M05 Carbon-14 decay

Isotopes of carbon
Carbon has three isotopes, (isotopes are nuclides of the same element containing different numbers of neutrons in the nucleus) :
has 8 neutrons and is a radioactive isotope and its slow rate of decay enables it to be used in the dating of plant and animal matter in so-called radiocarbon dating;
(so called C-12) has 6 neutrons, and is the most common isotope;
has 7 neutrons and has a natural abundance of 1.1% of , but its low natural abundance is utilised in the analysis of organic molecules by NMR spectroscopy.

Carbon-14 is continuously formed in the upper atmosphere by the collision of neutrons with nitrogen atoms:

These neutrons are produced when cosmic rays strike atoms and split them into fragments. Simultaneously with the formation of in the upper atmosphere, the already present is decaying to by the emission of electrons:

The ratio of ¹⁴C/¹²C in the air remains constant, due to the fact that the rate of production of C-14 by the above reaction is equal to the rate of decay. In the nucleus of a neutron splits into a proton and an electron:

Radiocarbon dating
Plants assimilate CO₂, which has this same ratio ¹⁴C/¹²C, during photosynthesis. Any living creature which consumes plants thus takes in the same ratio of C-14 /C-12. The activity of carbon in living tissue is thus 0.007µCi per kg. When a plant or animal dies, however, the assimilation of carbon-14 stops. The quantity of C-14 decays with time, and the radioactivity that remains can be used to calculate the age of the plant or animal material. After 5,750 years half of the radioactive C-14 has decayed (this time is known as the half-life). Fossils and other materials of natural origin (e.g. the parchment of the Dead Sea Scrolls) have been dated using this method. The method is most accurate for ages up to 5,000 years, but it has been used for objects up to 45,000 years old