Lighting the Dark with Tritium

Exit signs. Watch dials. Gun sights.

Each of these items may emit glowing light – light that neither come from a battery nor requires electricity. This light source is emitted from tritium, a radioactive form of hydrogen. Most important, this tritium light source provides an important safety function during power outages or emergencies when exit signs or a lit clock face may lead someone to safety.

Low concentrations of tritium are found naturally in air and water, as a liquid or gas. This hydrogen isotope is produced in the atmosphere, and, as a result, tritium is found in very small amounts in groundwater throughout the world. People are exposed to small amounts of tritium every day since it is widely dispersed in the environment and in the food chain, according to the Environmental Protection Agency (EPA)’s website. Additionally, tritium is a by-product of nuclear reactor operations that is closely and safely monitored. It is odorless, colorless and tasteless, weighing less than air.
 

In exit signs, tritium gas is contained in sealed glass vials lined with a light-emitting phosphorous compound; the isotope emits low-energy Beta particles (radiation that does not penetrate paper or clothing) that cause the lining to glow that red or green that you see in the exit sign.

How do you know when an exit sign contains tritium? According to the Nuclear Regulatory Commission’s website, there are several factors.

•   When lights are off, tritium will make the word EXIT glow green or red.
•   The sign should have a permanent warning label that mentioned tritium, 3H or H-3.
•   The sign should display a three-bladed radiation warning symbol.
•   It should also state “Caution-Radioactive Materials.”

Tritium has a half-life of approximately 12.3 years, so tritium illumination devices, such as exit signs or watch tiles, lose about half their brightness in that time frame. Facilities that use tritium exit signs are considered “general licensees,” which means they do not need a specific license to use the signs (manufacturers and distributors of these signs are “specific licensees” that apply for and receive a radioactive material license from a regulatory body). Despite the type of license received, regulatory requirements must be followed by all parties, especially involving proper disposal of signs. A general licensee must transfer a tritium exit sign to a specific licensee, such as a manufacturer, distributor, licensed radioactive waste broker or licensed low-level radioactive waste disposal facility, according to the NRC’s website. Exit signs cannot be disposed of as normal trash in order to prevent unwanted groundwater contamination in landfills.

More information on tritium and tritium exit signs can be found on the NRC’s website.