The unstable carbon-14 gradually decays to carbon-12 at a steady rate. Scientists measure the ratio of carbon isotopes to be able to estimate how far back in time a biological sample was active or alive.
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.
The amount of carbon-14 in the air has stayed the same for thousands of years.
There is a small amount of radioactive carbon-14 in all living organisms because it enters the food chain.
Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.
Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere.
A copy of this paper may be found in the Radiocarbon Home Page The radiocarbon age of a sample is obtained by measurement of the residual radioactivity. T (National Institute of Standards and Technology; Gaithersburg, Maryland, USA) Oxalic Acid I (C). The activity of 1890 wood is corrected for radioactive decay to 1950.
This is calculated through careful measurement of the residual activity (per gram C) remaining in a sample whose age is Unknown, compared with the activity present in Modern and Background samples. Thus 1950, is year 0 BP by convention in radiocarbon dating and is deemed to be the 'present'.
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.
Radiocarbon dating uses isotopes of the element carbon. Cosmic rays – high energy particles from beyond the solar system – bombard Earth’s upper atmosphere continually, in the process creating the unstable carbon-14. Because it’s unstable, carbon-14 will eventually decay back to carbon-12 isotopes.
To understand radiocarbon dating, you first have to understand the word Although an element’s number of protons cannot change, the number of neutrons can vary slightly from each atom.
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.
A detailed description of radiocarbon dating is available at the Wikipedia radiocarbon dating web page.