Re: Why does a smoke detector have radioactive material inside it?

Jaime:
     I am assuming that the smoke detectors in England are constructed 
pretty much the same as in the USA.  A large percentage of them use the 
radioactive material as the means of detecting smoke.  To answer your 
question, the inscription says "danger" because it is possible that the 
radioactive material could get into your body, and in that case it would 
probably not be a good thing.  I would recommend that you not tamper with 
the smoke detector, especially the part containing the radioactive 
material.
     Now, you didn't exactly ask this question, but I will try to explain 
both how the detector works and why you shouldn't play around with the 
radioactive source.  In the USA, most currently produced smoke detectors 
use a radioactive source of Americium-241 (Am-241).  This source emits 
lots of alpha particles (and some gamma rays).  The alpha particles only 
have a range in air of about 2 inches.  The alpha particle has a 
characteristic of creating a lot of ionization (removal of electrons from 
atoms) in air over its fairly short path length.  It also has the 
characteristic of being able to be completetly absorbed in a very small 
mass of material.
     In a smoke detector, there are two chambers, each with an identical 
alpha particle source at one end.  In each of the chambers are also two 
electrodes, one of positive charge and one negative.  They are 
continuously collecting the electrons liberated by the ionization taking 
place in the chambers due to the alpha particle bombardment.  Since the 
chambers are identical, the small electric current formed by the ions is 
exactly the same.  Actually, I SHOULD have said that the two chambers are 
ALMOST identical.  One of them is sealed shut and the other is open to the 
atmosphere.
    In the event of a fire, smoke and other combustion products become 
entrained in the air.  They cannot get into the sealed chamber, but they 
do enter the chamber that is open to the atmosphere.  Smoke particles are 
very, very small, but they are large enough to absorb some of the alpha 
particles' energy.  This leaves less energy for ionization of the air, and 
so the current across the electrodes in the open chamber drops, while the 
current in the closed chamber remains the same.  There is an electronic 
circuit monitoring the currents in the two chambers.  When the currents 
become unbalanced by a certain amount, an alarm is triggered.  These 
devices are very sensitive, and operate for many months on a single 9-volt 
battery.  In the USA, many thousands of lives have been saved by smoke 
detectors, and I am sure the same is true in England.
     The radioactive source, Am-241, has a half-life of around 400 years, 
so it is likely to last for a lot longer than the smoke detector itself.  
Because it is electroplated onto a stainless steel disc, it is unlikely to 
become displaced even in a fire.  The only way the source could become a 
threat to the health of humans would be if the radioactive material were 
somehow removed from the disc and ingested or inhaled by a person.  That 
would be very difficult, but not impossible, to happen.  That is why there 
is a warning on the smoke detector.  And, when you are ready to discard 
the smoke detector for any reason, it is a good idea to return it to the 
manufacturer.  They are usually happy to receive the detectors and are 
equipped to handle the Am-241 in a responsible manner.
     If you have any further questions, Jaime, please write back to MadSci!!  
Thanks very much for your question.