Thursday, March 19, 2009

Nobel Laureate: Willard Libby


The Nobel Prize in Chemistry 1960.

"for his method to use carbon-14 for age determination in archaeology, geology, geophysics, and other branches of science"

Willard Frank Libby (1908 - 1980) won the Nobel Prize in Chemistry for using 14C decay to date organic material. Libby set out to study cosmic rays. He, and others, determined that one of the effects of cosmic ray is to produce carbon-14 atoms from nitrogen-14 atoms in the upper atmosphere.

He determined that the rate of production of carbon-14 and its rate of disintegration (half-life ~5600 years1) has reached an equilibrium. No matter where you find carbon, in the ocean, the atmosphere, or the biosphere, its radioactivity corresponds to about 14 disintegrations per minute per gram.

Living things incorporate this equilibrium mixture of 14C and 12C. Thus, we, like all other living things, are radioactive and this level of radioactivity can be measured using techniques that Willard Libby developed. When living things die, they stop incorporating carbon and the existing 14C continues to decay. As time goes on, the level of radioactivity declines with a half-life of ~5600 years. The age of organic material can be determined directly by measuring the remaining radioactivity of extracted carbon.

That's the basis of radiocarbon dating. Libby confirmed the feasibility of the technique by dating Egyptian artifacts, tree rings of known age, and the dead sea scrolls (labeled "Bible" in the figure). The results confirmed that radiocarbon dating works.

The results were published in the late 1940's. Since then, the technology has improved considerably. Today, scientists measure 14C directly using mass spectrometry so they don't have to wait for it to decay. Detailed calibration curves have been worked out to take account of the fact that cosmic ray intensity has varied somewhat over the past few thousand years.

With current technology, reliable dates back as far as 60,000 years can be obtained. This is about the limit of radiocarbon dating because the half-life of Carbon-14 is so short compared to more long-lived isotopes.

The presentation speech highlights the importance of radiocarbon dating in a number of disciplines.
Nobel Laureates
Libby's dating method soon attracted attention from the scientific world, and it was not long before carbon-14 laboratories were set up in many countries. Today, some forty institutions carry on investigations in this field, nearly half of them in America. Also here, in Sweden, we have such institutions, and their investigations have given results of great value. All age determinations - nowadays several thousand every year - are published in a general review, and thus made rapidly available throughout the world. The literature in this field has grown from year to year, and at present covers an impressive area.

One of the scientists who suggested Libby as a candidate for the Nobel Prize has characterized his work in the following way: "Seldom has a single discovery in chemistry had such an impact on the thinking in so many fields of human endeavour. Seldom has a single discovery generated such wide public interest".

Professor Libby. The idea you had 13 years ago of trying to determine the age of biological materials by measuring their carbon-14 activity was a brilliant impulse. Thanks to your great experimental skill, acquired during many years devoted to the study of weakly radioactive substances, you have succeeded in developing a method that is indispensible for research work in many fields and in many institutes throughout the world. Archaeologists, geologists, geophysicists, and other scientists are greatly indebted to you for the valuable support you have given them in their work. The Swedish Academy of Sciences desires to join those who offer you grateful thanks for what you have done for the benefit of so many sciences, and has decided to award you this year's Nobel Prize for Chemistry. May I congratulate you on behalf of the Academy, and ask you to receive the prize from the hands of His Majesty the King.

1. The modern value is 5730±40 years.

[Photo Credit: University of California History Digital Archives, Copyright © 2006 The Regents of the University of California.]

The images of the Nobel Prize medals are registered trademarks of the Nobel Foundation (© The Nobel Foundation). They are used here, with permission, for educational purposes only.

1 comment:

  1. The late Prof. Libby was something of a hardline conservative who advocated the construction of backyard fallout shelters, and took his own advice by building one. Interestingly enough, during one of the Southern California brush fires in the Santa Monica mountains, both his house and the fallout shelter were burned to the ground, indicating that retiring to it in response to a nuclear attack on the City of Los Angeles would not have done him much good.