MadSci Network: Earth Sciences

Re: How long does it take to do carbon dating on an object?

Date: Thu Dec 7 21:17:26 2006
Posted By: Dr. Nagesh N Bhat, Scientific Officer
Area of science: Earth Sciences
ID: 1162438445.Es

Carbon-14 is a radionuclide created by neutrons from cosmic rays through a 
nuclear reaction as follows.

14N+ n --> 14C+p

14C is a radioactive isotope. It decays by emitting beta rays. It has a 
half-life of about 5500 years. The proportion of 14C is about 1 in 
1,000,000,000,000 of natural 12C. Living systems, both animals and plants 
continuously uptake carbon atoms in the form of gaseous CO2 and food. 
There exists equilibrium between production of 14C, its uptake by living 
system and its decay. However, when an animal or tree dies, the uptake 
stops but the decay continues. The rate of decay of 14C gives an 
indication about concentration of 14C in the sample.

With such a long half-life and very low concentration in archeological  
samples, it is difficult to determine the concentration. Background 
suppressed spectrometers with sample size of few grams are used for this 
purpose. Typically, about 1  3 days of counting time is required. In 
spite of many uncertainties, this method gives fairly good idea about the 
age of the sample up to about 60000 years. 

As you can guess, due to many nuclear tests since last few decades, this 
technique may fail to give us proper indication in future.

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