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When cosmic rays enter the atmosphere, they undergo various transformations, including the production of neutrons.
The resulting neutrons ( but attempts to directly measure the production rate in situ were not very successful.
In connection with building the Borexino solar neutrino observatory, petroleum feedstock (for synthesizing the primary scintillant) was obtained with low Since many sources of human food are ultimately derived from terrestrial plants, the carbon that comprises our bodies contains carbon-14 at almost the same concentration as the atmosphere.
This resemblance is used in chemical and biological research, in a technique called carbon labeling: carbon-14 atoms can be used to replace nonradioactive carbon, in order to trace chemical and biochemical reactions involving carbon atoms from any given organic compound.
These are relatively low energies; the maximum distance traveled is estimated to be 22 cm in air and 0.27 mm in body tissue.
One of the frequent uses of the technique is to date organic remains from archaeological sites.
Plants fix atmospheric carbon during photosynthesis, so the level of C level for the calculation can either be estimated, or else directly compared with known year-by-year data from tree-ring data (dendrochronology) up to 10,000 years ago (using overlapping data from live and dead trees in a given area), or else from cave deposits (speleothems), back to about 45,000 years before the present.