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Other corrections must be made to account for the proportion of throughout the biosphere (reservoir effects).
Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s.
In 1939, Martin Kamen and Samuel Ruben of the Radiation Laboratory at Berkeley began experiments to determine if any of the elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research.
They synthesized Libby and several collaborators proceeded to experiment with methane collected from sewage works in Baltimore, and after isotopically enriching their samples they were able to demonstrate that they contained .
Cosmic rays enter the earth's atmosphere in large numbers every day.
For example, every person is hit by about half a million cosmic rays every hour.
The carbon-14 atoms that cosmic rays create combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis.
For more information on cosmic rays and half-life, as well as the process of radioactive decay, see How Nuclear Radiation Works.
Since the calibration curve (Int Cal) also reports past atmospheric concentration using this conventional age, any conventional ages calibrated against the Int Cal curve will produce a correct calibrated age.