Over 25 isotopes of Polonium (Po) have been discovered, with the most common being Po-210. Although it is found naturally, the amount available is so exceedingly small (about 1 part in 1015 concentration of earth crust) that any useful quantity is man-made. [1] Po-210 is created by bombarding Bismuth-209 with neutrons (typically in a nuclear reactor or particle accelerator) resulting in Bismuth-210 which then decays into Po-210. [2] Other Po isotopes, such as Po-208 and Po-238, can also be created but are more expensive and less useful due to their longer half-lives and thus reduced specific activity (2.9 years, 590 Ci/g and 88 years, 15 Ci/g respectively). [2] Po-210 is an alpha emitter with a half-life of 140 days and a specific activity of 4,500 Ci/g. [2] By taking advantage of its high radioactivity, Po-210 has been used in three predominant applications: as a static eliminator, as a heat/energy source, and as a neutron source when combined with a low atomic weight element. Although Po-210 is still found in some of these applications today, most of its uses are historical due to advancements in technology and concerns about safety. In addition to these uses, Po-210 has also been linked to use as a poison.
Static eliminators ranging from handheld brushes to large machines have been built using Po-210. A small amount of the Po-210 is plated onto a thin foil which is then inserted into the device. Since Po-210 is an alpha emitter, the air around the device is ionized resulting in the neutralization of static electricity in the environment. Large devices have been used in areas where a significant amount of static electricity is generated (paper and plastic sheet production) and areas extremely sensitive to small amounts of static electricity (silicon chip production). Due to the short half-life of Po-210 these devices must be replaced periodically. Although effective, safety concerns and cost have led to the use of beta emitters or non-radioactive methods as an alternative.
The high alpha emission rate of some Po isotopes makes them a compact and lightweight source of energy. For example, one gram of Po-210 produces 137W of heat and 1/2 gram of Po-210 will reach 500 °C. [2] This makes Po-210 a good energy source candidate in spacecraft/satellites where volume and weight are critical. Traditionally, Po-238 has been used as an energy source in space exploration but other isotopes, including Po-210, have been investigated as well. [2] There has also been interest in coupling a Po source with a thermoelectric cell to produce power for spacecraft. However, this solution is losing traction since the short half-life of most Po isotopes precludes their use in long missions.
When combined with a low-atomic weight element (most commonly beryllium), Po-210 can be utilized as a neutron source. Alpha particles emitted from Po-210 are absorbed by the second element, emitting a neutron in the process. The polonium-beryllium pair was used in early nuclear weapons as a neutron initiator. The two elements were kept separated by materials such as nickel and gold until a chemical explosion caused a mixing of the two. [3] This neutron initiator was typically placed in the core of a fission fuel (such as plutonium) so that the onset of neutron emission coincided with the fuel reaching super-criticality (through compression). However, as with other uses of Po-210, the short half-life made it impractical to use since the shelf life of any weapon would be directly related to the half-life. Modern neutron emitters utilize a small-scale linear accelerator, or "neutron tube," which has a much longer shelf life and the added benefit of being able to control the neutron emission rate. [4]
The use of Po-210 as a poison was made famous in the popular media with the murder of Alexander Litvinenko in 2006. [5] Since Po-210 is an alpha emitter, it is relatively harmless exterior to the body. However, if it is consumed or inhaled, the alpha radiation can cause severe damage to internal cells. It was reported that Litvinenko was poisoned with a small amount of Po-210 and initial medical tests did not determine the cause since Po-210 is a very weak gamma emitter (typical radiation measuring equipment relies on gamma radiation). Since it is extremely difficult to produce any appreciable amount of Po-210 and is typically easily traced, it is unlikely to become a common weapon or poison.
© Chris Yu. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. C. Brasted, "Polonium (Po)," Encyclopedia Britannica, 2010.
[2] R.C. O'Brien, et. al., "Safe Radioisotope Thermoelectric Generators and Heat Sources for Space Applications," J. Nuc. Materials, Vol 377, No. 3, 2008.
[3] R. L. Miller, "Under The Cloud: The Decades of Nuclear Testing," New York: Free Press, 1986.
[4] G. Bauwens, A Noblet, and G. Sylin, "A Compact Accelerator-Type Neutron Generator," J. Phys E.: Sci Instrum., Vol 18, 1985.
[5] D. Schorn, " Who Killed Alexander Litvinenko?," CBS News, 7 Jan 07.