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History

Promethium - (Prometheus, who, according to mythology, stole fire from heaven).   The existence of promethium was first predicted by Bohuslav Brauner in 1902; this prediction was supported by Henry Mosely in 1914, who developed an x-ray method for determining integer atomic numbers of elements and 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).

Historical claims for the discovery of element 61 create an interesting trail from around 1925 in Florence (suggested name: florentium) to America in 1926 (suggested name: illinium).  None of the claims, however, could be substantiated and today we know they were not simply a result of fleetingly small samples but rather poor work.

While spectral lines of promethium are evident in the light from some stars, it now seems apparent that no promethium is found in accessible areas of the earth--hence the difficulty in finding any.  Initial attempts at synthesis of element 61 in a cyclotron at Ohio State University in 1941 led to the suggested name cyclonium. But the recognized synthesis and identification finally came at Oak Ridge.

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.  They discovered promethium while analyzing the byproducts of uranium fission that were produced in a nuclear reactor located at Clinton Laboratories in Oak Ridge, Tennessee.  Today, Clinton Laboratories is known as Oak Ridge National Laboratory.  The name promethium is derived from Prometheus in Greek mythology, who stole the fire of the sky and gave it 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.

Characteristics

Promethium has been identified in the spectrum of the star HR 465 in Andromeda, and possibly HD 101065 (Przybylski's Star) and HD 965.  This element is being formed recently near the star's surface, for no known isotope of promethium has a half-life longer than 17.7 years.   ¹⁴⁵Pm has a specific activity of 940 Ci/g.   It is a soft beta emitter; although no gamma rays are emitted, X-radiation can be generated when beta particles impinge on elements of a high atomic number, and great care must be taken in handling it.  Promethium salts luminesce in the dark with a pale blue-or greenish glow, due to their high radioactivity.  Ion-exchange methods led to the preparation of about 10 g of promethium from atomic reactor fuel processing wastes in early 1963.  Little is yet generally known about the properties of metallic promethium.  Two allotropic modifications exist.

Occurrence

Promethium has one semi-stable isotope, ¹⁴⁵Pm, it is a soft beta emitter; it does not emit gamma rays, but beta particles impinging on elements of high atomic numbers can generate X-rays.  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.

Today, promethium is still recovered from the byproducts of uranium fission. It can also be produced by bombarding neodymium-146 with neutrons.   Neodymium-146 becomes neodymium-147 when it captures a neutron. Neodymium-147, with a half-life of 11 days, decays into ¹⁴⁷Pm through beta decay.  Promethium does not occur naturally on earth, although it has been detected in the spectrum of a star in the constellation Andromeda.

Applications

The element has applications as a beta source for thickness gages, and it can be absorbed by a phosphor to produce light.  Light produced in this manner can be used for signs or signals that require dependable operation; it can be used as a nuclear-powered battery by capturing light in photocells which convert it into electric current.  Such a battery, using ¹⁴⁷Pm, would have a useful life of about 5 years.  It is being used for fluorescent lighting starter sand coatings for self-luminous watch dials.  Promethium shows promise as a portable X-ray source, and it may become useful as a heat source to provide auxiliary power for space probes and satellites.  Other uses:

• Light source for signals that require reliable, independent operation (using phosphor to absorb the beta radiation and produce light). 
• Promethium (III) chloride (PmCl₃) mixed with zinc sulfide (ZnS) was used for a while as luminous paint for watches after radium was discontinued.  Still used for some luminous paint applications.

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.

Compounds

More than 30 promethium compounds have been prepared.  Most are colored.  ¹⁴⁷Pm is available upon special order from the Idaho National Engineering, Laboratory, Idaho Falls, ID, or from the Westinghouse Hanford Co., Richland, WA. 

Isotopes

38 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 ¹⁴⁸Pm^m (T_½ 41.29 days), ¹⁵²Pm^m2 (T_½ 13.8 minutes) and ¹⁵²Pm^m (T_½ 7.52 minutes).  ¹⁴⁵Pm decays into neodymium-145 through electron capture.

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

Beside technetium, promethium is one of the two elements with atom number less than 83 that solely has unstable isotopes, which is a rarely occurring effect of the liquid drop model and stabilities of neighbor element isotopes.

There are no significant commercial uses of the metal and so very little has been produced except for theoretical studies.

Precautions

Promethium must be handled with great care because of its high radioactivity.   In particular, promethium can emit X-rays during its beta decay.

Note that its half-life is less than that of plutonium-239 by a factor of multiple thousands to tens of thousands.  Promethium has no biological role.