This plot shows the level of carbon-14 in the atmosphere as measured in New Zealand (red) and Austria (green), representing the Northern and Southern Hemispheres, respectively.Aboveground nuclear testing almost doubled the amount of carbon-14 in the atmosphere. The black arrow shows when the Partial Test Ban Treaty was enacted that banned aboveground nuclear tests. A special kind of radiocarbon dating: Bomb radiocarbon dating.
Most radiocarbon dating today is done using an accelerator mass spectrometer, an instrument that directly counts the numbers of carbon 14 and carbon12 in a sample.
A detailed description of radiocarbon dating is available at the Wikipedia radiocarbon dating web page.
Atoms of the same element that have different numbers of neutrons are called isotopes. Most carbon on Earth exists as the very stable isotope carbon-12, with a very small amount as carbon-13.
Here’s an example using the simplest atom, hydrogen. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
We find that about 18 such halvings are required for the p MC value to drop below 0.001 (Figures 1 and 2).
(We could “round up” the value of 0.0007 p MC at 17 half-lives to 0.001 p MC, but the 0.00038 p MC at 18 half-lives is definitely below the detection threshold.) Since each half-life is 5,730 years, this means that no C has even been detected in diamonds, which some scientists claim are billions of years old!
As we mentioned above, the carbon-14 to carbon-12 ratio in the atmosphere remains nearly constant.
It’s not absolutely constant due to several variables that affect the levels of cosmic rays reaching the atmosphere, such as the fluctuating strength of the Earth’s magnetic field, solar cycles that influence the amount of cosmic rays entering the solar system, climatic changes, and human activities.
In principle, this decay rate may be used to “date” the time since an organism’s death.