Others measure the subatomic particles that are emitted as an isotope decays.Some measure the decay of isotopes more indirectly.
Some of these are found in nature; the rest are produced artificially as the direct products of nuclear reactions or indirectly as the radioactive descendants of these products.
Each “parent” radioactive isotope eventually decays into one or at most a few stable isotope “daughters” specific to that parent. Some radioactive isotopes are present as terrestrial radiation.
Only hydrogen-3 (tritium), however, is a radioactive isotope; the other two are stable.
More than 1,800 radioactive isotopes of the various elements are known.
For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.