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).
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.
This technique has allowed the measurement of radiocarbon in samples of much less than a milligram, or more than a thousand times less material than is needed for the older counting methods.
This has led to a great increase in the use of C dating in applications to artwork, where conservation of the work requires removal of the smallest sample possible.
When a C atom decays, it emits a beta particle, which can be counted in a gas by the electrical pulse it generates.
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.
Radiocarbon ages are then quoted as "years before present" (BP).
The formula used for this calculation is: Radiocarbon age (years BP) = -C in 1950 AD (pre-bomb) material.
In the 1950s, gas-counting methods were perfected, and later, liquid scintillation counting has also been used, as we will discuss later.
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.
About one carbon nucleus in a trillion contains two extra neutrons, giving a mass of 14.
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.
We can equally well use a different standard if we know its relation to "modern," or 1950 AD.