Warning : The radioactive element inside smoke detectors (Americium-241) does present a health hazard if ingested, so anybody experimenting with one should not touch or remove the actual sample from the detector.
The smoke detector with the plastic cover removed.
The smoke detector with the metal shield removed. The sample of americium-241 is in the small hole in the centre of the metal area of the unit.
The counter is set to x10 scale and reads 310 counts per second (this is a high count) when placed as close to the sample as possible.
It is important to use a counter with a mica end window that can register Alpha particles. Americium-241 emits alpha particles and low energy gamma waves and has a half life of 432 years.
The americium exists in the detector in the form of an oxide that decays into neptunium-237.