Researcher uses carbon dating ancient items
In this way, calibration tables have been developed that eliminate the discrepancy.
Despite its usefulness, radiocarbon dating has a number of limitations.
The radioactive isotope carbon-14 is created in the upper atmosphere when cosmic-ray particles from outer space strike nitrogen atoms and transform them into radioactive carbon.
Some of the carbon-14 might find its way into living creatures.
On April 26, 2007 this facility celebrated 25 years of operation, during which time it had processed over 75,000 radiocarbon measurements on objects ranging from the Dead Sea Scrolls to the Shroud of Turin.
Their commercial rate (in 2008) is 5.00 per sample, which somewhat limits its accessibility to chronically under-funded archeological research projects.
The proportion of carbon-14 to carbon-12 in the atmosphere therefore remains relatively stable at about 1.5 parts per billion.
One of the implied assumptions in radiocarbon dating is that levels of atmospheric carbon-14 have remained constant over time.
It can theoretically be used to date anything that was alive any time during the last 60,000 years or so, including charcoal from ancient fires, wood used in construction or tools, cloth, bones, seeds, and leather.
This turns out not to be exactly true, and so there is an inherent error between a raw "radiocarbon date" and the true calendar date.
To correct for this, scientists have compared radiocarbon dates from objects who's age is known by other means, such as artifacts from Egyptian tombs, and growth rings from ancient trees.
Conversely, contamination by newer plant matter carried by flowing water or intruding plant roots may result in a date that is much too young. The original technique was based on counting the number of individual radioactive decay events per unit of time, using a device similar to a Geiger counter.
Archaeologists are acutely aware of these and other potential difficulties, and take extreme care in the selection and handling of objects to be dated. In the 1970s a new technique was developed called Accelerator-based Mass Spectrometry (AMS), which counts the number of carbon-14 atoms directly.