They use a weak source of alpha radiation.
Alpha radiation is placed in the detector, close to two electrodes.
The source of alpha radiation causes ionisation and a current flows between the electrodes.
If there is a fire, the smoke will absorb the radiation and this means the current will stop and the alarm will sound.
Tracers in medicine
In tracers, beta and gamma radiation with a short half life are used.
Certain radioactive isotopes can be injected into people and it can be followed round their body using an external detector. A computer converts the reading to a display showing where the strongest reading is coming from.
A well known example is the use of iodine-131, which is absorbed by the thyroid gland. It gives out radiation which can detect whether the thyroid gland is working or not.
All isotopes which are taken into the body must be gamma or beta emitters so that the radiation can pass out of the body.
They should only last a few hours so the radioactivity inside the patient quickly dissapears (they should have a short half life)
Radiotherapy is the treatment of cancer using gamma rays.
High doses of gamma rays will kill living cells so this is used to kill cancer cells.
The gamma rays have to be directed carefully at the right dosage as it should only kill the cancer cells, not the normal cells.
However, damage still happens to normal cells making cancer patients feel very ill.
It is worth it in the end as by killing off the cancer, it can save a persons life.
Sterilisation of food and surgical instruments
The sterilisation of food and surgical instruments is done using gamma rays.
Food can be exposed to a high dose of gamma will kill all microbes, keeping the food fresh for longer.
Medical instruments can be sterilised in the same way.
The advantage of using radiation rather than boiling is that it doesn't involve high temperatures so this means fresh apples and plastic instruments can be sterilised without damaging them.
The food is NOT radioactive afterwards so it's okay to eat.
The isotope used for these needs has to be very strong emitter of gamma rays with a resonably long half - life (at least a few months) so it doesn't need to be replaced often.