polonium

bismuth ← polonium → astatine Te ↑ Po ↓ Uuh 84Po Periodic table
Appearance
silvery
General properties
Name, symbol, number	polonium, Po, 84
Element category	metalloid
Group, period, block	16, 6, p
Standard atomic weight	(209) g·mol−1
Electron configuration	[Xe] 6s2 4f14 5d10 6p4
Electrons per shell	2, 8, 18, 32, 18, 6 (Image)
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
Specific heat capacity	(25 °C) 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	(300 K) ? 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 ε, β+ 1.401 208Bi 209Po syn 103 y α 4.979 205Pb ε, β+ 1.893 209Bi 210Po trace 138.376 d α 5.307 206Pb
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Polonium (pronounced /pɵˈloʊniəm/, po-LOH-nee-əm) 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 metalloid,^[1] polonium is chemically similar to bismuth^[2] and tellurium, and it occurs in uranium ores. Polonium has been studied for possible use in heating spacecraft. It is unstable; all isotopes of polonium are radioactive. Polonium is very volatile; it will almost completely vaporize at room temperatures.

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 138.376 days; it decays directly to its stable daughter isotope, ²⁰⁶Pb. A milligram of ²¹⁰Po emits about as many alpha particles per second as 4.5 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. A single gram of ²¹⁰Po generates 140 watts of power.^[3] Because it emits many alpha particles, which are stopped within a very short distance in dense media and release their energy, ²¹⁰Po has been used as a lightweight heat source to power thermoelectric cells in artificial satellites; for instance, ²¹⁰Po heat source was also used in each of the Lunokhod rovers deployed on the surface of the Moon, to keep their internal components warm during the lunar nights.^[4] Some anti-static brushes contain up to 500 microcuries (20 MBq) of ²¹⁰Po as a source of charged particles for neutralizing static electricity in materials like photographic film.^[5]

The majority of the time ²¹⁰Po decays by emission of an alpha particle only, not by emission of an alpha particle and a gamma ray. About one in 100,000 alpha emissions causes an excitation in the nucleus which then results in the emission of a gamma ray.^[6] This low gamma ray production rate (and the short range of alpha particles) makes it difficult to find and identify this isotope. Rather than gamma ray spectroscopy, alpha spectroscopy is the best method of measuring this isotope.

Solid state form

The alpha form of solid polonium.

Polonium is a radioactive element that exists in two metallic allotropes. The alpha form has a simple cubic crystal structure with an edge length of 335.2 picometres; the beta form is rhombohedral.^{[7] [8]} The structure of polonium has been characterized by X-ray diffraction ^[9][10] and electron diffraction.^[11]

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. The hydrogen compound PoH₂ is liquid at room temperature (melting point −36.1°C, boiling point 35.3°C). Halides of the structure PoX₂, PoX₄ and PoX₆ are known. The two oxides PoO₂ and PoO₃ are the products of oxidation of polonium.^[12]

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).^[13] 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.

It has been reported that some microbes can methylate polonium by the action of methylcobalamin.^[14][15] 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 the polonium will follow the same biochemical pathways as selenium and tellurium.

Compounds

This section requires expansion.

Polonium has no common compounds, only lab created ones.

History

Also tentatively called "Radium F", polonium was discovered by Marie Skłodowska-Curie and her husband Pierre Curie in 1898^[16] and was later named after Marie Curie's native land of Poland (Latin: Polonia)^[17][18] 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.^[19]

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, and then within a few years, also isolated radium.

Detection

Intensity against photon energy for three isotopes.

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.

Intensity against alpha energy for four isotopes, note that the line width is narrow and the fine details can be seen.

Intensity against alpha energy for four isotopes, note that the line width is wide and some of the fine details can not be seen. This is for liquid scintillation counting where random effects cause a variation in the number of visible photons generated per alpha decay.

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 for 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 100 micrograms 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.^[20][21][22]

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.^[23] 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.^[24][25]

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.^[26]

Applications

When it is mixed or alloyed with beryllium, polonium can be a neutron source: beryllium releases a neutron upon absorption of an alpha particle that is supplied by ²¹⁰Po. It has been used in this capacity as a neutron trigger or initiator for nuclear weapons. However, a license is needed to own and operate this form of neutron source.^[27] Other uses include the following.

• Devices that eliminate static charges in textile mills and other places.^[28] However, beta particle sources are more commonly used and are less dangerous. A non-radioactive alternative is to use a high-voltage DC power supply to ionise air positively or negatively as required.^[29]
• ²¹⁰Po can be used as an atomic heat source to power radioisotope thermoelectric generators via thermoelectric materials.^[30]
• Because of its very high toxicity, polonium can be used as a poison (see, for example, Alexander Litvinenko poisoning).
• Polonium is also used to eliminate dust on film.^[31]

Toxicity

Overview

By mass, polonium-210 is around 250,000 times more toxic than hydrogen cyanide (the actual LD₅₀ for ²¹⁰Po is about 1 microgram for an 80 kg person (see below) compared with about 250 milligrams for hydrogen cyanide^[32]). The main hazard is its intense radioactivity (as an alpha emitter), which makes it very difficult to handle safely: one gram of Po will self-heat to a temperature of around 500 °C (932 °F).^[3] Even in microgram amounts, handling ²¹⁰Po is extremely dangerous, requiring specialized equipment and strict handling procedures. 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 if the polonium is outside the body.

Acute effects

The median lethal dose (LD₅₀) for acute radiation exposure is generally about 4.5 Sv.^[33] The committed effective dose equivalent ²¹⁰Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if inhaled.^[34] Since ²¹⁰Po has an activity of 166 TBq (4486.5 Ci) per gram^[34] (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 microcuries, 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^[35]) 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.^[36][37]

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.^[33] 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^[38] of the estimated 15,000-22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.^[39] Tobacco smoking causes additional exposure to polonium.^[40]

Regulatory exposure limits

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).^[41] The target organs for polonium in humans are the spleen and liver.^[42] 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)."^[43]

Famous poisoning cases

Notably, the murder of Alexander Litvinenko, a Russian dissident, in 2006 was announced as due to ²¹⁰Po poisoning^[44][45] (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.^[46] The Polonium Restaurant (a Polish restaurant in Sheffield, England, owned by Boguslaw Sidorowicz and named after his folk band in the late 1970s) experienced increased interest and business as a result of internet searches for the phrase polonium restaurant.^[47][48][49]

It has also been suggested that Irène Joliot-Curie was the first person ever to die from the radiation effects of polonium (due to a single intake) in 1956.^[50] She was accidentally exposed to polonium in 1946 when a sealed capsule of the element exploded on her laboratory bench. A decade later, on 17 March 1956, she died in Paris from leukemia which may have been caused by that exposure.

According to the book The Bomb in the Basement, several death cases in Israel during 1957-1969 were caused by ²¹⁰Po.^[51] 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.^[52]

Treatment

It has been suggested that chelation agents such as British Anti-Lewisite (dimercaprol) can be used to decontaminate humans.^[53] 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.^[54]

Commercial products containing polonium

No nuclear authority has asserted that a commercial product was a likely source for the poisoning of Litvinenko. However, as Prof. Peter D. Zimmerman says, "Polonium 210 is surprisingly common. ...Polonium sources with about 10 percent of a lethal dose are readily available—even in a product sold on Amazon.com."^[55]

Potentially lethal amounts of polonium are present in anti-static brushes sold to photographers.^[56] Many of the devices are available by mail order. General Electric markets a static eliminator module with 500 µCi (20 MBq), roughly 2.5 times the lethal dose of ²¹⁰Po if 100%-ingested, for US$71;^[57] Staticmaster sells replacement units with the same amount (500 µCi) of ²¹⁰Po for US$36.^[58] 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"^[59] which means that a buyer need not be registered by any authorities: the general license "is effective without the filing of an application with the Commission or the issuance of a licensing document to a particular person."

If these sources were used to collect the amount of polonium likely used in the poisoning—and one could devise a method of separating the polonium from its protective casing—it would take 10–100 modules for price of US$360 to US$7,100. That such a thing could be done is extremely difficult according to the manufacturers^{[citation needed]} and would be highly dangerous to anyone attempting to do so without some special equipment like a glovebox.

Sometimes sources of polonium used in industry are stolen or lost. According to the Nuclear Regulatory Commission (NRC), there were registered at least 8 cases of loss of control of potentially lethal polonium sources in the USA during 2006.^{[citation needed]}

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 available to the public in the United States by mail order from a company called United Nuclear as 'needle sources' for laboratory experimentation. It would require about 15,000 of these ²¹⁰Po sources, at a total cost of about $1 million, to obtain a toxic quantity of polonium. They typically sell between four and eight sources per year.^[60][61]

According to some estimates,^[62] the cost of the quantity of pure polonium-210 used to kill Litvinenko would be around £20 million (US$39 million).^[63] However, this estimation is based on retail prices of commercially available demonstration radiation sources with very small activities and cannot be considered as reasonable.

Tobacco

The presence of polonium in tobacco smoke has been known since the early 1960s.^[64][65] 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.^[22]

Radioactive polonium-210 contained in phosphate fertilizers is absorbed by the roots of plants (such as tobacco) and stored in its tissues.^[66][67][68] Tobacco plants fertilized by rock phosphates contain polonium-210, which emits alpha radiation estimated to cause about 11,700 lung cancer deaths annually worldwide.^[22][69][70]

See also

• Decay chain
• Polonium halo
• Copernicium

Notes

External links

• Build a pocket-sized ion chamber, useful for detecting Polonium
• Chemistry in its element podcast (MP3) from the Royal Society of Chemistry's Chemistry World: Polonium