Promethium (Pm) has no stable isotopes, and does not exist in nature, except in trace quantities as a product of spontaneous fission of 238U and 235U and alpha decay of 151Eu. It is a synthetic element, first produced in 1945. Thirty-eight radioisotopes 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 365 days, and the majority of these have half-lives that are less than 30 seconds. This element also has 18 meta states with the most stable being 148mPm (t1/2 41.29 days), 152m2Pm (t1/2 13.8 minutes) and 152mPm (t1/2 7.52 minutes).
Contents
The isotopes of promethium range in atomic weight from 125.95752 u (126Pm) to 162.95368 u (163Pm). The primary decay mode before the longest-lived isotope, 145Pm, is electron capture, and the primary mode after is beta decay. The primary decay products before 145Pm are isotopes of neodymium and the primary products after are isotopes of samarium.
Since promethium does not exist in nature, a relative atomic mass cannot be given.
Stability of promethium isotopes
Promethium is one of the two elements of the first 82 elements that have no stable isotopes; the other is technetium (Z = 43). This is a rarely occurring effect of the liquid drop model.
Promethium-147
Promethium-147 has a half-life of 2.62 years, and is a fission product produced in nuclear reactors via beta decay from neodymium-147. The isotopes 142Nd, 143Nd, 144Nd, 145Nd, 146Nd, 148Nd, and 150Nd are either stable or nearly so, so the isotopes of promethium with those masses cannot be produced by beta decay and therefore are not fission products in significant quantities. 149Pm and 151Pm have half-lives of only 53.08 and 28.40 hours, so are not found in spent nuclear fuel that has been cooled for months or years.
Promethium-147 is used as a beta particle source and a radioisotope thermoelectric generator (RTG) fuel; its power density is about 2 watts per gram. Mixed with a phosphor, it was used to illuminate Apollo Lunar Module electrical switch tips and painted on control panels of the Lunar Roving Vehicle.