14c dating calibration
Most carbon atoms have six positively charged protons and six uncharged neutrons.
Animals (and humans) get their carbon atoms primarily from what they eat (i.e., plants).
The Mass Number for any isotope is the addition of all the protons and neutrons in the nucleus.
Looking at the first isotope in the chart, Carbon 9 has 9 (protons neutrons).
The ratio of radiocarbon to stable carbon atoms in the atmosphere has varied in the past.
This is because the amount and strength of cosmic radiation entering the earth's atmosphere has varied over time.
Radiocarbon dating works by precisely measuring the ratio of radiocarbon to stable carbon in a sample. The tree-ring chronologies have been constructed by counting the annual rings in living trees and matching patterns in these rings to older wood and dead trees.