How alpha particles cause ionisation of air molecules in the smoke detector?
Smoke detectors contain a radioactive substance that emits alpha particles. These particles are charged and have enough energy to liberate electrons from their atoms when the collide with them. Atoms that have lost electrons like this are called ions, and the process is called ionization. In a smoke detector the ionized air permits a constant current to flow in a circuit. Smoke that enters the detector absorbs the alpha particles, which reduces the ionization and interrupts this current, which in turn sets off the alarm.
