Where is the Carbon-14?

14C is radioactive with a half-life of 5730 yr. It is produced in the upper atmosphere by bombardment of atmospheric 14N by neutrons from cosmic rays. Although present in too small an amount to change the average atomic mass of carbon, there is enough 14C in living organisms to produce a decay rate of 15.3 disintegrations per minute per gram of carbon. When organisms die, the carbon-14 in the atmosphere no longer keeps the ratio of 14C/12C constant, and the ratio goes down with time. Measuring the 14C/12C ratio in formerly living material allows us to determine its age up to about 25,000 years.