Ninety-nine percent of these also contain six neutrons.The 6 proton 6 neutron atoms are said to have a mass of 12 and are referred to as "carbon-12." The nuclei of the remaining one percent of carbon atoms contain not six but either seven or eight neutrons in addition to the standard six protons.
The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.
"Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure.
We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 19)].
The method was developed by Willard Libby in the late 1940s and soon became a standard tool for archaeologists.
Libby received the Nobel Prize in Chemistry for his work in 1960.