Thulium: The X-Ray Visioned Rare Earth?

Given that it is the rarest of the rare earth metals, is thulium more famous for its portable X-ray related use than any of its rather relatively “obscure” applications? 

By: Ringo Bones 

Even though in terms of its abundance in the Earth’s crust, thulium does truly qualify as the rarest of the rare earth metals, although in truth, it is only slightly scarcer than the halogen iodine. As a reminder to the uninitiated, the term “rare earth” is actually a misnomer – the widespread use of the term arose near the end of the 19^th Century when the chemists who first discovered these rare earth elements used to prepare the elements’ oxides, which were, at first, taken from the elements themselves. Thulium was discovered as a distinct chemical element back in 1879 by Per Theodore Cleve. Atomic number 69 and chemical symbol Tm, its name is derived from Thule or Northland. For the first half of the 20^th Century, thulium was only known as mere “impurity” in your run-of-the-mill misch metal alloys destined for pocket cigarette-lighter flint production. 

Thulium became more well-known to the unsuspecting public when in 1954, a portable X-ray unit was developed which employs radioactive thulium as its source of radiation – produced by irradiating a pure sample of the metal inside a nuclear reactor. Like those small nuclear reactors often found in some Ivy League university physics labs that costs 200 US dollars an hour to run. A small amount – usually button-sized specimen - of this thulium radioisotope that gives off X-rays is encased in a lead-lined compartment which affords full protection to personnel involved in its operation, produces X-ray radiographs without the use of electricity, water or darkroom facilities. The unit, which weighs only 40 pounds, is simple to operate and produces a finished radiograph ready for inspection in five to ten minutes. The “hot” thulium radioisotope used in portable X-ray machines is replaced every few months or so since it spontaneously decays into a more stable element that no longer gives off X-rays.