Uses of mass spectrometry in radioactive dating
Libbys measurements on C, using samples of several grams of carbon-black powder (see Anderson et al., 1946).
Radiocarbon ages are then quoted as "years before present" (BP).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.In the 1950s, gas-counting methods were perfected, and later, liquid scintillation counting has also been used, as we will discuss later.Large sample sizes were needed for both counting methods, which limited their usefulness in such applications as studies of artwork, where only small samples could be taken.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.
The number of applications of AMS today is large, and so we will focus on a general overview of some interesting applications that will give some flavor for the variety of uses of the method.
These methods relied on the observation of a decay of the radioactive carbon atoms.
When a C atom decays, it emits a beta particle, which can be counted in a gas by the electrical pulse it generates.
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.
For historical reasons, uncalibrated radiocarbon measurements are often referred to a half-life of 5568 years.