Polonium

bismuth ← polonium → astatine
Te
↑
Po
↓
Uuh
Appearance
silvery
General properties
Name, symbol, number polonium, Po, 84
Pronunciation  /pɵˈloʊniəm/ po-loh-nee-əm
Element category metalloid
Group, period, block 16, 6, p
Standard atomic weight (209)
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
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
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
· r

Polonium ( /pɵˈloʊniəm/ po-loh-nee-əm) is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth[1] and tellurium, and it occurs in uranium ores. Polonium has been studied for possible use in heating spacecraft. As it is unstable, all isotopes of polonium are radioactive. There is disagreement as to whether polonium is a post-transition metal or metalloid.[2][3]

Contents

Characteristics

Isotopes

Main article: Isotopes of polonium

Polonium has 33 known isotopes, all of which are radioactive. They have atomic masses that range from 188 to 220 u. 210Po (half-life 138.4 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 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.[4] 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.[5] 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.[6]

About one in 100,000 alpha emissions causes an excitation in the nucleus which then results in the emission of a gamma ray.[7] Because of its relatively high rate of alpha emissions, only about 1 in 100,000 result in releasing a gamma ray. But it is the alpha particles, not the side effect of an occasional gamma ray, that results in 210Po decay. Low gamma output renders gamma detection nearly impossible, with any emitted gamma nearly indistinguishable from background radiation. At 4.001 u, the alpha particle is too massive to penetrate most barriers, including intact human epidermis. If the skin is broken however, or the alpha emitter is ingested or inhaled, the high charge on the alpha particle will result in severe cellular damage. The high alpha decay of polonium renders alpha detection as the preferred method of quantifying this isotope in the laboratory.

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 picometres; the beta form is rhombohedral.[8][9] The structure of polonium has been characterized by X-ray diffraction [10][11] and electron diffraction.[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][14] 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. 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.[15]

It has been reported that some microbes can methylate polonium by the action of methylcobalamin.[16][17] 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

Polonium has no common compounds, only synthetically created ones.

Oxides

  • PoO2
  • PoO3

Hydrides

Halogen Compounds

History

Also tentatively called "Radium F", polonium was discovered by Marie Skłodowska-Curie and her husband Pierre Curie in 1898[18] and was named after Marie Curie's native land of Poland (Latin: Polonia)[19][20] 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.[21]

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.

Because of the small quantities present in nature, isolation of polonium from natural sources is complicated. The largest ever isolated batch from 37 tonnes of residues from radium production yielded only 40 Ci of polonium-210.[22]

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 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.[23][24][25]

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.[26] 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.[27][28]

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.[29]

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.[30] Other uses include the following.

Toxicity

Overview

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[35]). 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).[4] Even in microgram amounts, handling 210Po is extremely dangerous, requiring specialized equipment (a.o., alpha glove box under depression and 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, bearing chemically resistant and "intact" gloves is a mandatory precautions 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.[36] The committed effective dose equivalent 210Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if inhaled.[37] Since 210Po has an activity of 166 TBq per gram (4,500 Ci/g)[37] (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[38]) 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.[39][40]

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.[36] 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[41] of the estimated 15,000-22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.[42] Tobacco smoking causes additional exposure to polonium.[43]

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).[44] The target organs for polonium in humans are the spleen and liver.[45] 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)."[46]

Famous poisoning cases

Notably, the murder of Alexander Litvinenko, a Russian dissident, in 2006 was announced as due to 210Po poisoning[47][48] (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.[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

Potentially lethal amounts of polonium are present in anti-static brushes sold to photographers.[55] 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",[56] 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.[57][58]

Occurrence in humans and the biosphere

The natural occurrence of polonium-210 is widespread in the biosphere, including in human tissues. This is a consequence of a mechanistic chain involving natural radioactive fallout. Radium-226 in the Earth's crust decays with the production of radon-220, some of which, during its 3.6-day half-life, diffuses into the atmosphere. Here it decays with the production of polonium-210, much of which, during its 138-day half-life, is washed back down to the Earth's surface, thus entering the biosphere.[59][60][61]

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.[62] More recent evidence suggests that this behaviour results from polonium substituting for sulphur in S-containing amino-acids or related molecules[63][64] and that similar patterns of distribution occur in human tissues.[65] 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.[66]

Tobacco

The presence of polonium in tobacco smoke has been known since the early 1960s.[67][68] 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.[25]

Radioactive polonium-210 contained in phosphate fertilizers is absorbed by the roots of plants (such as tobacco) and stored in its tissues.[69][70][71] Tobacco plants fertilized by rock phosphates contain polonium-210, which emits alpha radiation estimated to cause about 11,700 lung cancer deaths annually worldwide.[25][72][73]

Food

Polonium is also found in the food chain, especially in seafood.[74][75]

See also

Notes

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