Polonium-210

Polonium-210 (²¹⁰Po, Po-210, historically radium F) is an isotope of polonium. It undergoes alpha decay to stable ²⁰⁶Pb with a half-life of 138.376 days (about months), the longest half-life of all naturally occurring polonium isotopes (^210–218Po).

In 1943, ²¹⁰Po was studied as a possible neutron initiator in nuclear weapons, as part of the Dayton Project. In subsequent decades, concerns for the safety of workers handling ²¹⁰Po led to extensive studies on its health effects.

In the 1950s, scientists of the United States Atomic Energy Commission at Mound Laboratories, Ohio explored the possibility of using ²¹⁰Po in radioisotope thermoelectric generators (RTGs) as a heat source to power satellites. A 2.5-watt atomic battery using ²¹⁰Po was developed by 1958. However, the isotope plutonium-238 was chosen instead, as it has a longer half-life of 87.7 years.

Polonium-210 was used to kill Russian dissident and ex-FSB officer Alexander V. Litvinenko in 2006, and was suspected as a possible cause of Yasser Arafat's death, following exhumation and analysis of his corpse in 2012–2013. The radioisotope may also have been used to kill Yuri Shchekochikhin, Lecha Islamov and Roman Tsepov.

Decay properties

²¹⁰Po is an alpha emitter that has a half-life of 138.376 days; (that is, a milligram is 5 curies). A few curies of ²¹⁰Po emit a blue glow caused by excitation of surrounding air.

²¹⁰Po occurs in minute amounts in nature, where it is the penultimate isotope in the uranium series decay chain. It is generated via beta decay from ²¹⁰Pb and ²¹⁰Bi.

The astrophysical s-process is terminated by the decay of ²¹⁰Po, as the neutron flux is insufficient to lead to further neutron captures in the short lifetime of ²¹⁰Po. Instead, ²¹⁰Po alpha decays to ²⁰⁶Pb, which then captures more neutrons to become ²¹⁰Po and repeats the cycle, thus consuming the remaining neutrons. This results in a buildup of lead and bismuth, and ensures that heavier elements such as thorium and uranium are only produced in the much faster r-process.

Production

Deliberate

Although ²¹⁰Po occurs in trace amounts in nature, it is not abundant enough (0.1 ppb) for extraction from uranium ore to be feasible. Instead, most ²¹⁰Po is produced synthetically, through neutron bombardment of ²⁰⁹Bi in a nuclear reactor. This process converts ²⁰⁹Bi to ²¹⁰Bi, which has a half-life of five days and beta decays to ²¹⁰Po. Through this method, it was reported in February 2007 that approximately of ²¹⁰Po was produced in Russia and shipped to the United States every month for commercial applications. 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 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, used for neutralizing static electricity on materials like photographic film, contain a few microcuries of ²¹⁰Po as a source of charged particles. ²¹⁰Po was also used in initiators for atomic bombs through the (α,n) reaction with beryllium. Small neutron sources reliant on the (α,n) reaction also usually use polonium as a convenient source of alpha particles due to its comparatively low gamma emissions (allowing easy shielding) and high specific activity.

Hazards

²¹⁰Po is extremely toxic; it and other polonium isotopes are some of the most radiotoxic substances to humans. With one microgram of ²¹⁰Po being more than enough to kill the average adult, it is 250,000 times more toxic than hydrogen cyanide by weight. This is a consequence of its ionizing alpha radiation, as alpha particles are especially damaging to organic tissues inside the body. However, ²¹⁰Po does not pose a radiation hazard when kept outside the body. The alpha particles it produces cannot penetrate the outer layer of dead skin cells.

The toxicity of ²¹⁰Po stems entirely from its radioactivity. It is not chemically toxic in itself, but its solubility in aqueous solution as well as that of its salts poses a hazard because its spread throughout the body is facilitated in solution.

In the environment, ²¹⁰Po can accumulate in seafood. It has been detected in various organisms in the Baltic Sea, where it can propagate in, and thus contaminate, the food chain. Heavy smokers may be exposed to the same amount of radiation (estimates vary from 100 µSv per year) as individuals in Poland were from Chernobyl fallout traveling from Ukraine.

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