Physicists Unravel Five Decade-Old Mystery Surrounding Carbon-14 Dating (2/27/2008)

The mystery has dogged scientists for 50 years about why carbon-14 dating works. And now, physicist Ruprecht Machleidt and a team of researchers have helped close a gap in the theory of carbon dating that has, until now, gone unsolved.

Machleidt, professor of physics at the University of Idaho, has pursued the solution to this mystery for his entire professional career. Carbon dating, a method used primarily to discover the age of fossils, relies on the slow transition of carbon into nitrogen to give a fossil a timeline, thus narrowing down the origin of the life form that eventually became fossilized.

"Without carbon dating, the history and the investigation of evolution of the past 60,000 years would be in deep trouble," he said.

However, for 50 years, physicists didn't know exactly why carbon dating worked the way it did.

"The reason why the half-life of carbon-14 is so long is non-trivial, and that's why it took 50 years to unravel the mystery," Machleidt said.

The confusion comes from the half-lives of different carbon atoms, or the process by which they decay into nitrogen atoms. Since decay is a predictable process, scientists have the ability to use it as a reference for time.

Most carbon isotopes, which are slightly modified versions of carbon-12, the standard carbon atom, decay very quickly. Some decay in less than a second, while others take up to a few minutes. Carbon-14, however, has a half-life of 5,730 years, after which only half of the original carbon remains, while the other half has converted into nitrogen-14. It then takes another 5,730 years for the carbon to be cut in half again, and so on. By this measurement, life forms that have high amounts of carbon in them can be dated back to about 60,000 years, Machleidt said, which has given us the most detailed and important accounts of evolution in the earth's recent history.

Machleidt's research revealed the reason why carbon-14 has such a long half-life, while its sibling atoms, carbon-11, 12, 13, 15 and so on, decay so quickly. Their findings were published this month in the journal Physical Review Letters of the American Physical Society, in a paper titled "Shell model description of the C-14 dating beta decay with Brown-Rho-scaled NN interactions."

Machleidt's research team included Jeremy W. Holt, Gerald E. Brown and Tom T.S. Kuo from Stony Brook University and Jason D. Holt from TRIUMF, a nuclear physics lab located at the University of British Columbia.

Essentially, the Stony Brook researchers injected into Machleidt's existing research a theory about how the nucleus of carbon-14 works, which converted Machleidt's theory into fact. The results finally proved how specific forces within carbon-14's nucleus slow down its transition into nitrogen.

Machleidt said while the discovery doesn't necessarily change anything in nuclear physics, it proves that nuclear theory can answer questions without forcing physicists to embark on lengthy tests that often cost billions of dollars and result in large amounts of radioactive waste. He said it also proves the reliability of carbon-dating, which has been called into question by those who denounce its use in the theory of evolution.

"On scientific grounds, our work lends more credibility to carbon dating because we now understand it better," he said.

Machleidt said that while this discovery represents a great leap forward in his own research, he now can set his sights on other goals that may aide in the search for clean nuclear power.

"There is a lot more to do," he said. "The Idaho Nuclear Laboratory is embarking on the development of fourth-generation nuclear reactors. An important aspect of this new generation is breeding and recycling the nuclear fuel, which is highly radioactive and dangerous stuff." To handle the fuel safely, Machleidt says "very elaborate and expensive facilities are needed."

Until now, there was no reason to have confidence in the findings of nuclear theorists because there was no proof that theory gave real, tangible results, Machleidt said.

"Hundreds of isotopes are produced in a nuclear reactor, most of which are unstable, which is why they are radioactive," he said. "One needs to know the half-lives of those isotopes to know what one is facing and to know what method to use to separate them. One can find this out experimentally, but that means conducting experiments with highly radioactive materials which, again, is tedious, expensive, and dangerous."

Enter nuclear physics. "Our research results prove that we can calculate the properties of certain radioactive isotopes precisely, which means instead of putting humans at risk and spending millions of dollars on dirty experiments, theoreticians can sit down at computers and calculate those half-lives and other properties of the isotopes produces in a reactor, which is a cheap, clean and fast alternative to what used to be standard in nuclear physics," he said.

Read Machleidt's published research paper on carbon-14 online at the American Physical Society's Web site: prl.aps.org

Note: This story has been adapted from a news release issued by the University of Idaho

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