In November 2006, it was reported in the press that the former Russian KGB agent Alexander Litvinenko was the victim of a "poison attack" with polonium-210, as a result of which he died on 23 November 2006. In the course of the public debate about this incident, many questions arose about the occurrence, origin, effects etc. of polonium-210. On this page we want to help answer some of these questions.

1 Occurrence
2 Physical properties
3 Radioactive decay
4 Specific activity
5 Absorption into the body, biological HWZ
6 Radiation exposure
6.1 External radiation exposure
6.2 Internal radiation exposure
7 Detection of polonium-210 in the body
8 Literature

Polonium (chemical symbol Po) is a radioactive chemical element discovered by Marie Curie with the atomic number Z = 84. All polonium isotopes are radioactive. The lightest isotope (Po-190) has the mass number A = 190, the heaviest (Po-218) the mass number A = 218.

The physical half-life (HWZ) of most polonium isotopes ranges from fractions of a second to a few days. Only three isotopes have comparatively long half-lives. These are Po-208 (HWZ 2.9 years), Po-209 (HWZ 102 years) and Po-210 (HWZ 138.4 days).

Of these isotopes, it is mainly Po-210 that occurs in nature, as it is continuously produced by the radioactive decay of the isotope uranium-238 (U-238), which also occurs in nature. The other two isotopes can be produced artificially, for example by bombarding lead or bismuth with alpha particles or protons. However, this production method is so complex (and therefore expensive) that it is not technically used.

Polonium-210 only occurs in very small quantities in nature. In the earth's crust, its concentration is approx. 0.2× ^10-9 mg/kg, i.e. approx. 0.2 billionths of a milligram per kilogramme. In uranium ores, the concentration is approx. ^10-4 mg/kg, i.e. approx. 1/10,000th of a milligram per kilogramme.

Significant quantities of Po-210 (in the milligram range) are produced by neutron bombardment (symbol n) of bismuth (chemical symbol Bi, also "bismuth"). This requires a very high neutron flux, which is only available in nuclear reactors. The production process is:

n + Bi-209 → Bi-210 → (ßdecay) Po-210.

Polonium is a silver-coloured heavy metal that occurs in a solid state in two different modifications (lattice structures). At room temperature, it is known as α-polonium with a density of 9.32 g/cm³. The melting temperature of polonium is 254 °C, the boiling temperature 962 °C.

For comparison: the heavy metal lead (chemical symbol Pb, Z = 82) has a density of 11.35 g/cm³, a melting temperature of 327 °C and a boiling temperature of 1749 °C.

Polonium-210 decays via alpha decay into the stable lead isotope 206 (Pb-206). The HWT of this process is 138.4 days. The alpha radiation emitted during decay has an energy of 5.304 MeV (MeV: mega-electron volts). In 0.0012 % of all decays, the alpha radiation has a lower energy of 4.516 MeV and gamma radiation with an energy of 0.803 MeV is emitted in addition to the alpha radiation.

Due to its comparatively short HWZ, polonium-210 has a very high specific activity. It is 1.67×10¹⁴ Bq/g (becquerels per gram). One microgram of Po-210 (1 µg, 1 millionth of a gram) therefore has an activity of 167 million becquerels (167 MBq). This means that 167 million alpha decays occur in 1 µg of Po-210 per second.

For comparison: the specific activity of plutonium-239 (Pu-239) is 2.3×10⁹ Bq/g. 1 µg of plutonium-239 therefore has an activity of only 2,300 becquerels.

Polonium-210 can enter the human body via the air we breathe (inhalation) or via food (ingestion). When ingested with food, about 10 % of the ingested activity is absorbed by the body. This activity is distributed throughout the body and is deposited in higher concentrations in the spleen, liver and kidneys.

Once ingested, an amount of activity is only slowly excreted by the body, mainly in the urine. The so-called biological half-life is 50 days. This means that the polonium concentration in the body would fall by half within 50 days as a result of metabolic processes alone. Added to this is the decrease in concentration due to radioactive decay with a physical half-life of 138.4 days. Together, this results in an effective half-life of approx. 37 days.

The alpha radiation of polonium-210 can already be completely shielded by thin layers of material (e.g. paper or clothing). Possible radiation exposure to the human body from outside can therefore only be caused by gamma radiation. However, as only 0.0012 % of all Po-210 decays produce gamma radiation, the radiation dose (absorbed radiation energy per mass) is low in the event of external exposure.

For example: if a person is one metre away from one microgram of polonium-210 for one hour, the resulting external radiation dose is approx. 2.4× ^10-10 Sv (Sievert). This is about 1/1000 of the natural radiation dose per hour. Transporting polonium-210 is therefore completely harmless for the carrier.

The situation is completely different if polonium-210 has been absorbed (incorporated) by the human body. In this case, the alpha radiation released during radioactive decay is completely absorbed within the body, resulting in a high radiation dose.

If, for example, one microgram of polonium-210 enters the body through respiration, the resulting radiation dose ("effective dose equivalent") is approximately (100 - 550) Sv, depending on the chemical compound in which the polonium is present. If one microgram of Po-210 is ingested with food, the radiation dose is approximately 200 Sv. Such high radiation doses lead to death within a short time due to so-called acute radiation damage.

Even much smaller radiation doses of a few sieverts can cause death through acute radiation damage. Accordingly, less than 0.1 micrograms mixed into the victim's food is sufficient for a fatal "poison attack" with polonium-210.

Direct detection of polonium-210 in the body is hardly possible as the alpha radiation cannot leave the human body and the intensity of the gamma radiation is too low. Detection is therefore carried out indirectly by analysing the excreted urine.

At the concentrations under discussion (activity intake in the range of 0.1 micrograms), such detection is easily possible in specialised laboratories. Addresses of such laboratories can be obtained from the Federal Office for Radiation Protection: www.bfs.de/.

1. Lide, D. R. [Ed.]: "CRC Handbook of Chemistry and Physics (CD-ROM Version 2006)", Taylor and Francis, Boca Raton, FL, 2006
2. Firestone, R. B; Baglin, C. M.; Chu, S. Y. [Eds.]: "Table of Isotopes", Wiley, New York and others, 1999
3. Magill, J.; Galy, J.: "Radioactivity, Radionuclides, Radiation", Springer, <st1:place w:st="on"><st1:state w:st="on">Berlin</st1:state></st1:place> and others, 2005
4. "Dosiskoeffizienten bei innerer Strahlenexposition für Einzelpersonen der Bevölkerung", published in the Federal Gazette of 28 August 2001, Supplement 160 a/b. PDF versions are available here.
5. Vogt, H.-G.; Schultz, H.: "Grundzüge des praktischen Strahlenschutzes", Carl Hanser Verlag, Munich, 1992
6. Ordinance on protection against the harmful effects of ionising radiation (Radiation Protection Ordinance, StrSchV)