Libby calculated the half-life of c14 as 5568 ± 30 years.This means that half of the c14 has decayed by the time an organism has been dead for 5568 years, and half of the remainder has decayed by 11,136 years after death, etc.
Carbon-14 is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms.
The ensuing atomic interactions create a steady supply of c14 that rapidly diffuses throughout the atmosphere.
Plants take up c14 along with other carbon isotopes during photosynthesis in the proportions that occur in the atmosphere; animals acquire c14 by eating the plants (or other animals).
During the lifetime of an organism, the amount of c14 in the tissues remains at an equilibrium since the loss (through radioactive decay) is balanced by the gain (through uptake via photosynthesis or consumption of organically fixed carbon).
However, when the organism dies, the amount of c14 declines such that the longer the time since death the lower the levels of c14 in organic tissue.
This is the clock that permits levels of c14 in organic archaeological, geological, and paleontological samples to be converted into an estimate of time.The measurement of the rate of radioactive decay is known as its half-life, the time it takes for half of a sample to decay. Since carbon is fundamental to life, occurring along with hydrogen in all organic compounds, the detection of such an isotope might form the basis for a method to establish the age of ancient materials. Libby, a Professor of Chemistry at the University of Chicago, predicted that a radioactive isotope of carbon, known as carbon-14, would be found to occur in nature.Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.In contrast, methane made from petroleum products had no measurable radioactivity.