Topics: The method - radiocarbon WEB-info
The atmosphere contains many stable carbon atoms and relatively few radiocarbon atoms. The ratio of radiocarbon to stable carbon atoms in the atmosphere has varied in the past. This is because the amount and strength of cosmic radiation entering the earth's atmosphere has varied over time. (This, in turn, is caused by variations in the magnetic fields of the earth and sun, for example.) Although the ratio of radiocarbon to stable carbon in the atmosphere has varied over time, it is quite uniform around the globe at any given time because the atmosphere mixes very quickly and constantly.
When an organism dies (whether plant or animal) its intake of carbon atoms ceases. The starting ratio of radiocarbon to stable carbon is locked in at that point. From then on, the ratio of radiocarbon to stable carbon will decrease, because the unstable radiocarbon atoms will slowly decay. After about 50,000 years, the radiocarbon concentration remaining is too small to be measured for the purpose of radiocarbon dating.
Radiocarbon dating works by precisely measuring the ratio of radiocarbon to stable carbon in a sample. This is done in one of three ways: 1. Gas Proportional Counting, 2. Liquid Scintillation Counting, and 3. Accelerator Mass Spectrometry. The purpose in each of these methods is to determine the ratio of radiocarbon to stable carbon in the sample. From this measurement the age in radiocarbon years is calculated.