| TRITIUM LIGHT SOURCES |
  A self-luminous tube made for use in an exit sign was filled with up to 20 curies of H-3. It contains a phosphor that glows in the dark (right) as it is excited by beta radiation from the tritium fill gas. The tube is 7" long and about 3/8" in diameter.  Modern 20-year self-powered exit sign contains approximately 20 Ci of H-3 within 17 glass tubes. Self-luminous exit
signs, watches, gunsights, and keychains often are powered by the
radioactive decay of H-3 (tritium). In terms of total activity,
exit sign light sources like the one pictured above are undoubtedly the
most radioactive items available to the general public in the United
States. The
20 Ci light source at top, intended for installation in a self-luminous
exit sign, was made in 1986. The tube still glows brightly enough
to read by at night. Underneath are several pictures of a modern
(2002) self-powered exit sign made by SRB Technologies, Inc.
This particular model contains up to 20 Ci of tritium gas distributed
among 17 separate ZnS-coated glass tubes of various sizes. The
service life is quoted at 20 years. A yellow sticker inside
the faceplate (bottom photo) warns of the radioactive contents, and
also warns that breaking the sign housing to get the tubes out is in
violation of the law.
The low-energy beta emissions from tritium (18 keV max) do not penetrate the glass tubes, and even if they did, are too weak to be detected by a Geiger counter. However, low-energy x-rays (bremsstrahlung) created when the beta particles are stopped in the walls ARE emitted by these devices, and CAN be detected by a thin-window Geiger tube. The single-tube light source shown above reads about 60 CPM above background with a pancake GM probe. These light sources are very safe...so long as they are not broken! Breaking a self-powered exit sign and huffing the entire contents could theoretically earn you a whole-body equivalent dose in excess of 1000 rem (calculated by the dose conversion factor method), which is likely to cause an awfully gnarly case of radiation sickness. In reality, when such a device breaks, the radioactive contents are dispersed and diluted in the air to such an extent that overexposure is very unlikely. |
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