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Radium was used to make self-luminous paint from the early 20th century to about 1970. Promethium briefly replaced radium as a radiation source. Tritium is the only radiation source used in radioluminescent light sources today due to its low radiological toxicity and commercial availability.
Radioluminescent paint is a self-luminous paint that consists of a small amount of a radioactive isotope (radionuclide) mixed with a radioluminescent phosphor chemical. The radioisotope continually decays, emitting radiation particles which strike molecules of the phosphor, exciting them to emit visible light.
Radium was used in luminous paint until the 1960s, when it was replaced with the other radioisotopes mentioned above due to health concerns. [2] In addition to alpha and beta particles , radium emits penetrating gamma rays , which can pass through the metal and glass of a watch dial, and skin.
In Bloomsburg, it continued to produce items with luminescent paint using radium, strontium-90 and cesium-137 such as watch dials, instrument gauge faces, deck markers, and paint. [8] It ceased radium processing altogether in 1968, spinning off those operations as Nuclear Radiation Development Corporation, LLC, based in Grand Island, New York.
The luminous paint used on the dials contained a mixture of zinc sulfide activated with silver, and powdered radium, a product that the Radium Dial Company named Luma. However, unlike the US Radium Corporation, Radium Dial Company was specifically set up to only paint dials, and no other radium processing took place at the premises.
Radium, like radon, is radioactive and is found in small quantities in nature and is hazardous to life if radiation exceeds 20-50 mSv/year. Radium is a decay product of uranium and thorium. [2] Radium may also be released into the environment by human activity: for example, in improperly discarded products painted with radioluminescent paint.
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An inert, absorbent coating for laser heat treatment has also been developed that eliminates the fumes generated by conventional paint coatings during the heat-treating process with CO 2 laser beams. One consideration crucial to the success of a heat treatment operation is control of the laser beam irradiance on the part surface.