Promethium

Promethium is a chemical element with the symbol Pm and atomic number 61. It is notable for being the only other radioactive element besides technetium which is followed by chemical elements that have stable isotopes.

Notable characteristics
Promethium's longest lived isotope 145Pm is a soft beta emitter with a half-life of 17.7 years. It does not emit gamma rays, but beta particles impinging on elements of high atomic numbers can generate X-rays, and a sample of 145Pm does produce some such soft X-ray radiation in addition to beta particles.

Pure promethium exists in two allotropic forms, and its chemistry is similar to other lanthanides. Promethium salts luminesce in the dark with a pale blue or greenish glow, due to their high radioactivity. Promethium can be found in traces in some uranium ores, as a fission product. Newly made promethium is also seen in the spectra of some stars.

Applications
Uses for promethium include:


 * As a beta radiation source for thickness gauges.
 * As a light source for signals that require reliable, independent operation (using phosphor to absorb the beta radiation and produce light).
 * In a nuclear battery in which photocells convert the light into electric current, yielding a useful life of about five years, using Pm-147.
 * Promethium(III) chloride (PmCl3) mixed with zinc sulfide (ZnS) was used for time as a major luminous paint for watches after radium was discontinued. This mixture is still occasionally used for some luminous paint applications (though most such uses with radioactive materials have switched to tritium for safety reasons).
 * Promethium has a possible future uses in portable X-ray sources, and as auxiliary heat or power sources for space probes and satellites (although the alpha emitter plutonium-238 has become standard for most space-exploration related uses – see Radioisotope thermoelectric generators).

History
The existence of promethium was first predicted by Bohuslav Brauner in 1902; this prediction was supported by Henry Moseley in 1914, who found a gap for a missing element which would have atomic number 61, but was unknown (however, Moseley of course had no sample of the element to verify this). Several groups claimed to have produced the element, but they could not confirm their discoveries because of the difficulty of separating promethium from other elements. Promethium was first produced and proved to exist at Oak Ridge National Laboratory (ORNL) in 1945 by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell by separation and analysis of the fission products of uranium fuel irradiated in the Graphite Reactor; however, being too busy with defense-related research during World War II, they did not announce their discovery until 1947. The name promethium is derived from Prometheus, the Titan, in Greek mythology, who stole the fire from Mount Olympus and brought it down to mankind. The name was suggested by Grace Mary Coryell, Charles Coryell's wife, who felt that they were stealing fire from the gods.

In 1963, ion-exchange methods were used at ORNL to prepare about 10 grams of promethium from nuclear reactor fuel processing wastes.

Today, promethium is still recovered from the byproducts of uranium fission; it can also be produced by bombarding 146Nd with neutrons, turning it into 147Nd which decays into 147Pm through beta decay with a half-life of 11 days.

Occurrence
Promethium can be formed as a product of uranium fission. Only trace amounts can be found in naturally occurring ores: a sample of pitchblende has been found to contain promethium at a concentration of four parts per quintillion (1018) by mass.

Promethium has also been identified in the spectrum of the star HR 465 in Andromeda, and possibly HD 101065 (Przybylski's star) and HD 965.

Compounds
Promethium compounds include:


 * Chlorides
 * PmCl3
 * Bromides
 * PmBr3
 * Oxides
 * Pm2O3

Isotopes
Thirty-six radioisotopes of promethium have been characterized, with the most stable being 145Pm with a half-life of 17.7 years, 146Pm with a half-life of 5.53 years, and 147Pm with a half-life of 2.6234 years. All of the remaining radioactive isotopes have half-lives that are less than 364 days, and the majority of these have half lives that are less than 27 seconds. This element also has 11 meta states with the most stable being 148Pmm (T½ 41.29 days), 152Pmm2 (T½ 13.8 minutes) and 152Pmm (T½ 7.52 minutes).

The isotopes of promethium range in atomic weight from 127.9482600 u (128Pm) to 162.9535200 u (163Pm). The primary decay mode before the longest-lived isotope, 145Pm, is electron capture, and the primary mode after is beta minus decay. The primary decay products before 145Pm are neodymium (Nd) isotopes and the primary products after are samarium (Sm) isotopes.

Stability of promethium isotopes
Besides technetium, promethium is one of the two elements with atomic number less than 83 that have only unstable isotopes, which is a rarely occurring effect of the liquid drop model and stabilities of neighbor element isotopes.

Precautions
Promethium must be handled with great care because of its high radioactivity. In particular, promethium can emit X-rays during its beta decay. Its half-life is less than that of plutonium-239 by a factor of about 1350, and its biological toxicity is correspondingly higher. Promethium has no biological role.