Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer (such as in the argon-argon scheme).
Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes.
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 half-life is the time it takes for half of the parent atoms to decay.This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable.This radioactivity can be used for dating, since a radioactive 'parent' element decays into a stable 'daughter' element at a constant rate.The amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism.This method faces problems because the cosmic ray flux has changed over time, but a calibration factor is applied to take this into account.The relationship between the two is: T = 0.693 / λ Many different radioactive isotopes and techniques are used for dating.