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Wednesday, October 3, 2012

Willard Libby

Libby was an authority on radioactivity and radioactive decay. His greatest legacy is the science of radioactive dating, by which the age of ancient organic material can be determined. For this discovery, Libby was awarded the 1960 Nobel Prize in chemistry.
Libby was the son of a farmer. He attended the University of California at Berkeley, earning his bachelor's degree in 1931 and his Ph.D. in 1933. While engaged in his graduate studies, Libby became fascinated with radioactivity. During these years Ernest O. Lawrence (1901-1958) was perfecting his cyclotron particle accelerator at Berkeley. Libby quickly became familiar with nuclear theories and technology and during his doctoral research he constructed the first Geiger counter seen in the United States. The idea of carbon-14 as a means for determining age came to Libby relatively early, particularly after Serge Korff's 1939 experiments showing how radioactive carbon-14 was created in the atmosphere by cosmic rays. Unfortunately, Libby's research was interrupted by the onset of World War II, during which he was enlisted to aid the Army in its race to construct an atomic bomb.
After the war when he returned to an academic position at the University of Chicago's Institute for Nuclear Studies, Libby was ready to begin his most important work. Libby knew that all living things carried traces of carbon-14: plants absorbed it during photosynthesis, and animals that ate plants would absorb it during digestion. As long as a creature was alive it would constantly replenish its supply of carbon-14; however, when it died the supply would begin to undergo radioactive decay. Libby hoped that the age of an organic material could be determined by measuring how much carbon-14 was left in its cells. Because the amounts of carbon-14 were very small, Libby's team was forced to construct a Geiger counter of unprecedented accuracy. They began by surrounding the device with steel walls 8 inches (20 cm) thick. Then he placed eleven smaller Geiger counters around the main sensor. This array was designed specifically to detect any cosmic rays strong enough to penetrate the steel armor; when such a ray was detected, the main sensor was turned off long enough for the ray to pass, so that it would not affect the overall reading. In order to make the carbon-14 easier to detect, the sample was first burned (converting it into pure carbon ) and later converted into a gas.
In his first tests Libby used samples whose ages were known, such as a piece of wood from an Egyptian tomb that was about 4,600 years old. As the accuracy of his method improved he chose items of greater and greater age, such as tree ring samples from 3000-6000 b.c. Meanwhile, scientists worldwide were beginning to take notice of Libby's experiments, intrigued by a dating method that could reach back nearly 50,000 years.
In the last forty years, Libby's method of radiocarbon dating has been used on more than 100,000 samples and in eighty different laboratories. In addition to plant and animal life, this dating method has been used to verify the age of such diverse artifacts as the Dead Sea Scrolls (2,100 years), a charcoal sample from an ancient South Dakota campsite (7,000 years), and a pair of sandals from an Oregon cave (9,300 years). Modern methods have improved the accuracy of carbon-14 dating to nearly 70,000 years, with an uncertainty of plus-or-minus ten percent.


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