Thulium

Thulium is the least abundant lanthanide that has stable isotopes, though "rare" is relative — it is still more common in the crust than silver or gold. Bombard it with neutrons and you get thulium-170, a 128-day beta emitter whose bremsstrahlung gives a portable X-ray source needing no power supply, used for radiographing welds and castings where a proper X-ray set cannot be dragged. Thulium-doped fibre lasers around 2000 nm have become the standard for cutting and welding polymers and for surgical lithotripsy, and Tm³⁺ supplies the blue component in the anti-counterfeiting phosphors printed on euro banknotes. It is named for Thule, the classical name for the far north.