This discovery meant that there are three naturally occurring isotopes of carbon: Whereas carbon-12 and carbon-13 are stable isotopes, carbon-14 is unstable or radioactive.
Carbon-14 is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms.
The sample must be destroyed in order to measure its c14 content.
The first measurements of radiocarbon were made in screen-walled Geiger counters with the sample prepared for measurement in a solid form.
The collagen fraction usually yields more reliable dates than the apatite fraction (see Dates on bones).
In addition to various pre-treatments, the sample must be burned and converted to a form suitable for the counter.
This is the clock that permits levels of c14 in organic archaeological, geological, and paleontological samples to be converted into an estimate of time.
All of these counter types measure the C-14 content by monitering the rate of decay per unit time.Most samples require chemical pre-treatment to ensure their purity or to recover particular components of the material.The objective of pre-treatment is to ensure that the carbon being analyzed is native to the sample submitted for dating.For example, it was once standard practice to simply burn whole bones, but the results were eventually seen to be unreliable.Chemical methods for separating the organic (collagen) from the inorganic (apatite) components of bone created the opportunity to date both components and compare the results.
Libby calculated the half-life of c14 as 5568 ± 30 years.