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temperature midpoint (on the Celsius scale)
The symbol for the element thulium.
The symbol for thulium.
A soft, silver-gray metallic element of the lanthanide series. One of its artificial radioactive isotopes is used as a radiation source in small, portable x-ray machines. Atomic number 69; atomic weight 168.934; melting point 1,545°C; boiling point 1,727°C; specific gravity 9.3; valence 2, 3. See Periodic Table.
(Tm), chemical element, rare-earth metal of the lanthanoid series of the periodic table. One of the rarest of the rare-earth elements, though more abundant than silver, thulium has few commercial uses. Natural thulium is wholly composed of the stable isotope thulium-169. Bombarded by neutrons, it becomes radioactive thulium-170 (128-day half-life), which ejects soft gamma radiation (0.084 million electron volts) resembling an X ray. It is useful in small portable X-ray units suitable for photographing bony tissues and for examining thin-walled machine parts and has been used by archaeologists to examine markings and symbols on ancient metallic artifacts.