Promethium (61Pm) is an artificial element, except in trace quantities as a product of spontaneous fission of 238U and 235U and alpha decay of 151Eu, and thus a standard atomic weight cannot be given.
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 all stable with respect to beta decay, so the isotopes of promethium with those masses cannot be produced by beta decay and therefore are not fission products in significant quantities (they could only be produced directly, rather than along a beta-decay chain).
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.
[5] 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.