Polonium

Polonium
84Po
Te ↑ Po ↓ Lv Periodic table bismuth ← polonium → astatine
Appearance
silvery
General properties
Name, symbol, number	polonium, Po, 84
Pronunciation	/ p ɵ ˈ l oʊ n i ə m / po-LOH-nee-əm
Element category	post-transition metal post-transition metal status is disputed (see article text)
Group, period, block	16 (chalcogens), 6, p
Standard atomic weight	(209)
Electron configuration	[Xe] 6s2 4f14 5d10 6p4 2, 8, 18, 32, 18, 6
History
Discovery	Pierre Curie and Marie Curie (1898)
First isolation	Willy Marckwald (1902)
Physical properties
Phase	solid
Density (near r.t.)	(alpha) 9.196 g·cm−3
Density (near r.t.)	(beta) 9.398 g·cm−3
Melting point	527 K, 254 °C, 489 °F
Boiling point	1235 K, 962 °C, 1764 °F
Heat of fusion	ca. 13 kJ·mol−1
Heat of vaporization	102.91 kJ·mol−1
Molar heat capacity	26.4 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K)       (846) 1003 1236
Atomic properties
Oxidation states	6, 4, 2, −2 ( amphoteric oxide)
Electronegativity	2.0 (Pauling scale)
Ionization energies	1st: 812.1 kJ·mol−1
Atomic radius	168 pm
Covalent radius	140±4 pm
Van der Waals radius	197 pm
Miscellanea
Crystal structure	cubic
Magnetic ordering	nonmagnetic
Electrical resistivity	(0 °C) (α) 0.40 µΩ·m
Thermal conductivity	? 20 W·m−1·K−1
Thermal expansion	(25 °C) 23.5 µm·m−1·K−1
CAS registry number	7440-08-6
Most stable isotopes
Main article: Isotopes of polonium
iso NA half-life DM DE ( MeV) DP 208Po syn 2.898 y α 5.215 204Pb β+ 1.401 208Bi 209Po syn 103 y α 4.979 205Pb β+ 1.893 209Bi 210Po trace 138.376 d α 5.307 206Pb

Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie and Pierre Curie. A rare and highly radioactive element with no stable isotopes, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Applications of polonium are few, and include heaters in space probes, antistatic devices, and sources of neutrons and alpha particles. Because of its position in the periodic table, polonium is sometimes classified as a metalloid. However, other sources say that on the basis of its properties and behaviour, it is "unambiguously a metal".

Characteristics

Isotopes

Polonium has 33 known isotopes, all of which are radioactive. They have atomic masses that range from 188 to 220 u. ²¹⁰Po (half-life 138.376 days) is the most widely available. ²⁰⁹Po (half-life 103 years) and ²⁰⁸Po (half-life 2.9 years) can be made through the alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron.

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 of ²¹⁰Po emits about as many alpha particles per second as 5 grams of ²²⁶Ra. A few curies (1 curie equals 37 gigabecquerels, 1 Ci = 37 GBq) of ²¹⁰Po emit a blue glow which is caused by excitation of 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. However, it is the alpha particles, not the side effect of an occasional gamma ray, that results in ²¹⁰Po decay. The gamma radiation level from polonium is below the normal background.

Solid state form

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, with an edge length of 335.2 picometers; the beta form is rhombohedral. The structure of polonium has been characterized by X-ray diffraction and electron diffraction.

²¹⁰Po (in common with ²³⁸Pu) 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, even though the melting point of polonium is 254 °C (489 °F) and its boiling point is 962 °C (1763 °F). 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.

Chemistry

The chemistry of polonium is similar to that of tellurium and bismuth. 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.

It has been reported that some microbes can methylate polonium by the action of methylcobalamin. This is similar to the way in which mercury, selenium and tellurium are methylated in living things to create organometallic compounds. As a result when considering the biochemistry of polonium one should consider the possibility that polonium will follow the same biochemical pathways as selenium and tellurium.

Compounds

Polonium has no common compounds, only synthetically created ones, and more than 50 of those are known. 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.

Polonium hydride (PoH₂) is a volatile liquid at room temperature prone to dissociation. The two oxides PoO₂ and PoO₃ are the products of oxidation of polonium.

Halides of the structure PoX₂, PoX₄ and PoX₆ are known. They are soluble in the corresponding hydrogen halides, i.e., PoCl_X in HCl, PoBr_X in HBr and PoI₄ in HI. 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.

Other polonium compounds include acetate, bromate, carbonate, citrate, chromate, cyanide, formate, hydroxide, nitrate, selenate, monosulfide, sulfate and disulfate.

Polonium compounds

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

Oxides PoO PoO2 PoO3

Hydrides PoH2

Halogen compounds PoX2, e.g. polonium dichloride, PoCl2 PoX4, e.g. polonium tetrachloride, PoCl4 PoX6, e.g. polonium hexafluoride, PoF6

History

Also tentatively called "Radium F", polonium was discovered by Marie and Pierre Curie in 1898 and was named after Marie Curie's native land of Poland (Latin: Polonia) Poland at the time was under Russian, Prussian, and Austrian 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.

This element was the first one discovered by the Curies while they were investigating the cause of pitchblende radioactivity. The pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than both the uranium and thorium put together. This spurred the Curies on to find additional radioactive elements. The Curies first separated out polonium from the pitchblende in July 1898, and five months later, also isolated radium.

In the United States, Polonium was produced as part of the Manhattan Project during World War II. It was a critical part of the implosion-type nuclear weapon design used in the Fat Man bomb on Nagasaki in 1945. Polonium and Beryllium were the key ingredients of the ' Urchin' detonator at the centre of the bomb's spherical Plutonium pit. The Urchin ignited the nuclear chain reaction at the moment of prompt-criticality to ensure the bomb did not fizzle.

Much of the basic physics of Polonium was classified until after the war. The fact that it was used as an initiator was classifed until the 1960s.

The Atomic Energy Commission and the Manhattan Project funded human experiments using polonium on 5 people at the University of Rochester between 1943 and 1947. The people were administered between 9 and 22 microcuries of Polonium to study its excretion.

Detection

Gamma counting

By means of radiometric methods such as gamma spectroscopy (or a method using a chemical separation followed by an activity measurement with a non-energy-dispersive counter), it is possible to measure the concentrations of radioisotopes and to distinguish one from another. In practice, background noise would be present and depending on the detector, the line width would be larger which would make it harder to identify and measure the isotope. In biological/medical work it is common to use the natural ⁴⁰K present in all tissues/body fluids as a check of the equipment and as an internal standard.

Alpha counting

The best way to test for (and measure) many alpha emitters is to use alpha-particle spectroscopy as it is common to place a drop of the test solution on a metal disk which is then dried out to give a uniform coating on the disk. This is then used as the test sample. If the thickness of the layer formed on the disk is too thick then the lines of the spectrum are broadened, this is because some of the energy of the alpha particles is lost during their movement through the layer of active material. An alternative method is to use internal liquid scintillation where the sample is mixed with a scintillation cocktail. When the light emitted is then counted, some machines will record the amount of light energy per radioactive decay event. Due to the imperfections of the liquid scintillation method (such as a failure of all the photons to be detected, cloudy or coloured samples can be difficult to count) and the fact that random quenching can reduce the number of photons generated per radioactive decay it is possible to get a broadening of the alpha spectra obtained through liquid scintillation. It is likely that these liquid scintillation spectra will be subject to a Gaussian broadening rather than the distortion exhibited when the layer of active material on a disk is too thick.

A third energy dispersive method for counting alpha particles is to use a semiconductor detector.

From left to right the peaks are due to ²⁰⁹Po, ²¹⁰Po, ²³⁹Pu and ²⁴¹Am. The fact that isotopes such as ²³⁹Pu and ²⁴¹Am have more than one alpha line indicates that the nucleus has the ability to be in different discrete energy levels (like a molecule can).

Occurrence and production

Polonium is a very rare element in nature because of the short half-life of all its isotopes. It is found in uranium ores at about 0.1 mg per metric ton (1 part in 10¹⁰), 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.

Because of the small abundance, isolation of polonium from natural sources is a very tedious process. The largest batch was extracted in the first half of the 20th century by processing 37 tonnes of residues from radium production. It contained only 40 Ci (9 mg) of polonium-210. Nowadays, polonium is obtained by irradiating bismuth with high-energy neutrons or protons.

Neutron capture

Synthesis by (n,γ) reaction

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

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.

Proton capture

Synthesis by (p,n) and (p,2n) reactions

It has been found that the longer-lived isotopes of polonium can be formed by proton bombardment of bismuth using a cyclotron. Other more neutron-rich isotopes can be formed by the irradiation of platinum with carbon nuclei.

Applications

Polonium-based sources of alpha particles were produced in the former Soviet Union. Such sources were applied for measuring the thickness of industrial coatings via attenuation of alpha radiation. 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 energy. Therefore, ²¹⁰Po is used as an atomic heat source to power radioisotope thermoelectric generators via thermoelectric materials. For instance, 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).

The alpha particles emitted by polonium can be converted to neutrons using beryllium oxide, at a rate of 93 neutrons per million alpha particles. Thus Po-BeO mixtures or alloys are used as a neutron source, for example in a neutron trigger or initiator for nuclear weapons and for inspections of oil wells. About 1500 of such sources with an individual activity of 1850 Ci have been used annually in the Soviet Union.

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 prior to the application of coatings (such as automotive). Alpha particles emitted by polonium ionize air molecules that neutralize charges on the nearby surfaces. However, 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.

Toxicity

Overview

Polonium is highly dangerous and has no biological role. By mass, polonium-210 is around 250,000 times more toxic than hydrogen cyanide (the actual LD₅₀ for ²¹⁰Po is less than 1 microgram for an average adult (see below) compared with about 250 milligrams for hydrogen cyanide). The main hazard is its intense radioactivity (as an alpha emitter), which makes it very 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 of the body. Meanwhile, 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.

Acute effects

The median lethal dose (LD₅₀) for acute radiation exposure is generally about 4.5 Sv. The committed effective dose equivalent ²¹⁰Po is 0.51 µSv/ Bq if ingested, and 2.5 µSv/Bq if inhaled. Since ²¹⁰Po has an activity of 166 TBq per gram (4,500 Ci/g) (1 gram produces 166×10¹² decays per second), a fatal 4.5 Sv (J/kg) dose can be caused by ingesting 8.8 MBq (238 micro curies, µCi), about 50 nanograms (ng), or inhaling 1.8 MBq (48 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) is somewhat less damaging than an instantaneous dose. It has been estimated that a median lethal dose of ²¹⁰Po is 0.015 GBq (0.4 mCi), or 0.089 micrograms, still an extremely small amount.

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. 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 of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon. Tobacco smoking causes additional exposure to polonium.

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). The target organs for polonium in humans are the spleen and liver. 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. In the US, a tracking system run by the Nuclear Regulatory Commission will be 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)."

Polonium and its compounds must be handled in a glove box, which is further enclosed in another box, maintained at a slightly lower 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.

Well-known poisoning cases

Notably, the murder of Alexander Litvinenko, a Russian dissident, in 2006 was announced as due to ²¹⁰Po poisoning (see Alexander Litvinenko poisoning). According to Prof. Nick Priest of Middlesex University, an environmental toxicologist and radiation expert, speaking on Sky News on December 2, Litvinenko was probably the first person ever to die of the acute α-radiation effects of ²¹⁰Po. However, according to William Dunkerley, a senior fellow at the American University in Moscow, the London coroner has yet to rule that polonium was actually the cause of Litvinenko's death, or that he was even a victim of homicide.

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.

According to the book The Bomb in the Basement, several death cases in Israel during 1957–1969 were caused by ²¹⁰Po. 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.

Abnormally high concentrations of ²¹⁰Po have been detected in July 2012 in clothes and personal belongings of the Palestinian leader Yasser Arafat, who died in 2004 of uncertain causes. However, 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." On 27 November 2012 Arafat's body was exhumed and samples were taken for separate analysis by experts from France, Switzerland and Russia. Results are expected by April 2013.

Treatment

It has been suggested that chelation agents such as British Anti-Lewisite ( dimercaprol) can be used to decontaminate humans. 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 after 5 months.

Detection in biological specimens

Polonium-210 may be quantitated 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.

Commercial products containing polonium

Some anti-static brushes contain up to 500 microcuries (20 MBq) of ²¹⁰Po as a source of charged particles for neutralizing static electricity. In USA, the devices with no more than 500 µCi of (sealed) ²¹⁰Po per unit can be bought in any amount under a "general license", which means that a buyer need not be registered by any authorities.

Tiny amounts of such radioisotopes are sometimes used in the laboratory and for teaching purposes—typically of the order of 4–40 kBq (0.1–1.0 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 are retailed by scientific supply companies. The actual polonium is a layer of plating which in turn is plated with a material such as gold. This allows the alpha radiation (used in experiments such as cloud chambers) while preventing the polonium from being released and presenting a toxic hazard. According to United Nuclear, they typically sell between four and eight sources per year.

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 gas radon-222, some of which, during its 3.6-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.

As early as the 1920s Lacassagne, using polonium provided by his colleague Marie Curie, showed that the element has a very specific pattern of uptake in rabbit tissues, with high concentrations particularly in liver, kidney and testes. More recent evidence suggests that this behaviour results from polonium substituting for sulfur in sulfur-containing amino-acids or related molecules and that similar patterns of distribution occur in human tissues. 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.

Tobacco

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

Radioactive polonium-210 contained in phosphate fertilizers is absorbed by the roots of plants (such as tobacco) and stored in its tissues. Tobacco plants fertilized by rock phosphates contain polonium-210, which emits alpha radiation estimated to cause about 11,700 lung cancer deaths annually worldwide.

Food

Polonium is also found in the food chain, especially in seafood.