Tritium radioluminescence 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, light is emitted through the process of phosphorescence. The overall process of using a radioactive material to excite a phosphor and ultimately generate light is called radioluminescence. As tritium illumination requires no electrical energy, it has found wide use in applications such as emergency exit signs, illumination of wristwatches, and portable yet reliable sources of low intensity light which will not degrade human night vision. Gun sights for night use and small lights used mostly by military personnel fall under the latter application.
History Tritium was found to be an ideal energy source for self-luminous compounds in 1953 and the idea was patented by Edward Shapiro on 29 October 1953, in the US (2749251 – Source of Luminosity).
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), or tritium lamp. The tritium in a gaseous tritium light source undergoes beta (β) 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 tritium. 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 β 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 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. Various preparations of the phosphor compound can be used to produce different colors of light. For example, doping zinc sulfide phosphor 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. The rate of β emissions decreases by half in each half-life (12.33 years). Also, phosphor degradation will cause the brightness of a tritium tube to drop by more than half in that period. The more tritium 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 is due to the lower concentration and high cost of tritium; this smaller size is usually the brightest and is used mainly in keychains available commercially.
Uses
These light sources are most often seen as "permanent" illumination for the hands of wristwatches intended for diving, nighttime, or combat use. They are also used in glowing novelty keychains and in self-illuminated exit signs. They are favored by the military for applications where a power source may not be available, such as for instrument dials in aircraft, compasses, and sights for weapons. In the case of solid tritium light sources, the tritium replaces some of the hydrogen atoms in the paint, which also contains a phosphor such as zinc sulfide. Tritium lights or beta lights were formerly used in fishing lures. Some flashlights have slots for tritium vials so that the flashlight can be easily located in the dark. Tritium is used to illuminate the iron sights of some small arms. The reticle on the SA80's optical SUSAT sight as well as the LPS 4x6° TIP2 telescopic sight of a PSL rifle, contains a small amount of tritium for the same effect as an example of tritium use on a rifle sight. The electrons emitted by the radioactive decay of the tritium cause phosphor to glow, thus providing a long-lasting (several years) and non-battery-powered firearms sight that is visible in dim lighting conditions. The tritium glow is not noticeable in bright conditions such as during daylight, however; consequently some manufacturers have started to integrate fiber optic sights with tritium vials to provide bright, high-contrast firearms sights in both bright and dim conditions. In addition to its widespread use in watch dials and weapon sights, tritium has also found niche applications in the jewelry industry. Its self-illuminating properties allow it to glow continuously for years without requiring an external power source, making it suitable for glow-in-the-dark rings and other accessories. These pieces are especially favored in contexts where both aesthetics and low-light visibility are desired.
Safety
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