RADIO CARBON DATING
The principle of radioactive decay is applied in the technique of radioactive dating, a process widely used by scientists to determine the age of materials and artifacts.
Radioactive dating is defined as the method of determining the age of biological or geological samples by using the radioactive technique.
Radioactive C-14 atoms exist naturally in very minute quantities. C-14 is everywhere around us: in our clothes, in the food we eat, even in the air we breathe. The ratio of radioactive C-14 atoms to stable C-12 atoms in the atmosphere has remained constant over thousands of years. Although C-14 naturally decays, it is also continually being formed. C-14 atoms are formed when neutrons from the cosmic radiation collide with N-14 atoms in the atmosphere. Thus, the decay of C-14 is reasonably balanced with its production, resulting in a constant ratio of C-14 to C-12.
Carbon dioxide (CO2) molecules in the air contain both isotopes of carbon. This CO2
is continually used by plants to grow. Because the ratio of C-14 to C-12 in atmospheric CO2 is constant, the intake of CO2 by a plant results in a constant ratio of the two isotopes in the plant’s body while it is alive. However, when the plant dies it no longer takes in CO2. As a result, the C-14 decaying in the dead plant can not be replenished by CO2, resulting in the decrease in activity of C-14 with time. Because animals eat plants and inhale air, the activity of C-14 decreases once they die, since the C-14 cannot be replenished. In order to determine the radioactivity of C-14, a small portion of the sample is burnt, so that carbon present in it reacts with oxygen to form CO2. The CO2 that contains C-14 is radioactive, and the amount is measured using a radiation counter. Burning is done to facilitate measuring the level of C-14. C-14 has a half-life of about 5730 years. This means that in a given sample of a carbon-containing substance, (without the C-14 being replenished) the activity of C-14 decreases by half every 5730 years. Suppose someone discovers an ancient manuscript and finds that the activity of C-14 in the paper is half of that found in living trees. This would mean that the manuscripts would be about 5730 years old.
(Credits: http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/cardat.html)
The use of radioactive C-14 for dating was first done by William Libby, at the University of Chicago, USA, in 1947.
The age of a given sample can be determined using ,
A = A0 e–λt
In calculating the age of a sample, we make the following assumption:
The activity of C-14 in a living sample has remained same over thousands of years.
In the above equations A0 is the activity of the sample, when it is living. A is the activity of the sample whose age is to be determined. For example, if we wish to determine the age of a given piece of wood, A0 is the activity for a fresh wood piece taken from a presently living tree. A is the activity of the sample whose age is to be determined.
The relatively short half-life of C-14 (5730 years) means that the amount of C-14 remaining in materials and objects older than about 30,000 years is too small to be measured experimentally. Thus, carbon dating has following two limitations:
l It cannot measure the age of an object older than about 30,000 years.
l It cannot measure the age of non-living objects like rocks and minerals.