Natural 14C Production

Carbon-14 is formed in the upper parts of the atmosphere from secondary neutrons formed by cosmic ray interactions with the atmosphere. The neutrons interact with common nitrogen  

The 14C atoms are oxidized upon production and mixed with atmospheric C02. With the latter the 14C is introduced into plants and into surface and 

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The concentration of 14C is expressed in relation to the 14C 12C ratio in an international standard (oxalic acid). The 14C concentration in the bulk carbon of the standard is defined as 100% modern carbon (pmc). Thus a tree that died will contain only 50 pmc after 5730 years (one half-life). The 14C concentration of a sample is measured in specialized counting laboratories. 

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