![Gaseous tritium light source](https://upload.wikimedia.org/wikipedia/commons/5/51/Gaseous_tritium_light_source.jpg)
Tritium radioluminescence is the use of gaseous
tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...
, a radioactive
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...
of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
, to create visible light. Tritium emits
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
s through
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
and, when they interact with a phosphor material, light is emitted through the process of
phosphorescence
Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluo ...
. The overall process of using a radioactive material to excite a phosphor and ultimately generate light is called
radioluminescence
Radioluminescence is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as alpha particles, beta particles, or gamma rays. Radioluminescence is used as a low level light source for night illumi ...
. As tritium illumination requires no electrical energy, it has found wide use in applications such as emergency
exit sign
An exit sign is a pictogram or short text in a public facility (such as a building, aircraft, or boat) denoting the location of the closest emergency exit to be used in case of fire or other emergency that requires rapid evacuation. Most rele ...
s, illumination of wristwatches, and portable yet very reliable sources of low intensity light which won't degrade human night vision. Gun sights for night use and small lights (which need to be more reliable than battery powered lights, yet not interfere with night vision or be bright enough to easily give away one's location) used mostly by military personnel fall under the latter application.
History
Tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...
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
![Glowing keychain traser](https://upload.wikimedia.org/wikipedia/commons/6/6c/Glowing_keychain_traser.jpg)
Tritium lighting is made using glass tubes with a
phosphor layer in them and
tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...
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
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
).
The tritium in a gaseous tritium light source undergoes
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
, releasing electrons that cause the phosphor layer to
phosphoresce.
During manufacture, a length of
borosilicate glass
Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
tube that has had the internal surface coated with a
phosphor-containing material is filled with radioactive tritium. The tube is then sealed at the desired length using a
carbon dioxide laser
The carbon-dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed. It was invented by Kumar Patel of Bell Labs in 1964 and is still one of the most useful types of laser. Carbon-dioxide lasers are the highest-power contin ...
. Borosilicate is preferred for its strength and resistance to breakage. In the tube, the tritium gives off a steady stream of
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
s 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
Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rathe ...
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 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. Being an unstable isotope with a
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...
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/m
2 and red appears the least bright. For comparison, most consumer desktop
liquid crystal displays
A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but i ...
have luminances of 200 to 300 cd/m
2. 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.
Uses
![FN57SIGHTS03](https://upload.wikimedia.org/wikipedia/commons/7/72/FN57SIGHTS03.jpg)
These light sources are most often seen as "permanent" illumination for the hands of
wristwatch
A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached by ...
es intended for diving, nighttime, or combat use. They are also used in glowing novelty
keychain
A keychain (also key fob or keyring) is a small ring or chain of metal to which several keys can be attached. The length of a keychain allows an item to be used more easily than if connected directly to a keyring. Some keychains allow one or b ...
s and in self-illuminated
exit sign
An exit sign is a pictogram or short text in a public facility (such as a building, aircraft, or boat) denoting the location of the closest emergency exit to be used in case of fire or other emergency that requires rapid evacuation. Most rele ...
s. They are favored by the military for applications where a power source may not be available, such as for instrument
dials in aircraft,
compass
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself wit ...
es, 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
vial
A vial (also known as a phial or flacon) is a small glass or plastic vessel or bottle, often used to store medication as liquids, powders or capsules. They can also be used as scientific sample vessels; for instance, in autosampler devices i ...
s 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
A reticle, or reticule also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the screen of an oscilloscop ...
on the
SA80
The SA80 (Small Arms for the 1980s) is a British family of 5.56×45mm NATO service weapons used by the British Army. The L85 Rifle variant has been the standard issue service rifle of the British Armed Forces since 1987, replacing the L1A1 Se ...
's optical
SUSAT
{{Other uses, Susat (disambiguation){{!Susat
The Sight Unit Small Arms, Trilux, or SUSAT, is a 4× telescopic sight, with tritium-powered illumination utilised at dusk or dawn. The full name of the current model is the SUSAT L9A1. The sight i ...
sight as well as the LPS 4x6° TIP2 telescopic sight of a
PSL rifle
The PSL ( ro, Puşcă Semiautomată 7,62 mm cu Lunetă, model 1974, "scoped semi-automatic rifle") is a Romanian military designated marksman rifle. It is also called PSL-54C, Romak III, FPK and SSG-97 (''Scharfschützengewehr'' 1997). Though sim ...
, 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. As a result, some manufacturers have started to integrate
fiber optic
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...
sights with tritium vials to provide bright, high-contrast firearms sights in both bright and dim conditions.
Safety
![Tritium-exit-sign](https://upload.wikimedia.org/wikipedia/commons/2/25/Tritium-exit-sign.jpg)
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
The Health Protection Agency (HPA) was a non-departmental public body in the United Kingdom. It was an organisation that was set up by the UK government in 2003 to protect the public from threats to their health from infectious diseases and envi ...
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
The International Commission on Radiological Protection (ICRP) is an independent, international, non-governmental organization, with the mission to protect people, animals, and the environment from the harmful effects of ionising radiation. Its r ...
, 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 beta radiation threat 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
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
. According to a report by the
Organisation for Economic Co-operation and Development, any external radiation from a gaseous tritium light device is solely due to bremsstrahlung, usually in the range of 8–14 keV. The bremsstrahlung dose rate can not be calculated from the properties of tritium alone, as the dose rate and effective energy is dependent on the form of containment. A bare, cylindrical vial GTLS constructed of 0.1 mm thick glass that is 10 mm long and 0.5 mm in diameter will yield a surface dose rate of 100 millirads per hour per curie. If the same vial were instead constructed of 1 mm thick glass and enclosed in a plastic covering that is 2–3 mm thick, the GTLS will yield a surface dose rate of 1 millirad per hour per curie. The dose rate measured from 10 mm away will be two orders of magnitude lower than the measured surface dose rate. Given that the half-value thickness of 10 keV photon radiation in water is about 1.4 mm, the attenuation provided by tissue overlaying blood-forming organs is considerable.
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
Biological half-life (also known as elimination half-life, pharmacologic half-life) is the time taken for concentration of a biological substance (such as a medication) to decrease from its maximum concentration ( Cmax) to half of Cmax in the bl ...
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 mostly harmless. If a tritium tube breaks, one should leave the area and allow the gas to diffuse into the air. Note that it is lighter than air and will float upwards like
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
. Tritium exists naturally in the environment, but in very small quantities.
Legislation
Because tritium is used in
boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released ...
s and
thermonuclear weapon
A thermonuclear weapon, fusion weapon or hydrogen bomb (H bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a low ...
s (though in quantities several thousand times larger than that in a keychain), consumer and safety devices containing tritium for use in the United States are subject to certain possession, resale, disposal, and use restrictions. In the US, devices such as self-luminous exit signs, gauges, wristwatches, etc. that contain small amounts of tritium are under the jurisdiction of the
Nuclear Regulatory Commission, and are subject to possession, distribution, and import and export regulations found in 10
CFR Parts, 30, 32, and 110. They are also subject to regulations for possession, use, and disposal in certain states. Luminous products containing more tritium than needed for a wristwatch are not widely available at retail outlets in the United States.
They are readily sold and used in the UK and US. They are regulated in England and Wales by environmental health departments of local councils. In Australia products containing tritium are licence exempt if they contain less than tritium and have a total activity of less than , except for in safety devices where the limit is total activity.
See also
*
List of light sources
This is a list of sources of light, the visible part of the electromagnetic spectrum. Light sources produce photons from another energy source, such as heat, chemical reactions, or conversion of mass or a different frequency of electromagnetic ener ...
*
Radium Girls
References
External links
Cleanup of a broken tritium signLuminor 2020 – Debunking Panerai's fictional history of tritium-based lume (Perezcope.com)
{{Artificial light sources
Lighting
Nuclear technology