Here is more about thulium. After radioactive promethium, thulium is the second rarest of the lanthanide (rare earth) elements. It it used in the delightfully named “Holmium-chromium-thulium triple-doped yttrium aluminium garnet” infrared lasers and, activated by neutrons in a nuclear reactor to produce the isotope thulium-170, as a radioactive source in portable X-ray equipment.
