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 4+1⁄2 months), the longest half-life of all naturally occurring polonium isotopes (210–218Po). First identified in 1898, and also marking the discovery of the element polonium, 210Po is generated in the decay chain of uranium-238 and radium-226. 210Po is a prominent contaminant in the environment, mostly affecting seafood and tobacco. Its extreme toxicity is attributed to intense radioactivity, mostly due to alpha particles, which easily cause radiation damage, including cancer in surrounding tissue. The specific activity of 210Po is 166 TBq/g, i.e., 1.66×1014 Bq/g. At the same time, 210Po is not readily detected by common radiation detectors, because its gamma rays have a very low intensity. Therefore, 210Po can be considered as a quasi-pure alpha emitter.
History
In 1898, Marie and Pierre Curie discovered a strongly radioactive substance in pitchblende and determined that it was a new element; it was one of the first radioactive elements discovered. Having identified it as such, they named the element polonium after Marie's home country, Poland. Willy Marckwald discovered a similar radioactive activity in 1902 and named it radio-tellurium as it was chemically extracted with its homolog tellurium, and at roughly the same time, Ernest Rutherford identified the same activity in his analysis of the uranium decay chain and named it radium F (originally radium E). By 1905, Rutherford concluded that all these observations were due to the same substance, 210Po. Further discoveries and the concept of isotopes, first proposed in 1913 by Frederick Soddy, firmly placed 210Po as the penultimate step in the uranium series. 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; it decays directly to stable 206Pb. 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 decays results in the emission of a gamma ray.
84 210 P o → 138.376 d 82 206 P b + 2 4 H e {\displaystyle \mathrm {^{210}_{\ 84}Po\ {\xrightarrow[{138.376\ d}]{}}\ _{\ 82}^{206}Pb\ +_{\ 2}^{\ 4}He} }
This low gamma ray production rate makes it difficult to use for identification of the isotope; rather than gamma ray spectroscopy, alpha spectroscopy is the best method of measuring it. Owing to its much shorter half-life, a milligram of 210Po emits as many alpha particles per second as 5 grams of 226Ra (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 8 grams (0.28 oz) of 210Po was produced in Russia and shipped to the United States every month for commercial applications.
Byproduct The production of polonium-210 is a downside to reactors cooled with lead-bismuth eutectic rather than pure lead. However, given the eutectic properties of this alloy, some proposed Generation IV reactor designs still rely on lead-bismuth.
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