Tritium illumination

Tritium illumination is the use of gaseous tritium, a radioactive isotope of hydrogen, to create visible light. Tritium emits electrons through beta decay, and, when they interact with a phosphor material, fluorescent light is created, a process called radioluminescence. As tritium illumination requires no electrical energy, it found wide use in applications such as emergency exit signs and illumination of wristwatches. More recently, many applications using radioactive materials have been replaced with photoluminescent materials.

Design

Tritium lighting is made using glass tubes with a phosphor layer in them and tritium gas inside the tube. Such a tube is known as a "gaseous tritium light source" (GTLS), or beta light, (since the tritium undergoes beta decay).

The tritium in a gaseous tritium light source undergoes beta decay, releasing electrons that cause the phosphor layer to fluoresce.

During manufacture, a length of borosilicate glass tube that has had the inside surface coated with a phosphor-containing material is filled with the radioactive tritium. The tube is then fused with a carbon dioxide laser at the desired length. Borosilicate is preferred for its strength and resistance to breakage. In the tube, the tritium gives off a steady stream of electrons due to beta decay. These particles excite the phosphor, causing it to emit a low, steady glow. Tritium is not the only material that can be used for self-powered lighting. Other beta particle-emitting radioisotopes can also serve. Radium was used to make self-luminous paint from the early years of the 20th Century until approximately 1970, but it has been replaced by tritium, which is less hazardous.