Absolute dating radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium–argon dating (K–Ar dating). Potassium-40 is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argon,
Isotope geochemistry successfully used in forensic science to fingerprint bullets, because each batch of ammunition has its own peculiar Pb/Pb vs Pb/Pb ratio. Samarium–neodymium is an isotope system which can be utilised to provide a date as well as isotopic fingerprints of geological materials, and various other materials including archaeological finds (pots, ceramics). Sm decays to produce Nd with a half life of 1.06x10 years. Dating is achieved usually by trying to produce an isochron of several minerals within a rock specimen. The initial Nd/Nd ratio is determined. This initial ratio is modelled relative to CHUR - the Chondritic Uniform Reservoir -