How do smoke alarms work by using alpha radiation?

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I know that the alpha radiation ionises the air and causes a current to flow but how does this affect the detector/ count rate and how does the smoke prevent a current flowing?

I am soooooooo confused!!!

Please help!

Posted Fri 12th October, 2012 @ 17:37 by Priya Patel

1 Answer

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This explains it quite well:

Ionization Detectors

Ionization detectors have an ionization chamber and a source of ionizing radiation. The source of ionizing radiation is a minute quantity of americium-241 (perhaps 1/5000th of a gram), which is a source of alpha particles (helium nuclei). The ionization chamber consists of two plates separated by about a centimeter. The battery applies a voltage to the plates, charging one plate positive and the other plate negative. Alpha particles constantly released by the americium knock electrons off of the atoms in the air, ionizing the oxygen and nitrogen atoms in the chamber. The positively-charged oxygen and nitrogen atoms are attracted to the negative plate and the electrons are attracted to the positive plate, generating a small, continuous electric current. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so they do not reach the plate. The drop in current between the plates triggers the alarm.

Hope it helps! 

Answered Sat 13th October, 2012 @ 19:12 by Joanne