Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide.Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14.
Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy.Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating.The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.After another 5,730 years only one-quarter of the original carbon-14 will remain.
After yet another 5,730 years only one-eighth will be left.
By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.
The relatively short half-life of carbon-14, 5,730 years, makes the reliable only up to about 50,000 years.
In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history).
Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics.
Other radiometric dating techniques are available for earlier periods.