Traser
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Traser is the generic name for glass tubes with a phosphor layer in them and Tritium (a Hydrogen isotope) gas inside the tube. They are most often seen as a type of glowing novelty keychain and in self-illuminated exit signs. They are also favored by the military for critical applications where there must be illumination regardless of circumstances. They were invented in the 1960s as a reliable self-powered light source for NATO. A traser consists of a section of tubular borosilicate glass, coated on the inside with a colored layor of phosphor. Borosilicate is preferred because it is a type of glass noted for its strength and resistance to breakage.
During manufacture, a long piece of tube is filled with the radioactive Tritium. Heat is then used to seal off small sections of the tube as, which are then also closed at either end. 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. In theory one could use any beta-particle emitting substance. In practice Tritium is preferred because it is not very hazardous.
Trasers give off a small amount of light; not enough to be seen in daylight, but plenty to be visible in the dark from several meters away. The average traser, if made correctly, can have a useful life of about 10-20 years. Tritium is the expensive component in trasers, so manufacturers are tempted to cut back on it. Tritium is an unstable isotope. It has a half life of about twelve and a half years. Thus, any traser will lose half its brightness every twelve and a half years. The more Tritium that is initially placed in the tube, the longer its useful life will be.
Note: Because Tritium is an integral part of thermonuclear devices (though in quantities several thousand times larger than that in the keychains), trasers are considered dual-use technology in the U.S.A., and are therefore illegal for export. However, trasers 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, eg. lightpanels based on light-emiting capacitor technology, are appearing recently.
Health Concerns: While the presence of a radioactive substance would seem dangerous, traser keychains are hardly a health concern. Tritium emits beta particles, which cannot pass through the glass of the tube. Tritium is only mildly radioactive, and in any case each keychain contains only a minute quantity of the gas; even direct short-term exposition to small amounts are pretty harmless. If a traser vial should break, you should leave the area and allow the gas to diffuse into the air. Tritium exists naturally, but in very small quantities.
See also
- Cleanup of a broken tritium sign (http://hps.org/publicinformation/ate/q3753.html)
- Radioluminiscent items (http://www.orau.org/ptp/collection/radioluminescent/radioluminescent.htm)de:Traser