The ionization chamber and circuit board weigh 26.7 grams and the piezo disc weighs 10.3 grams. This payload is a bit much for the CQAR prototype, but is within the ducted fan's payload constraints. The sensitivity of the smoke alarm varied; the alarm went off with a exstinguised and lit match and a candle, but did not go off with the exhuast from a car engine. The alarm also went off when translating a match (extinguished) under the smoke detector despite the relative speed.
An ionization chamber is very simple. It consists of two plates with a voltage across them, along with a radioactive source of ionizing radiation, like this:
The alpha particles generated by the americium have the following property: They ionize the oxygen and nitrogen atoms of the air in the chamber. To "ionize" means to "knock an electron off of." When you knock an electron off of an atom, you end up with a free electron (with a negative charge) and an atom missing one electron (with a positive charge). The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent. When smoke enters the ionization chamber, it disrupts this current -- the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn.