Radium is a dial coated with glowing paint that contains radium-226, a radioactive isotope. It is found on watches, clocks and instruments. Mixed with zinc sulphide, the radium made dials and hands glow. Old radium dials stay radioactive after the glow has faded.
How does radium paint glow?
Radium paint mixes radium with a luminescent material. Zinc sulphide powder is the one used most often. The glow comes from combining the two. Radiation from the radium puts energy into the zinc sulphide crystals, and the crystals give that energy off as light. A typical self-luminous watch with radium paint holds about one microgram of radium.
Why do old radium dials stop glowing?
The glow weakens with age. The radiation that makes the crystals shine also damages them, so over time they give off less light. The radium itself lasts far longer. Its half-life is around 1,600 years, so a dial that no longer glows is still radioactive. In the 1970s radium was replaced by tritium-based luminous material, which was considered safer. Radioactive luminous paints made today use tritium or promethium-147 instead of radium.
Where it comes from
The word radium dates to around 1899. It comes from French and is built on the Latin radius, meaning ray. The ORAU Health Physics Museum credits William J. Hammer with inventing radioluminescent paint by combining radium and zinc sulphide. The paint was first applied to the dials and hands of clocks and watches. It was also used on aircraft switches and on instrument dials and panels. In 1914 von Sochocky and Willis set up the Radium Luminous Material Corporation, which made radium paint.
Checked