Tritium Exit Sign (ca. 1970s)
This is a self-luminous exit sign (ca. 8 1/2" x 15 1/2" x 1") manufactured by Brandhurst Co., Ltd., of England and distributed by Brandhurst Inc., of Connecticut. Brandhurst's license expired in 1989 and is no longer in business.
The NRC regulations dealing with the manufacture of such a device are found in 10 CFR 32.51. Possession of this sign requires an NRC (or Agreement State) General License. For information regarding the license refer to10 CFR 31.5.
The sign employs several sealed borosilicate glass tubes arranged into the word "EXIT." The tubes are positioned in the parabolic channels of a backing material that also serves as a reflector. Each tube is coated on the inside with a thin layer of a phosphor (probably zinc sulfide) and filled with tritium gas. The low energy beta particles from the tritium (half-life 12 years) strike the phosphor and cause it to glow. Because no electrical power source is required, such signs are used where there is no power or the power could fail. Perhaps the most familiar location to find these signs is inside aircraft. This particular sign at one time contained close to 25 curies of tritium.
In an ideal world, these signs are disposed of by returning them to the manufacturer, who would inevitably charge a fee for this service. Some manufacturers will even accept signs produced by other companies, but the fee will be greater.