Radioactive Dating

Radioactive Dating

 

Þ    Rocks are made up of many individual crystals, each comprising several different chemical elements like iron, magnesium, silicon, etc. Although most elements are stable, atoms and some elements are unstable in their natural state as they undergo radioactive decay. Radiometric dating is the process of determining the age of rocks from the decay of such radioactive elements.

 

Þ    There are several techniques of radioactive dating, each using a different radioactive element or a different way of measuring them.

 

Þ    The radiocarbon method was developed by Willard F. Libby for which he received the Bobel Prize in Chemistry in 1960. This method has been used to date samples as old as 50,000 years.

 

Þ    Radiocarbon dating can be done on ancient samples of wood, charcoal, bones, peat and organic – bearing sediments, besides carbonate deposits such as tufa, caliche, and marl; and dissolved carbon dioxide and carbonates present in oceans, lakes and ground – water sources

 

Þ    Potassium – Argon dating is a viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. The radioactive isotope of Potassium, Potassium – 40 (K – 40) decays to the gas Argon (Ar – 40). By comparing the proportion of K – 40 to Ar – 40 in a sample of volcanic rock, and knowing the decay rate of K – 40, the date of formation of that rock can be determined.

 

Þ    Radiocarbon dating depends on measuring the amount of C – 14 in a sample and then calculating the age since death by considering the half – life.

 

Þ    Radiocarbon dating relies on a simple natural phenomenon. As the Earth's upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon – C – 14. As – C – 14 reacts identically to the stable forms of carbon, C – 12 and C – 13, if becomes attached to complex organic molecules through photosynthesis in plants and becomes part of their molecular makeup. C – 14 thus becomes fixed in the biosphere through food chain. The process of ingesting C – 14 continues as long as the plant or animal remains alive. However, when it dies, there is no replenishment of radioactive carbon, but only decay occurs, whereby C – 14 is converted back to N – 14.

 

Þ    The gas counting method and liquid scintillation counting are two important methods of radiocarbon dating.

 

Þ    The method of counting C – 14 used by Libby and his co – workers involved measuing radioactivity using modified Geiger counters. The next development in counting technology was the conversion of sample carbon of CO₂ gas for measurement in Gas Proportional counters.

 

Þ    The Accelerator Mass Spectrometry (AMS) method is for direct C14 isotope counting in just milligram – sized samples.

 

Þ    Using radiocarbon dating, archaeologists have been able to ascertain the timing of major prehistoric events live the development of agriculture in various parts of the world.