In a liquid scintillation counter, the beta particle excites the emission of light from a complex organic molecule or "scintillant." Because only about 13.5 decays per minute occur in one gram of modern carbon, it was necessary to use fairly large samples of several grams of carbon.
Carbon-14 is produced in the upper atmosphere by nuclear reactions induced by cosmic rays on nitrogen (see Fig. Nearly all the carbon in the atmosphere is present as carbon dioxide (CO in the atmosphere maintains an equilibrium with the biosphere and the oceans.
Because plants absorb carbon from the atmosphere during photosynthesis, and as animals eat plants, the animals will also contain the same level of C in a sample with that in "modern" material, defined as 1950 AD.
Accelerator mass spectrometry (AMS) is a technique for direct measurement of the concentration of radioisotopes.
A radiocarbon measurement can be obtained on a sample of ~0.5 mg of carbon, and measured to 40 years in uncalibrated radiocarbon age in a measurement time of 3040 minutes on each sample.
Accurate dating also had to wait for a good calibration of the radiocarbon time-scale in the 1960s, using an absolute chronology based on tree rings.
The radiocarbon time-scale has now been calibrated with tree rings to more than 10000 years before present, and beyond that using a coral chronology (Stuiver, et al., 1993).C produced in the atmosphere were always the same, then we could calculate a "radiocarbon age" using the equation we have discussed directly as an estimate of sample age. This was recognized soon after Libby published his first Curve of Knowns (Arnold and Libby, 1949).The cosmic rays striking the upper atmosphere fluctuate in intensity with time by a small amount due to changes in the magnetic fields of the sun and the earth.About one carbon nucleus in a trillion contains two extra neutrons, giving a mass of 14.This carbon-14 is radioactive and decays with a half-life of 5730 years.The practical use of accelerator mass spectrometry was shown in 1977 by two groups simultaneously at Mc Masversity and at the universities of Toronto and Rochester (N. The great advantage of using AMS is that we can measure the isotope ratio of C to stable carbon directly.