Tritium (Symbol T or 3H, also known as hydrogen-3) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of protium (by far the most abundant hydrogen isotope) contains one proton and no neutrons. Naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays. The name of this isotope is formed from the Greek word "tritos" meaning "third".
The emitted electrons from the radioactive decay of small amounts of tritium cause phosphors to glow so as to make self-powered lighting devices called betalights, which are now used in firearm night sights, watches, exit signs, map lights, knives and a variety of other devices. Commercial demand for tritium is 400 grams per year and the cost is approximately US $30,000 per gram.
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.
In Commonwealth military use, particularly when referring to illuminated sighting devices using tritium, the term TRILUX is often used
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 in the past to make self-luminous paint, but it has been replaced by tritium, which is less hazardous.
Various preparations of the phosphor compound can be used to produce different colors of light. Some of the colors that have been manufactured in addition to the common phosphors are green, red, blue, yellow, purple, orange, and white.
The types of GTLS used in watches give off a small amount of light: not enough to be seen in daylight, but enough to be visible in the dark from a distance of several meters. The average such GTLS has a useful life of 10–20 years. As the tritium component of the lighting is often more expensive than the rest of the watch itself, manufacturers try to use as little as possible. Being an unstable isotope with a half-life of 12.32 years, tritium loses half its brightness 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 colour and size, green are usually the brightest color and white usually the least, sizes can be found ranging from tiny tubes small enough to fit on the hand of a watch to ones the size of a pencil, when it comes to large ones (5mm diameter and up to 100mm long) they are usually only found in green and can surprisingly be not as bright as the standard 22.5mm x 3mm sized tritium, this smaller size is usually the brightest and is used mainly in key chains available commercially.
Safety: While these devices contain a radioactive substance, it is currently believed that self-powered lighting does not pose a significant health concern. A 2007 report by the UK government's Health Protection Agency Advisory Group on Ionizing Radiation declared the health risks of tritium exposure to be double that previously set by the International Commission on Radiological Protection, but encapsulated tritium lighting devices, typically taking the form of a luminous glass tube embedded in a thick block of clear plastic, prevent the user from being exposed to the tritium at all unless the device is broken apart.
Tritium presents no external radiation threat via beta radiation when encapsulated in non-hydrogen-permeable containers due to its low penetration depth, which is insufficient to penetrate intact human skin. However, GTLS devices do emit low levels of X-rays due to bremsstrahlung. The primary danger from tritium arises if it is inhaled, ingested, injected, or absorbed into the body. This results in the absorption of the emitted radiation in a relatively small region of the body, again due to the low penetration depth. The biological half-life of tritium—the time it takes for half of an ingested dose to be expelled from the body—is low, at only 12 days. Tritium excretion can be accelerated further by increasing water intake to 3–4 liters/day.
Direct, short-term exposure to small amounts of tritium is relatively harmless. If a tritium tube breaks, one should leave the area and allow the gas to diffuse into the air. Tritium exists naturally in the environment, but in very small quantities. Persons working with the gas face another hazard: tritium reacts with the oxygen in air, forming tritiated water. This moisture is readily ingested by the body. When in contact with any hydrocarbon the tritium atom replaces the natural hydrogen. This can even occur with the natural oil on a person's skin.