In contrast, from 1955 to 1963, atmospheric radiocarbon levels almost doubled.
Since then they have been dropping back toward natural levels.
To determine year of birth, the researchers focused on tooth enamel.
Adult teeth are formed at known intervals during childhood.
Radiocarbon levels in teeth formed before then contained less radiocarbon than expected, so when applied to teeth formed during that period, the method was less precise.
To determine year of death, the researchers used radiocarbon levels in soft tissues.
Although neutrons do not carry an electrical charge, they have a mass comparable to that of protons, so different isotopes have different atomic weight. Because of the different number of neutrons, carbon-12 and carbon-14 differ with respect to radioactivity. Carbon-14, on the other hand, undergoes radioactive decay:e (half-life is 5720 years)The other common isotope of carbon is carbon-13.