# How does Carbon-14 Dating Work?

Ever since its introduction more than 60 years ago by scientist Willard Libby, carbon-14 dating technique has been instrumental in determining the age of organic objects of historical and archeological origin.

ScienceStruck Staff

Last Updated: Mar 1, 2018

Did you know?

Carbon-14 dating technique has been successfully used on the Minoan ruins, Dead Sea Scrolls, and tombs of the Pharaohs.

To understand how carbon-14 dating works, we need to understand a few things about carbon and its isotopes, how it is formed, and the role it plays in our environment.

How Carbon-14 is formed?

All living organisms and our environment are made of carbon. The most common form of carbon in the atmosphere is carbon-12, which is a stable isotope of carbon. An unstable form of carbon isotope, carbon-14, also known as radiocarbon, is formed when cosmic rays hit atoms in the upper atmosphere of the earth. When cosmic rays collide with the atoms, it creates an energetic neutron, which, upon hitting a nitrogen atom (7 neutrons and 7 protons), creates a carbon-14 atom (6 protons and 8 neutrons) and a hydrogen atom (1 neutron, 0 proton). Carbon-14 is radioactive, and has a half life of about 5700 years.

How Carbon-14 Dating Works

Plants get their share of carbon atoms from the atmosphere. Animals and humans get it from what they eat. It is seen that at any given point of time, the ratio of stable carbon and radiocarbon in the atmosphere, is almost same as the ratio of stable carbon and radiocarbon in living organisms like plants, animals, and humans. When a living organism dies, its carbon intake ceases. From that point onwards, the unstable carbon-14 decays with its half life of almost 5700 years, and the amount of carbon-12 remains the same. Hence, gradually, the ratio of stable carbon to radioactive carbon also decreases. By comparing the ratio of carbon-12 and carbon-14 in a sample to that of a living organism, it is possible to determine the age of an archaeological object, with a fair accuracy. The ratio of carbon-12 and carbon-14 is determined by three techniques, namely gas proportional counting, liquid scintillation counting, and accelerator mass spectrometry.

Calculating with Carbon-14 Dating Technique

Formula to calculate the age of a sample by carbon-14 dating is as follows:

where,

**t = [ ln (N**_{f}/N_{o}) / (-0.693) ] x t_{1/2}where,

**ln**is the natural logarithm**N**is the percentage of carbon-14 in the sample compared to that in a living sample_{f}/N_{o}**t**is the half life of carbon-14_{1/2}**t**is the age of the sample

t = [ ln (.15) / (-.693) ] x 5700 years

= [ ln (-1.897) / (-.693) ] x 5700 years

= 1.3146 x 5700 years

= 7493 years old

Shortcomings

► Carbon dating can only be used to date objects up to 5700 years. Things older than that have only about 1% carbon-14 left in them and hence, it is not possible to calculate their age accurately.