In 2008 we could only calibrate radiocarbon dates until 26,000 years.
Isotopes of a particular element have the same number of protons in their nucleus, but different numbers of neutrons.This means that although they are very similar chemically, they have different masses.The pathway from the plant to the molecule may have been indirect or lengthy, involving multiple physical, chemical, and biological processes.Levels of C can represent either mixtures of modern and dead carbon or carbon that was fixed from the atmosphere less than 50,000 years ago.The rate at which C atoms, half of them will decay in 5730 years.
Since this rate is slow relative to the movement of carbon through food chains (from plants to animals to bacteria) all carbon in biomass at earth's surface contains atmospheric levels of C is present at atmospheric levels, the molecule must derive from a recent plant product.
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.
From these records a “calibration curve” can be built (see figure 2, below).
A huge amount of work is currently underway to extend and improve the calibration curve.
They have masses of 13 and 14 respectively and are referred to as "carbon-13" and "carbon-14." If two atoms have equal numbers of protons but differing numbers of neutrons, one is said to be an "isotope" of the other.