Researchers studying the labyrinthine nature of atomic nuclei say they have answered a question that has puzzled physicists for more than half a century: Why does the radioactive isotope known as carbon-14 decay so slowly? The discovery could lead to a better understanding of the workings of the strong nuclear force, one of the four fundamental forces of nature.
Within the menagerie of common isotopes--such as carbon-11, nitrogen-13, and oxygen-15--carbon-14 is a tortoise among hares, and a painfully slow tortoise at that. Whereas its cousins take mere minutes or hours to decay, only half of the carbon-14 component of a given substance is gone after 5730 years, having become nitrogen-14. This long half-life has made the isotope invaluable to archaeologists as a tool to determine the age of organic matter, whether plant or animal. By analyzing the ratio of carbon-14 to nitrogen-14, researchers can determine, within a narrow margin, when the sample in question last breathed or photosynthesized. That's because when an organism dies, it stops ingesting carbon, including carbon-14.
Yet the reason carbon-14 decays much more slowly than other isotopes has remained elusive, and researchers have argued for decades about the mechanism. No matter how hard they tried, no one was able to describe the mechanics of carbon-14 decay, says physicist Gerald Brown of Stony Brook University in New York. Brown is a member of the duo who in the early 1990s proposed an explanation for the peculiar behavior of the carbon-14 nucleus, called Brown-Rho scaling. "It's a much-maligned idea," says Brown.
The theory goes like this: The protons and neutrons within any element's nucleus are bound by the actions of subatomic particles called mesons, which whiz back and forth between them. The mesons carry two versions of the strong nuclear force, which keeps the nucleus from flying apart. According to Brown-Rho scaling, in other isotopes the difference in strength between the two versions--called the central force and the tensor force--is large enough to render the nuclei unstable, meaning they decay quickly. But in carbon-14, the two versions are nearly in balance, which allows the isotope to persist much longer. Eventually, however, the nucleus succumbs to instability, and the element transmutes into nitrogen-14.
Until now, however, no one has been able to confirm the theory mathematically. In an upcoming issue of Physical Review Letters, Brown and Stony Brook University colleague Jeremy Holt have completed new calculations that they say verify the idea. Holt's team reports that the sensitivity of carbon-14 decay to the tensor force is indeed behind the process. "This is the basic mechanism leading to the long lifetime we predict," Holt says.
The findings also reaffirm the first mention of the connection between the tensor force and carbon-14 decay, made by Israeli physicist Igal Talmi in 1954, notes physicist Larry Zamick of Rutgers University in Piscataway, New Jersey. But limitations remain, he says. The calculations cover only certain meson interactions within the shell of the carbon nucleus, Zamick says. So, he recommends that the team try to expand its conclusions to cover "a larger shell model."