Polonium (IPA: /pə(ʊ)ˈləʊniəm/) is a chemical element in the periodic table that has the symbol Po and atomic number 84. A rare radioactive metalloid, polonium is chemically similar to tellurium and bismuth and occurs in uranium ores. Polonium has been studied for possible use in heating spacecraft. It exists as a number of isotopes.
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 for nuclear weapons. Other uses include:
Devices that eliminate static charges in textile mills and other places.[1] However, beta sources are more commonly used and are less dangerous. Another alternative is to use a high voltage direct current power supply to ionize air positively or negatively.[2]
Brushes that remove accumulated dust from photographic films. The polonium used in these brushes is sealed and controlled thus minimizing radiation hazards.
As 210Po, a lightweight heat source to power thermoelectric cells.
Russian secret services allegedly use polonium for smudging currency bills so that they can trace them. [1]
Radioactive poison [3].
History
Also called "Radium F", polonium was discovered by Marie Curie and her husband Pierre Curie in 1898[4] and was later named after Marie's native land of Poland (Latin: Polonia).[5] [6] Poland at the time was under Russian, Prussian and Austrian domination, and not recognized as an independent country. It was Marie's hope that naming the element after her native land would add notoriety to its plight. 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 uranium and radium, was more radioactive than both radium and uranium put together. This spurred them on to find the element. The electroscope showed it separating with bismuth.
Occurrence
A very rare element in nature, polonium is found in uranium ores at about 100 micrograms per metric ton (1:1010). Its natural abundance is approximately 0.2% of the abundance of radium. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers.[8][9]
Synthesis by (n,g) reaction
In 1934 an experiment showed that when natural 209Bi is bombarded with neutrons, 210Bi, which is the parent of polonium, was created. Polonium may now be made in milligram amounts in this procedure which uses high neutron fluxes found in nuclear reactors. Only about 100 grams is produced each year, making polonium exceedingly rare.[10]
Synthesis by (p,n) and (p,2n) reactions
It has been found that by proton bombardment of bismuth using a cyclotron that the longer lived isotopes of polonium can be formed. Other more proton rich isotopes can be formed by the irradation of platinum with carbon nuclei.[11]
Isotopes
Polonium has 25 known isotopes, all of which are radioactive. They have atomic masses that range from 194 u to 218 u. 210Po 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. However these isotopes are expensive to produce.
All elements containing 84 or more protons are radioactive. Alpha decay is a common form of decay for these nuclei. The most stable isotopes with more than 84 protons are thorium-232 and uranium-238; which form an "island of stability" which renders them stable enough to be found in large quantities in nature, but heavier nuclei are more and more affected by spontaneous fission.
210Po
Polonium-210 is an alpha emitter that has a half-life of 138.376 days. A milligram of 210Po emits as many alpha particles as 5 grams of radium. 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.[12] 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. A 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. 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.[13]. Another use of Polonium-210 has come to light recently and that is its use as a radioactive poison. Former KGB spy Alexander V. Litvinenko was poisened with Polonium-210 on November first 2006. The majority of the time 210Po decays only by emission of an alpha particle, not by emission of an alpha particle and a gamma ray. About one in a 100,000 decays results in the emission of a gamma ray[14]. This low gamma ray production rate makes it more difficult to find and identify this isotope. Rather than gamma ray spectroscopy, alpha spectroscopy will be the best method of measuring this isotope.
Chemical characteristics
Polonium dissolves readily in dilute acids, but is only slightly soluble in alkalis. It is closely related chemically to bismuth and tellurium. 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).[15] 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 microbes can methylate polonium by the action of methylcobalamin.[16][17]
Solid state form
The alpha form of solid polonium is cubic with a distance of 3.352 Å between atoms. It is a simple cubic solid which is not interpenetrated.
The beta form of polonium is hexagonal; it has been reported in the chemical literature, along with the alpha form, several times.
Two papers report X-ray diffraction experiments on polonium metal.[18] [19] The first report of the crystal structure of polonium was done using electron diffraction.[20]
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 spectroscopy is 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 a 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
Polonium is a highly radioactive and toxic element and is very difficult to handle. Even in milligram or 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 fatal dose (LD50, the dose that leads to 50% risk of death) for acute radiation exposure is generally about 4 Sv [21]. One Bq of 210Po (i.e., an amount that produces one decay per second) causes a radiation dose of 0.51 µSv if ingested, and 2.5 µSv if inhaled [22]. Since 210Po radiates 166 TBq per gram[22], a fatal 4-Sv dose can be caused by ingesting 8 MBq (200 microcurie), about 50 nanograms (ng), or inhaling 1.6 MBq (40 microcurie), about 10 ng. One gram of 210Po could thus in theory poison 100 million people. In addition to the acute effects, short-term radiation exposure carries a long-term risk of death from cancer of approximately 8% per Sv [23].
In rats a dose of 1.45 MBq/kg (8.7 ng/kg) of 210Po tends to cause death in about 30 days[24]. By this measure, 210Po is 400,000 times more toxic than hydrogen cyanide [25].
Body burden limit
The maximum allowable body burden for ingested polonium is only 1,100 Bq (0.03 microcurie), which is equivalent to a particle weighing only 6.8 picograms. The maximum permissible concentration for airborne soluble polonium compounds is about 7,500 Bq/m3 (2 × 10-11 µCi/cm3). The biological half-life of polonium in humans is 30 to 50 days.[26] The target organs for polonium in humans are the spleen and liver.[2] 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 cesium or tritium (as T2O). A review of biological effects of fission products and actinides can be read at.[3]
Notably, the death in 2006 of Alexander Litvinenko has been announced as probably due to 210Po poisoning. [27] Generally, Po is the most lethal when it is ingested.
Treatment
It has been suggested that Chelation theraphy using dimercaprol can be used to decontaminate humans.
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