Carbon dating tree ring calibration
This method needs less than 1 mg of carbon and directly measures the abundance of the individual ions of carbon (14C, 12C and 13C).To obtain a radiocarbon age the sample activity or the 14C/12C ratio must be compared to a standard material of known age.A unique characteristic of 14C is that it is constantly formed in the atmosphere.Production and decay 14C atoms are produced in the upper atmosphere where neutrons from cosmic rays knock a proton from nitrogen-14 atoms.The main limitation of these techniques is sample size, as hundreds of grams of carbon are needed to count enough decaying beta particles.This is especially true for old samples with low beta activity.Hereafter these isotopes will be referred to as 12C, 13C, and 14C.14C is radioactive and has a half-life of 5730 years.
de Vries also postulated that the fluctuations were due to the production of 14C and how it changed during variations in cosmic ray production.
All radiocarbon laboratories either standardize to the US National Bureau of Standards Oxalic Acid I (OX-I) which is derived from Sugar Beets in 1955 or a secondary standard NBS OX-II (grown in 1977) or Australian National University Sucrose (ANU), which is sugar from the 1974 growing season in Australia.
Both the OX-II and ANU have been extensively cross-calibrated to OX-I and can be used to normalize a sample for radiocarbon dating.
When a plant or animal dies it no longer exchanges CO with the atmosphere (ceases to take 14C into its being). 14C decays by emitting an electron, which converts a neutron to a proton, converting it back to its original 14N form.
The History of Radiocarbon Dating Willard Libby invented radiocarbon dating in the late 1940s.