Others measure the subatomic particles that are emitted as an isotope decays.Some measure the decay of isotopes more indirectly.The atoms of some chemical elements have different forms, called isotopes.
All radiometric dating methods measure isotopes in some way.
Most directly measure the amount of isotopes in rocks, using a mass spectrometer.
For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.
One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains.
Absolute dating is the process of determining an age on a specified chronology in archaeology and geology.
Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy.
Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old.
Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.
These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.