Promethium has possible future uses in portable X-ray sources, what is significant about Promethium?

Question

Promethium has possible future uses in portable X-ray sources, and as auxiliary heat or power sources for space probes and satellites. what about promethium that makes it such a good source for this or why would scientists consider using it in the future?

Answer 1

The standard for space-based radioisotope thermal electric generators is plutonium 238. Its half life of 87.7 years and high energy density make it just about ideal for a long-duration space mission. (Using an isotope of < 10 yr half life results in a generator with too short of a useful life, anything longer than a few centuries and the output would be too low.) Also 238Pu is an alpha emitter so is very easily shielded. It has excellent energy density -- good for space missions. It is hard to see another isotope taking its place. (238Pu is no good for weapons so there are no proliferation issues. 239Pu is the bad guy.)

I did find a 40 year old patent to use promethium 147 as a beta and x-ray source. I doubt if anything came of it. In principle a beta source such as 147Pm could be used to generate x-rays by directing the beta emission (electrons) into a high-Z metal (such as tungsten). You would have an x-ray source without the usual high voltage supply and insulation. With a half life of 2.6 years it would be only good for about that long before you needed a fresh supply, sounds way too expensive.

Since that patent was written, high voltage power supplies have become much more efficient, reliable, compact and cheap (the miracle of modern electronics). So there are much more practical ways of producing x-rays than resorting to radioisotopes. (Radioisotopes have no OFF switch and have become a regulatory nightmare regardless if they pose a real threat or not.)

Other than all that, promethium is cool simply because it is a relatively low atomic weight element that has no stable or even long-life isotopes, so it does not exist naturally on earth. If a meteorite landed on earth containing even traces of promethium... that would be the news of the century.