Shedding Some Light On Tritium Illumination Devices

Shirley Xu
Health Physicist

Some radioactive materials are used to produce light. This is done by bombarding a special material known as a phosphor with the radiation (typically beta radiation) emitted by the radioactive material. Phosphor gets its name from the Greek words for “light” and “to bring.” The phenomenon is called “radioluminescence.”

Radioluminescence can be used to provide a low level light source to allow instruments or signs to be visible at night or for other situations where light is needed for long periods without electricity, such as emergency exit signs.

watchfacePaint with radium was the first radioluminescent product. Today, tritium is most commonly used, primarily on wristwatch faces and gun sights. Small tritium lights can be made by sealing tritium and a phosphor layer in small glass tubes. Such a tube is known as a “gaseous tritium light source” (GTLS), or more commonly, a beta light (since the tritium undergoes beta decay).

Tritium is a radioactive isotope with a half-life of about 12 years, which means the glass tube loses half its energy and some of its brightness in that period. So the types of GTLS used in watches generally have a useful life of 10 to 20 years. They give off a small amount of light: not enough to be seen in daylight, but enough to be visible in the dark. The more tritium that is initially placed in the tube, the brighter it is to begin with and the longer its useful life.

The NRC regulates devices that contain small amounts of tritium. Manufacturers and initial distributors of these devices need to have a distribution license issued by the NRC. They also need to have a separate license to possess and use the material. This license can be issued either by the NRC or the state. [There are 37 states that have agreements with us to regulate these types of radioactive materials. They are called Agreement States.] Anyone who initially buys one of these products from someone who has the proper licenses and subsequent owners of the product are exempt from the requirements for an NRC license.

Approval of these types of products would require extremely low risk of radiation exposures to members of the public from normal use, misuse or accidents. The NRC would also need to see the usefulness or benefits of the product. For example, items that could be mishandled, especially by children, will be approved only if they combine an unusual degree of utility and safety. Other countries have different regulatory requirements. That is why some tritium products available for sale internationally are not sold in the U.S.

These regulations can be found in 10 CFR Part 30 and Part 32.

EXIT — A Good Sign of Radiation

Maureen Conley
Public Affairs Officer

refresh leafMost people know radioactive energy can be harnessed to provide electricity and even to diagnose and treat certain illnesses. But would it surprise you to learn that radioactive materials also perform an important safety function by lighting emergency EXIT signs?

Look for the EXIT sign the next time you go to work, school, a sporting event, religious service, the movies, or the mall. If the sign glows green or red, chances are it contains a radioactive gas called tritium. The tritium, a radioactive isotope of hydrogen, is sealed into glass tubes lined with a chemical that glows in the dark. Tritium emits low-energy radiation that cannot penetrate paper or clothing and even if inhaled, it leaves the body relatively quickly. As long as the tubes remain sealed, the signs pose no health, safety, or security hazard.

exit3We estimate there are more than 2 million of these signs in use in the United States. To ensure safety in handling and the manufacturing process, we and our Agreement State partners regulate the manufacture and distribution of tritium EXIT signs. Companies have to apply for and receive a license before they can manufacture or distribute one of these signs.

But because the signs are designed to be inherently safe, the NRC does not require any special training before a building can display the signs. Users are responsible for meeting the requirements for handling and disposal of unwanted or damaged signs and for reporting any changes affecting the signs.

exit2Proper handling and disposal is the most important safety requirement for these signs. A damaged sign could contaminate the immediate area and require an expensive cleanup. That is why broken or unwanted signs must be return to a licensed manufacturer, distributer, radioactive waste broker or radioactive waste disposal facility.

Tritium EXIT signs are one of several types of radioactive consumer products that we allow. These products can be produced and sold ONLY if they have a benefit that outweighs any radiation risk. See our earlier blog post for more information on how we regulate these products.

REFRESH is an occasional series where we revisit previous blog posts.