Others measure the subatomic particles that are emitted as an isotope decays.Some measure the decay of isotopes more indirectly.When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.The table below shows characteristics of some common radiometric dating methods.Geologists choose a dating method that suits the materials available in their rocks. Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock.Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old.
However, there are radiometric dating methods that can be used on sedimentary rock, including luminescence dating.
All radiometric dating methods measure isotopes in some way.
Most directly measure the amount of isotopes in rocks, using a mass spectrometer.
For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes.
Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.