Thulium, THOO lee uhm, is one of the rare-earth elements. It occurs with other rare-earth elements in such minerals as gadolinite, euxenite, and xenotime. Portable X-ray units use radioactive thulium. Such units require no electrical equipment, and they need to be recharged with thulium only once every few months. The Swedish scientist Per Cleve discovered thulium in 1879. The element's name comes from Thule, the Latin word for the northernmost part of the inhabitable world.