The Chlorine-36 (36Cl) dating method is used to date rock samples. Some isotopes (called cosmogenic nuclides) which do not exist naturally on Earth are produced as a result of the nuclear reaction between particles from cosmic radiation and atoms in the Earth's crust exposed to this radiation. The concentration of cosmogenic nuclides produced in rock (in situ) increase according to the duration of its exposure, which enables an estimation to be made of the length of time a sample has been exposed to cosmic radiation and thus how long it has been present on the Earth's surface. The development of accelerator mass spectrometry and improvements in its sensitivity have made it possible to measure very small concentrations of these nuclides which are produced on the surface of the Earth in very small numbers (a few atoms per year).