Another way of expressing this is the half-life period (given the symbol T).
For geological purposes, this is taken as one year.
Any item that derives from a previously living organism—for example, a basket made of plant fibers or a scrap of animal hide—can be subjected to carbon-14 dating.
The technique is useful for dating items up to about 50,000 years old.
However, there is a limited range in Sm-Nd isotopes in many igneous rocks, although metamorphic rocks that contain the mineral garnet are useful as this mineral has a large range in Sm-Nd isotopes.
This technique also helps in determining the composition and evolution of the Earth's mantle and bodies in the universe.
The isotopes are then measured within the same machine by an attached mass spectrometer (an example of this is SIMS analysis).