User:Ryan pelo/Tritium radioluminescence

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 phosphoresce.

During manufacture, a length of borosilicate glass tube that has had the internal surface coated with a phosphor-containing material is filled with radioactive tritium. A commonly used phosphor is zinc-sulfide, which can be doped with other metals to produce different colors. The tube is then sealed at the desired length using a carbon dioxide laser. 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. Radium was used to make self-luminous paint from the early years of the 20th century until approximately 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.

Various preparations of the phosphor compound can be used to produce different colors of light. For example, doping zinc sulfide with different metals can change the emission wavelength. Some of the colors that have been manufactured in addition to the common phosphors are green, red, blue, yellow, purple, orange, and white.

The GTLSs used in watches give off a small amount of light: Not enough to be seen in daylight, but visible in the dark from a distance of several meters. The average such GTLS has a useful life of 10–20 years. Being an unstable isotope with a half-life of 12.32 years, the rate of beta emissions decreases by half in that period. Additionally, phosphor degradation will cause the brightness of a tritium tube to drop by more than half in that period. The more tritium that is initially placed in the tube, the brighter it is to begin with, and the longer its useful life. Tritium exit signs usually come in three brightness levels guaranteed for 10, 15, or 20 year useful life expectancies. The difference between the signs is how much tritium the manufacturer installs.

The light produced by GTLSs varies in color and size. Green usually appears as the brightest color with a brightness as high as 2 cd/m2 and red appears the least bright. For comparison, Most consumer desktop liquid crystal displays have luminances of 200 to 300 cd/m2. Sizes range from tiny tubes small enough to fit on the hand of a watch to ones the size of a pencil. Large tubes (5 mm diameter and up to 100 mm long) are usually only found in green, and can surprisingly be not as bright as the standard 22.5 mm × 3 mm sized tritium; this smaller size is usually the brightest and is used mainly in keychains available commercially.