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 (^{210–218}Po). 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 is 166 TBq/g, i.e., . At the same time, ²¹⁰Po is not readily detected by common radiation detectors, because its gamma rays have a very low energy. Therefore, 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.

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 beta decays to ²¹⁰Po with a five-day half-life. 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.

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.

Applications

A single gram of ²¹⁰Po generates 140 watts of power (as heat). 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. Intake of ²¹⁰Po occurs primarily through contaminated air, food, or water, as well as through open wounds. Once inside the body, ²¹⁰Po concentrates in soft tissues (especially in the reticuloendothelial system) and the bloodstream. Its biological half-life is approximately 50 days.

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. ²¹⁰Po is also known to contaminate vegetation, primarily originating from the decay of atmospheric radon-222 and absorption from soil.

In particular, ²¹⁰Po attaches to, and concentrates in, tobacco leaves. Elevated concentrations of ²¹⁰Po in tobacco were documented as early as 1964, and cigarette smokers were thus found to be exposed to considerably greater doses of radiation from ²¹⁰Po and its parent ²¹⁰Pb. Heavy smokers may be exposed to the same amount of radiation (estimates vary from 100 Ã‚µSv to 160 mSv per year) as individuals in Poland were from Chernobyl fallout traveling from Ukraine. As a result, ²¹⁰Po is most dangerous when inhaled from cigarette smoke.

References