Promethium is a chemical element with the symbol Pm and atomic number 61. A gap in the periodic table was pointed out between neodymium at 60 and samarium at 62 by Bohuslav Brauner. After two false claims of discovery in 1926, in 1938 a few atoms of promethium were created but intentional production and chemical proof did not happen until 1945 at Ohio State University when promethium was extracted from irradiated uranium fuel. A sample of the metal was only produced in 1963. Promethium was named for “Prometheus,” the Titan in Greek mythology who stole fire from the god and gave it to humanity.
Promethium is a metallic member of the cerium group of the lanthanide series. It forms salts with oxygen, water, chloride, nitrogen and sulfur.
Promethium has no stable isotopes; all of its thirty eight isotopes are radioactive. They vary in atomic weight from 126 to 163. They have half-lives that range from around 200 billionths of a second for Pm-129 to 17.7 years for Pm-145. Most of the isotopes of promethium decay by emission of positrons (anti-electrons). Pm-145 also emits protons.
Promethium is extremely rare in nature, minute quantities being produced in uranium ores by the spontaneous fission of uranium-238. This process has contributed about half a kilogram of promethium to the curst of the Earth. Pm-150 can be produced by rare beta decay of neodymium and europium decay can produce Pm-147 via an alpha particle. Europium decay has produced about 12 grams of promethium in the history of the Earth. Promethium has been found in the spectra of some stars.
Promethium-147 is the only isotope of promethium that has industrial applications. It is produced by bombardment of unranium-235 with thermal neutrons, bombardment of uranium-238 with fast neutrons to trigger fast fission, and by decay of artificially produced neodymium-146. Pm-147 has a relatively long half-life with radiation that has a shallow penetration and it produces no gamma rays. Glowing hands and numbers for watches and gauges contain a phosphor activated by Pm-147 which causes it to emits light. Because beta emission does not cause the phosphor to age, such applications have years of stable light release. Atomic batteries utilize a sandwich of semiconductor material and Pm0147 and has about a 5 year useful lifespan. It is also used in batteries that convert light to electricity for use in watches, calculators, radios, etc. Pm-147 is used as a starter in the new compact fluorescent lamps. It can also be used to measure the thickness of materials such as plastic, sheet metal, rubber, textiles and paper. It is used in electric blanket thermostats.
Promethium has no biological function but it is dangerous to living systems due to x-rays emitted during beta decay. Sealed devices containing promethium are safe but if the seal is broken, precautions should be taken when handling the device.
Seiko watches with promethium:
