Both types are availible Brian, the optical ones work by flashing an LED inside the smoke chamber. A photodiode will start the alarm if it sees the light. The labyrinth construction of the chamber ensures that the photodiode can only see the light scattered by the smoke. Naturally there is the obligatory microcontroller to mitigate false alarms and to provide self tests etc.
I have been interested in the furore over the lost capsule, but it did make me think about radioactive sources found in the home such as in smoke alarms. Now I know these are quite small and supposedly well shielded but what happens when these things are recycled? My local tip simply advised me to sling some broken ones into the electrical waste, should they be disposed of separately?
"As a private household you are permitted to dispose of your radioactive smoke alarm to household waste, however many local authorities would prefer that you remove the battery and take your smoke detector to your local household waste recycling centre."
They probably end up in landfill anyway. IIRC the isotopes in smoke alarms are all alpha radiation and its not very penetrating.
"Ionization smoke detectors use americium as a source of alpha particles. Alpha particles from the americium source ionize air molecules. This makes some particles positively charged and some negatively charged. Two charged plates inside of the ionization smoke detector create a flow of positively and negatively charged ions. The smoke alarm triggers when smoke breaks the constant flow of ions.
Alpha particles are very heavy and cannot travel very far. They can be shielded by a layer as thin as a layer of dead skin cells. Ionization smoke detectors have a small americium source encased in a layer of foil and ceramic, which stops the alpha particles from travelling outside of the smoke detector. Because of this shielding, the smoke detector poses no radiation health risk when they are properly handled.
There is no health threat from ionization smoke detectors as long as the detector is not damaged and used as directed. Do not tamper with your smoke detectors, as it could damage the shielding around the radioactive source inside of them. There are no special disposal instructions for ionization smoke detectors. They may be thrown away with household garbage, or your community may have a separate recycling program."
blah blah blah.
The Australians found that little capsule by the way.
I am not aware of the story, baton the second question, my smoke alarm says it has no radioactive parts inside. The one before that claimed it would wear out in 10 years as the source of the alpha radiation gradually reduced. The very first one I had had a big box inside with a yellow label and a skull and crossbones on it warning to not just throw it in the rubbish.
So from this I was under the opinion that these days some other way of detecting smoke was in use, perhaps optical rather than changes in the ionisation current.
Brian
The ones I have now have no user replicable battery. When the battery dies send Amazon a nice fiver or whatever and you shall have a new one. Brian
What happened to all of those betalite things inside dial trimphones then? These I gather had a radioactive gas inside and this made the coating on the glass glow to allow the numbers to be seen. Being beta and a gas, I'd have thought that these were more dangerous. I also recall a product that you pulled up a central stem on them and placed then near the edge of a record as you played it and it dissipated the static. That has to have been radiation, or was it snake oil?
Oh and what about all those millions of tin alarm clocks made in Hong Kong back in the 50s with those dobs of glowing green paint at the hour markers?
Brian
Possibly tritium which is the radioactive isotope of Hydrogen. Half-life 12 years. Only a small amount will be needed. Decays by beta emission to become Helium-3.
Poor sods doing those have prolly got cancer of the tongue or throat from licking their brushes.
They did in the 1940s. But they stopped them doing that. Cancers on the tongue and lips IIRC
Americium-241 has a half life of 432.2 years so it can't be that. I think the detector clogs up with dust, which is why, with modern detectors, you have to replace the whole thing not the battery.
I don't think the optical type would be anything like as sensitive as it would require visible smoke.
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