Polonium

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.

Applications

When it is mixed or alloyed with beryllium, polonium can be a neutron source. 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. However, beta sources are more commonly used and are less dangerous.
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.

History

Also called "Radium F", polonium was discovered by Marie Curie and her husband Pierre Curie in 1897 and was later named after Marie's homeland of Poland (Latin: Polonia). 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 home land would add notoriety to its plight. 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 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 radium's.

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 believed to be produced each year, making Polonium exceedingly rare.

Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers [1] [2].

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 ²³²Th and ²³⁸U; 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.

Polonium-210

This isotope of polonium 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 great deal of energy is released by its decay with a half a gram quickly reaching a temperature above 750 K. A few curies (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.^[1] Since nearly all alpha radiation can be easily stopped by ordinary containers and upon hitting its surface releases its 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. Polonium-210 is thought to have been used in the fatal poisoning of ex-KGB agent Alexander Litvinenko in London in November 2006

Chemical characteristics

Polonium dissolves readily in dilute acids, but is only slightly soluble in alkalis. It is closely related chemically to bismuth and tellurium. Polonium-210 (in common with ^{238^Pu}) has the ability to become airborne with ease (volatilize), 50% of a sample is vaporized in air in 45 hours at 328K even though its melting point is 527K and its boiling point is 1235K. 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.

N. Momoshima, L.X. Song, S. Osaki and Y. Maeda, Environ Sci Technol. 2001, 35, 2956-2960[3] report that microbes can methylate polonium. They also claim that methylcobalamin can also methylate polonium.

Solid state form

The alpha form of the Po solid is cubic (Po-Po distance is 3.352 Å), it is a simple cubic solid which is not interpenetrated. A myth has grown up from a single sentence in one of the original papers on the crystal structure as determined by X-ray powder diffraction. Below is a diagram of a triple interpenetrated cubic solid; while this is not the structure for Po, a reasonable number of real examples of such an interlocking network have been found.

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 Po metal.

R.J. Desando and R.C Lange, Journal of Inorganic and Nuclear Chemistry, 1966, 28, 1837-1846.
W.H Beamer and C.R. Maxwell, Journal of Chemical Physics, 1946, 14, 569-569

The first report of the crystal structure of Po was done using electron diffraction.

M.A. Rollier, S.B. Hendricks and L.R. Maxwell, Journal of Chemical Physics, 1936, 4, 648-652.

Tests

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 real life, background noise would be present and depending on the detector, the line width would be larger thus making it harder to make a identification and measurement of the isotope. In biological/medical work it is common to use the natural ^{40^{K present in all tissues/body fluids as a check of the equipment and as an internal standard.}}

^[2]^[3]

Toxicity

Polonium is a highly radioactive and toxic element and is very difficult to handle. Even in milligram or microgram amounts, handling polonium-210 is extremely dangerous, requiring specialized equipment and strict handling procedures. Alpha particles emitted by polonium material are absorbed by the body and will damage organic tissue easily if ingested or absorbed (though they do not penetrate the epidermis and hence are not hazardous if the polonium is outside the body).

A lethal dose of 10 sieverts, or about 525 microcuries, weighs just 12 millionths of a gram. A cube of pure polonium 210 about the size of the period at the end of this sentence—only 0.35 mm wide and weighing just 400 micrograms—would still be 3,370 times the lethal dose. (Calculations based on a committed effective dose equivalent (CEDE) of 5.14×10⁻⁷ sieverts per becquerel (1.9×10³ mrem/microcurie) for ingested ²¹⁰Po and a specific activity of 1.66×10¹⁴ Bq/gram (4.49×10³ curies/gram)[4].)

The maximum allowable body burden for ingested polonium is only 1,100 becquerels (0.03 microcurie), which is equivalent to a particle weighing only 6.8 × 10^-12 gram. Weight for weight, polonium is approximately 2.5 × 10¹¹ (250 billions) times as toxic as hydrogen cyanide. The maximum permissible concentration for airborne soluble polonium compounds is about 7,500 Bq/m³ (2 × 10^-11 µCi/cm³). The biological halflife of Polonium in humans is 30 to 50 days.[5]

Use as a poison

Polonium-210 caused the death of Alexander Litvinenko, a dissident former Russian FSB spy, on November 23, 2006 according to Pat Troop, chief executive of Britain's Health Protection Agency.^[4] Traces of polonium were found in several locations Litvinenko had visited shortly before becoming ill, as well as in his urine.^[5]

References

^ Polonium, Argonne National Laboratory
^ "Radiation tests after spy death". BBC. 24 November 2006. Retrieved 2006-11-24. {{cite web}}: Check date values in: |date= (help)
^ Template:Ru icon "История, которая развивалась чуть больше 3-х недель вокруг странной болезни бывшего сотрудника ФСБ Александра Литвиненко, сегодня завершилась трагически". Ekho Moskvy. November 24 2006. Retrieved 2006-11-24. {{cite web}}: Check date values in: |date= (help)
^ "Polonium-210 Difficult to Detect" AP
^ "Radiation found after spy's death", BBC News online

External links

References and External links verified 2006-11-25 unless noted.

[1] Polonium, Argonne National Laboratory

[BBC_Polonium-2] "Radiation tests after spy death". BBC. 24 November 2006. Retrieved 2006-11-24. {{cite web}}: Check date values in: |date= (help)

[3] Template:Ru icon "История, которая развивалась чуть больше 3-х недель вокруг странной болезни бывшего сотрудника ФСБ Александра Литвиненко, сегодня завершилась трагически". Ekho Moskvy. November 24 2006. Retrieved 2006-11-24. {{cite web}}: Check date values in: |date= (help)

[Poison-4] "Polonium-210 Difficult to Detect" AP

[BBC_News-5] "Radiation found after spy's death", BBC News online

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