Alpha radiation in a smoke detector
An ionisation smoke detector uses a tiny source of americium-241 to keep a small electric current flowing. Add smoke with the slider and watch what happens to that current.
Normal: ions are reaching the plates, so a steady current flows.
How the detector works
- The americium-241 source gives out alpha particles at random, all the time.
- Each alpha particle knocks electrons off air molecules as it travels. This is ionisation, and it makes positive ions and free electrons.
- A battery makes one plate positive and the other negative. Positive ions are pulled to the negative plate and electrons to the positive plate, so a small current flows in the circuit.
- Smoke particles catch the ions, so fewer of them reach the plates and the current falls.
- When the current drops below a set level, the circuit sets off the alarm.
Why an alpha source?
- Strongly ionising. An alpha particle (a helium nucleus) makes thousands of ions in a few centimetres of air, enough for a measurable current.
- Short range and easily stopped. Alpha particles travel only a few centimetres in air and are stopped by paper or the detector’s casing, so the radiation stays inside the unit.
- Long half-life. Americium-241 has a half-life of about 432 years, so its activity stays almost constant for the whole life of the detector.
Check your understanding
Why would a gamma source be a poor choice?
Gamma rays are weakly ionising, so they would make very few ions and a tiny current. They are also very penetrating, so they would pass straight through the casing and out into the room.
Why must the source have a long half-life?
If the half-life were short, the activity would fall quickly. Fewer alpha particles means fewer ions and a smaller current, and the detector would give false alarms even with no smoke.
Why does the current fall when smoke enters the chamber?
Ions stick to the smoke particles, so fewer ions reach the plates. Fewer charges moving each second means a smaller current.
This simulation is simplified. Ions are drawn much larger, slower and fewer than in a real detector, and the real current is only a few picoamps.