The total mass of the isotope is indicated by the numerical superscript.While the lighter isotopes C has decayed that what remains can no longer be measured. In 5,730 years half of the C in the atmosphere, and therefore in plants and animals, has not always been constant.The older a sample is, the less (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.The idea behind radiocarbon dating is straightforward, but years of work were required to develop the technique to the point where accurate dates could be obtained.In a stratigraphical context objects closer to the surface are more recent in time relative to items deeper in the ground.Although relative dating can work well in certain areas, several problems arise.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.
More recently is the radiocarbon date of 1950 AD or before present, BP.
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.
As a result it is always undergoing natural radioactive decay while the abundances of the other isotopes are unchanged.
Carbon-14 is most abundant in atmospheric carbon dioxide because it is constantly being produced by collisions between nitrogen atoms and cosmic rays at the upper limits of the atmosphere.