Re: Tritium Exit Signs

[bkolb@arinc.com]

>After 7 years or so, the tritium signs are marked to "expire" because of
>decay.  - How are these signs disposed of?
>        _ Does anyone have a rotation program for these things when they
>          expire?  Is it really necessary?

I've read through brochures and independent studies done on these items and
came away with the feeling that even if they break, they present less of a
radiation hazard than a cross-country flight in terms of potential exposure
(far less in terms of actual mortality).  The tubes can be individually
removed and must be sent back to the manufacturer for disposal.  New tubes
are shipped in special containers and are replaced by hand.  Radiation at
the surface of these devices is extremely small since it consists of very
weak beta that is almost entirely absorbed by the glass.  I once tried to
buy a replacement tube but the manufacturer was pretty wary and wouldn't
sell me a new one unless I returned the old one.  Similar lights have been
used for exit signs in aircraft, movie theaters, industrial plants, and
public facilities.  One local theater has hundreds of aisle lights that are
2 Ci apiece.  These are basically the same thing but embedded in a heavy
acrylic housing that can withstand being walked on, kicked, and vacuumed.
These must be returned to the manufacturer for disposal and have an
estimated life of 10-15 years.  Smaller versions of these tubes are used in
consumer items such as watches and rifle sights.

There is no non-electrical alternative except ZnS luminous paint, which
isn't as bright, has to be recharged by bright light, and fades more or
less rapidly depending on how it is doped.

bill