It's usually americium-241. It undergoes alpha-decay with a half-life of about 400 years. Am-241 is artificially manufactured; it doesn't occur in nature.
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The first smoke detectors are based on optical principles - photoelectric detectors.
Americium's normal phase is solid at room temperature. It is a silvery-white metal that is radioactive and is commonly used in smoke detectors.
Americium-241 is used in smoke detectors because it emits alpha particles that ionize the air inside the detector. This ionization process allows a small electric current to flow between two electrodes in the detector. When smoke enters the detector, it disrupts this current, triggering the alarm.
It might not seem obvious, but the "original use" of the element Americium was as a tool of scientific investigation. Americium is a synthetic element, so it was made in a nuclear reactor facility by exposing plutonium to the neutron flux in the operating core. It was produced in small quantities to investigate its nuclear, chemical and physical properties. That's what it was originally used for. You can read about it here: http://en.wikipedia.org/wiki/Americium
The element used as a target substance in Irene Joliot-Curie's experiments was beryllium.