Polonium

Polonium (pronounced /pəˈloʊniəm/) is a chemical element that has the symbol Po and atomic number 84. A rare and highly radioactive metalloid^[1], polonium is chemically similar to tellurium and bismuth, 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.

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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. Other uses include

• Devices that eliminate static charges in textile mills and other places.^[2] 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.^[3]
• ²¹⁰Po can be used as an atomic heat source to power radioisotope thermoelectric generators via thermoelectric materials.
• Because of its very high toxicity, polonium can be used as a poison (see, for example, Alexander Litvinenko poisoning).

History

Also tentatively called "Radium F", polonium was discovered by Marie Skłodowska-Curie and her husband Pierre Curie in 1898^[4] and was later named after Marie Curie's native land of Poland (Latin: Polonia).^[5][6] 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.^[7]

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.

Occurrence

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, though similar amount occur in everything from cherries to human tissue.^[8][9]

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. 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.^{[10] [11]}

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.^[12]

Isotopes

Polonium has 25 known isotopes, all of which are radioactive. They have atomic masses that range from 194u to 218u. 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

²¹⁰Po is an alpha emitter that has a half-life of 138.376 days; it decays directly to its 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) of ²¹⁰Po emit a blue glow which is caused by excitation of surrounding air. A single gram of ²¹⁰Po generates 140 watts of power.^[13] 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.^[14] Some anti-static brushes contain up to 500 microcuries of ²¹⁰Po as a source of charged particles for neutralizing static electricity in materials like photographic film.^[15]

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.^[16] 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.

Chemical characteristics

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 (M.P. -36.1°C to B.P. 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.^[17]

²¹⁰Po (in common with ²³⁸Pu) has the ability to become airborne with ease: if a sample is heated in air to 328 K (55°C, 131°F), 50% of it is vaporized in 45 hours, even though the melting point of polonium is 527 K (254°C, 489°F) and its boiling point is 1235 K (962°C, 1763°F).^[18] 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.^[19][20]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.

Solid state form

The alpha form of solid polonium has a simple cubic crystal structure with an edge length of 3.352 Å.

The beta form of polonium is rhombohedral; it has been reported in the chemical literature, along with the alpha form, several times. A picture of it is present on the web.^[21]

Two papers report X-ray diffraction experiments on polonium metal.^[22][23] The first report of the crystal structure of polonium was done using electron diffraction.^[24]

Tests

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).

Toxicity

Overview

By mass, polonium-210 is around 250,000 (2.5*10⁵) times more toxic than hydrogen cyanide (the actual LD₅₀ for ²¹⁰Po is about 1 microgram for an 80 kg person (see below) compared to about 250 milligram for hydrogen cyanide^[25]). 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. 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 (though 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.^[26] The committed effective dose equivalent ²¹⁰Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if inhaled.^[27] Since ²¹⁰Po has an activity of 166 TBq (4486.5 Ci) per gram^[27] (1 gram produces 166×10¹² decays per second), a fatal 4.5 Sv (J/kg) dose can be caused by ingesting 8.8 MBq/kg (238 microcuries, or about 24 mCi for 80 kg person), about 50 nanograms per kilogram (ng/kg), or inhaling 1.8 MBq/kg (48 microcuries, or about 4 mCi for 80 kg person), about 10 ng/kg. One gram of ²¹⁰Po could thus in theory poison 2 million (50 kg each) people of whom 1 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^[28]) is somewhat less damaging than an instantaneous dose. It has been estimated that a minimal lethal dose of ²¹⁰Po for an 80 kg person is 0.15 GBq (4 millicuries), or 0.89 micrograms, still an extremely small amount. ^[29][30]

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.^[26] 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^[31] of the estimated 15,000-22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.^[32] Tobacco smoking causes significant additional exposure to Po.^[33]

Regulatory exposure limits

The maximum allowable body burden for ingested ²¹⁰Po is only 1,100 Bq (0.03 microcurie), which is equivalent to a particle weighing only 6.8 picograms. The maximum permissible workplace concentration of airborne ²¹⁰Po is about 10 Bq/m³ (3 × 10^-10 µCi/cm³).^[34] The target organs for polonium in humans are the spleen and liver.^[35] 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 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."^[36]

Famous polonium poisoning cases

Notably, the murder of Alexander Litvinenko, a Russian dissident, in 2006 was announced as due to ²¹⁰Po poisoning ^[37][38] (see Alexander Litvinenko poisoning). According to Nick Priest, a 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.^[39]

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.^[40] 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 or may not 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.^[41] A leak was discovered at a Weizmann Institute laboratory in 1957. Traces of ²¹⁰Po were found on the hands of Prof. 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.

Treatment

It has been suggested that chelation agents such as British Anti-Lewisite (dimercaprol) can be used to decontaminate humans.^[42][43] 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.^[44]

See also

• Polonium - Radon Decay Chain [4]
• Polonium halo

References