal belongings by the Institut de Radiophysique in Lausanne, Switzerland.[115][116] Even though Arafat's symptoms were acute gastroenteritis with diarrhoea and vomiting,[117] the institute's spokesman said that despite the tests the symptoms described in Arafat's medical reports were not consistent with 210Po poisoning, and conclusions could not be drawn.[116] In 2013 the team found levels of polonium in Arafat's ribs and pelvis 18 to 36 times the average,[118][119] even though by this point in time the amount had diminished by a factor of 2 million.[120] Forensic scientist Dave Barclay stated, "In my opinion, it is absolutely certain that the cause of his illness was polonium poisoning. ... What we have got is the smoking gun - the thing that caused his illness and was given to him with malice."[117][118] Subsequently, French and Russian teams claimed that the elevated 210Po levels were not the result of deliberate poisoning, and did not cause Arafat's death.[121][122]

It has also been suspected that Russian businessman Roman Tsepov was killed with polonium. He had symptoms similar to Aleksander Litvinenko.[123]

Treatment

It has been suggested that chelation agents, such as British anti-Lewisite (dimercaprol), can be used to decontaminate humans.[124] 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 five months.[125]

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.[126]

Occurrence in humans and the biosphere

Polonium-210 is widespread in the

As early as the 1920s, French biologist

testes.[130] 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[131] and that similar patterns of distribution occur in human tissues.[132] 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.[133]

Tobacco

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.[52][134][135][136][137]

The presence of polonium in tobacco smoke has been known since the early 1960s.[138][139] 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.[52]

Food

Polonium is found in the food chain, especially in seafood.[140][141]

See also

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Bibliography

External links

  • Polonium at
    The Periodic Table of Videos
    (University of Nottingham)