Polonium (Molecule of the Month for November 2006)
Polonium is in the news this week since it has been linked to the fatal assination of the Russian ex-spy Alexander Litvinenko who was found "major dose" of radioactive polonium-210 in his body. Polonium is a very rare natural element, it is formed through the decay of radioactive uranium and thorium. Uranium ores contain only about 100 micrograms of the element per ton. Polonium-210 is a low-melting, fairly volatile metal, 50% of which is vaporized in air within two days. It is an alpha emitter with a half-life of 138.39 days. A milligram emits as many alpha particles as 5 g of radium. The energy released by its decay is so large (140W/g) that a capsule containing about half a gram reaches a temperature above 500C. A few micro grams of polonium exhibit a blue glow, caused by excitation of the surrounding gas. Polonium-210 is very dangerous to handle in even milligram or microgram amounts, and special equipment and strict control is necessary. Damage arises from the complete absorption of the energy of the alpha particle into tissue. The maximum permissible body burden for ingested polonium is only 0.03 microcuries, which represents a particle weighing only 0.000 000 000 068 g. Weight for weight it is about 250 billion times as toxic as hydrocyanic acid.
Nobel Prize winners Marie and Pierre Curie discovered the radioactive elements radium and polonium. Before this discovery, uranium and thorium were the only known radioactive elements. While studying uranium minerals (pitchblend from Joachimsthal, Bohemia). Marie Curie noticed two minerals were much more radioactive than uranium itself. She and her husband, Pierre, chemically separated the compounds in the minerals and found a substance 400 times more radioactive than uranium. Marie named this substance polonium, after her native country of Poland. It was the discovery of polonium and radium that lead Marie Curie to discovery of radioactivity. She later becoming the first two-time Nobel laureate and the only person with Nobel Prizes in two different fields of science (physics and chemistry). She is one of only two people who has been awarded a Nobel Prize in two different fields, the other being Linus Pauling. As of 2006 she remains the only woman to win two Nobel prizes. In 1949, scientists working on the Dayton Project discovered that when they bombarded natural bismuth - 209Bi - with neutrons, Po -210 was obtained. Milligram amounts of polonium may now be prepared this way, by using the high neutron fluxes of nuclear reactors. Polonium is available commercially on special order from the Oak Ridge National Laboratory.
Because almost all alpha radiation is stopped within the solid source and its container, giving up its energy, polonium has attracted attention for uses as a lightweight heat source for thermoelectric power in space satellites. The element has been used in devices for eliminating static charges in textile mills, etc.; however, beta sources are both more commonly used and less dangerous. It is also used on brushes for removing dust from photographic films. Polonium can be mixed or alloyed with beryllium to provide a source of neutrons and was used in the Manhattan Project to build an atomic bomb. Polonium-210 was vital to this program, because it was to be used in a neutron source that would ensure initiation of a chain reaction. Initiators made of polonium-210 and beryllium were located at the center of the fissile cores of early atomic weapons. The highly radioactive isotope of Polonium (Po-210) is a strong alpha emitter. Beryllium will absorb alphas and emit neutrons. This isotope of polonium has a half life of almost 140 days, and a neutron initiator using this material needs to have the polonium, which is generated in a nuclear reactor, to be replaced frequently. To supply the initiation pulse of neutrons at the right time, the polonium and the beryllium need to be kept apart until the appropriate moment and then thoroughly and rapidly mixed by the implosion of the weapon.
Update by Karl Harrison
(Molecule of the Month for November 2006 )