Neil, of Invercargill, asks :-

Can the gamma rays from a smoke alarm build up on the casing and cause it to be radioactive?

Murray Robertson, a physicist at the Ministry of Health's National Radiation Laboratory, responded.

No. The radioactive material in smoke detectors is a small quantity of americium-241. This emits alpha radiation which ionises the air in the ionisation chamber, making it electrically conducting as the essential mechanism for the smoke detector to function. It also emits soft (not very penetrating) gamma radiation. This plays no part in the operation of the smoke detector. The gamma radiation is more penetrating than the alpha radiation but is still almost totally absorbed in the wall of the ionisation chamber and in the plastic case of the instrument. This absorption does not induce any radioactivity in the absorbing material.

The only radioactivity to be found associated with the use of smoke detectors is the americium-241 itself. Our primary concern about this is not so much the radiation that this emits, but the possibility that this radioactive material itself may be released and dispersed in a fire or other accident. We have tested samples of the small (typically about 3mm diameter x 0.5mm thick) metal foil containing the radioactivity and we are well satisfied that no radioactive material will be released in severe fires and accidents. To release the radioactivity it would be necessary for the foil to reach a temperature in excess of 2600 degrees C, or for the foil to be immersed in aqua regia, a mixture of concentrated acids. More information can be found at our web site: http://www.nrl.moh.govt.nz