Promethium is also used to measure the thickness of materials by evaluating the amount of radiation from a promethium source that passes through the sample.
Where indefinite self-luminosity in darkness was required, safer radioactive promethium paint was initially used, later replaced by tritium which continues to be used today.
Lighter unstable isotopes of samarium primarily decay by electron capture to isotopes of promethium, while heavier ones convert through beta decay to isotopes of europium.