Tritium radioluminescence

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Radioluminescent 1.2 Curie 4" x .2" Tritium vials are simply tritium gas filled thin glass vials whose inner surface are coated with a phosphor. The "gaseous tritium light source" vial shown here is 1.5 years old. This is a 15 year certified vial. The vial would have been used for the top & bottom of the letter 'E' in EXIT.

Self-powered lighting is a generic term describing devices that emit light continuously without an external power source. Self-powered lighting is most frequently used on wristwatches (i.e. Night watches), gun sights, and certain emergency and tactical equipment.

Early self-powered lighting used radium paint, which posed serious health risks to the workers who processed and applied it as well as to the users of devices incorporating it. Recently, self-powered lighting has used tritium a gaseous, radioactive isotope of hydrogen emitting a 18.6 KeV beta particle.

Tritium lighting

Tritium lighting is made using glass tubes with a phosphor layer in them and tritium (a hydrogen isotope) gas inside the tube. Such a tube is known as a "gaseous tritium light source" (GTLS).

Physics behind the light

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

During manufacture, a length of borosilicate glass tube which has had the inside surface coated with a phosphor-containing compound is filled with the radioactive tritium. The tube is then fused with a CO2 LASER at the desired length. Borosilicate is preferred because it is a type of glass noted 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, emitting a low, steady glow. One could use any beta particle-emitting substance, but in practice tritium is preferred because it is not very hazardous.

Various preparations of the phosphorous compound can be used to produce different colors of light. Some of the colors that have been manufactured in addition to the common phosphorous green are red, blue, yellow, purple, and orange.

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. Relative to watches tritium is the expensive most component involved so manufacturers are tempted to cut back on it. Being an unstable isotope with a half-life of about 12.36 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. As found in self luminous exit signs tritium often comes in 3 three brightness levels providing 10, 15 & 20 years of certified life.

Uses of self-powered lighting

These light sources are most often seen as "permanent" illumination for the hands of wristwatches intended for diving, nighttime, or "tactical" use. They are additionally used in glowing novelty keychains and in self-illuminated exit signs. They are also favored by the military for critical applications where illumination of the glow-in-the-dark sort is desired but a light source may not be. Some uses of this sort are analog dials in aircraft and in compasses. They were invented in the 1960s as a reliable self-powered light source for NATO.

Legal issues

Because tritium in particular is an integral part of thermonuclear devices (though in quantities several thousand times larger than that in a keychain), devices containing tritium are considered dual-use technology in the U.S.A., and are therefore illegal for export. However, they are widely available in the U.K., most of Europe, Asia and Australia. Paradoxically, tritium exit signs are quite common within the U.S.A., especially in older buildings. Alternative technologies, e.g. lightpanels based on light-emitting capacitor technology, have appeared recently.

Health concerns

While tritium is much safer than its heavy counterpart radium this has lent itself to the propagation of a false sense of security. Contrary to popular belief weak radiation can be detected in very close proximity (6 inches) to non-contaminated tubes. Because tritium is an isotope of hydrogen it will readily adsorb onto the surface of most metals, plastic, ect. If exposure to tritium in amounts greater than that contained in a watch i.e. several or more tubes from an EXIT sign one should immediately evacuate the room and provide ventilation for several days. Tritium exists naturally, but in very small quantities.

References

External links

• Cleanup of a broken tritium sign
• Radioluminescent items

See also

• List of light sources