Susan Beckenholdt, age 14, of St. Louis, Mo., for her question:

WHAT IS CARBON DATING?

Carbon dating is the name given to a procedure that allows scientists to find the age of ancient objects by using radiocarbon, or carbon 14. Radiocarbon is a radioactive form of carbon.

By means of radiocarbon dating, scientists have been able to obtain a great deal of information about prehistoric man, prehistoric animals and changes in the earth's climate.

Radiocarbon is formed by cosmic rays. A scientist named W.F. Libby and his associates at the University of Chicago discovered the material in 1947 and developed it as an archaeological and geological calendar.

About one trillionth of the molecules of carbon dioxide in the air contain an atom of radioactive carbon 14. Human beings, animals and plants constantly absorb radiocarbon from the atmosphere. The radiocarbon in their tissues keeps disintegrating, but it is continually renewed from the air as long as they live.

When a tree is cut down, it dies and stops taking in radiocarbon. But the radiocarbon already in its wood goes on decaying at its constant rate. We can measure the radiations from the radiocarbon left in the wood used for the coffin of an ancient Egyptian king and learn how old the wood is. This can tell us approximately when the king died.

In dating an object by its radiocarbon content, the scientist first heats and burns a sample of the object to convert it to carbon dioxide. The carbon dioxide is then purified and changed chemically into pure carbon.

The scientist then measures the amount of radiocarbon in the sample, using sensitive instruments that count the radiations given off by the radiocarbon.

Radiocarbon continues to give off rays at a known rate, even after a living thing dies. Radiocarbon loses half its radioactivity during the first 5,700 years, and half the remainder every 5,700 years that follow.

All living plants and animals, including man, take in radiocarbon from the air. This substance is radioactive. After a living thing dies, the radiocarbon in its remains loses radioactivity at a fixed rate over thousands of years.

A radiation detector picks up and counts the rays given off by the radiocarbon in a sample of once living material. In this way, scientists can determine the age of the material up to about 40,000 years.

Radiocarbon can be produced artificially in an atomic reactor. Such radiocarbon is used in laboratories and hospitals to trace the element carbon through biological and industrial processes.