Nuclear radiation carbon dating
The source is sealed so that only gamma rays get out.
Therefore the irradiated substance is sterile but NOT radioactive.
During a fire, it's necessary to make sure that emergency exit signs remain illuminated, even if the power goes out. Others have used tritium, a beta-emitting isotope of hydrogen, with a half-life of 12.3 years.
No radioactive source particles are allowed to get in touch with the irradiated substance.
Some smoke detectors contain a small amount of Americium-241, an alpha emitter (and low energy gamma emitter) with a half life of 460 years.
It consists of an ionisation chamber linked to a simple electronic alarm circuit The Americium ionises the air between the plates, causing a current to flow.
The most common and accepted method of 'absolute geologic dating' (establishment of actual age) is based on the natural radioactivity of certain minerals found in rocks.
As the rate of radioactive decay of any particular isotope is known, the age of a specimen can be worked out from the ratio of the remaining isotope and its decay product.