Explore The World: What is the Role of Carbon 14 for Date Determination

Saturday, May 14, 2011

What is the Role of Carbon 14 for Date Determination

What is the Role of Carbon 14 for Date Determination
Carbon exists in nature in the form of three principal isotopes - C12, C13, both if which are stable, and C14 which is unstable or radioactive. The C12 constitutes 98.89% , C13 - 1.11% while C14 - represents only 0.00000000010%.
This means that one carbon 14 atom exists in nature for every 1,000,000,000,000 C12 atoms in living material.

Since 14C is unstable it decays back to Nitrogen N14 from which it was formed in the upper atmosphere, and as it decays it emits a weak beta particle (b ), or electron, which possesses an average energy of 160keV. The decay can be shown:

14C => 14N + b

Libby, Anderson and Arnold (1949) were the first to measure the rate of this decay. They found that after 5568 years, half the C14 in the original sample will have decayed and after another 5568 years, half of that remaining material will have decayed, and so on. The half-life (t 1/2) is the name given to this value which Libby measured at 5568±30 years. This became known as the Libby half-life time.

However, the use of this phenonemon is limited to 50 - 60 000 years i.e. 10 half-lives. After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. Beyond about 50 - 60 000 years, then, the limit of the technique is reached, and other radiometric techniques must be used.

The 14C is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen 14. The reaction is:

14N + n => 14C + p
Where n is a neutron and p is a proton.

The 14C formed is rapidly oxidised to 14CO2 and enters the earth's plant and animal lifeways through photosynthesis and the food chain.

Plants and animals which utilise carbon in biological foodchains take up 14C during their lifetimes. They exist in equilibrium with the C14 concentration of the atmosphere, that is, the numbers of C14 atoms and non-radioactive carbon atoms stays approximately the same over time. As soon as a plant or animal dies, they cease the metabolic function of carbon uptake; there is no more intaking of radioactive carbon, only decay.

By measuring the C14 concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the countrate or number of decay events per gram of Carbon. By comparing this with modern levels of activity and using the measured half-life it becomes possible to calculate a date for the death of the sample.

Libby and his team intially tested the radiocarbon method on samples from prehistoric Egypt. They chose samples whose age could be independently determined. A sample of acacia wood from the tomb of the pharoah Zoser, 3rd Dynasty, 2700-2600 BC was obtained and dated. Libby reasoned that since the half-life of C14 was 5568 years, they should obtain a C14 concentration of about 50% that which was found in living wood. The results they obtained indicated this was the case.

The C14 technique has been and continues to be applied and used in many, many different fields including hydrology, atmospheric science, oceanography, geology, palaeoclimatology, archaeology and biomedicine.

Later measurements of the Libby half-life indicated the figure was ca. 3% too low and a more accurate half-life was 5730±40 years. This is known as the Cambridge half-life. (To convert a "Libby" age to an age using the Cambridge half-life, one must multiply by 1.03).

Libby received the Nobel Prize in Chemistry in 1960.