How carbon-14 lives long and prospers

IOWA STATE (US) — Thirty million process hours on the Jaguar supercomputer was enough to explain carbon-14’s long, slow decay and how it is able to accurately date relics as far back as 60,000 years.

The computer, at Oak Ridge National Laboratory in Tennessee, has a peak performance of 2.3 quadrillion calculations per second, a speed that tops the list of the world’s top 500 supercomputers.

While the carbon dating technique is well known and understood (the ratio of carbon-14 to other carbon isotopes is measured to determine the age of objects containing the remnants of any living thing), the reason for carbon-14’s slow decay has not been understood.

Why, exactly, does carbon-14 have a half-life of nearly 6,000 years while other light atomic nuclei have half-lives of minutes or seconds? (Half-life is the time it takes for the nuclei in a sample to decay to half the original amount.)

“This has been a very significant puzzle to nuclear physicists for several decades,” says James Vary, professor of physics and astronomy at an Iowa State University. “And the underlying reason turned out to be a fairly exotic one.”

The reason involves the strong three-nucleon forces (a nucleon is either a neutron or a proton) within each carbon-14 nucleus. It’s all about the simultaneous interactions among any three nucleons and the resulting influence on the decay of carbon-14. And it’s no easy task to simulate those interactions.

The research project’s findings were recently published online by the journal Physical Review Letters.

In explaining the findings, Vary notes that two subatomic particles with different charges will attract each other. Particles with the same charges repel each other. What happens when there are three particles interacting that’s different from the simple addition of their interactions as pairs?

The strong three-nucleon interactions are complicated, but it turns out a lot happens to extend the decay of carbon 14 atoms.

“The whole story doesn’t come together until you include the three-particle forces,” Vary says. “The elusive three-nucleon forces contribute in a major way to this fact of life that carbon-14 lives so long.”

The three-particle forces work together to cancel the effects of the pairwise forces governing the decay of carbon-14, says Pieter Maris, postdoctoral research associate. As a result, the carbon-14 half-life is extended by many orders of magnitude—making it such a useful tool for determining the age of objects.

To get that answer, the researchers needed a billion-by-billion matrix and a computer capable of handling its 30 trillion non-zero elements. They also needed to develop a computer code capable of simulating the entire carbon-14 nucleus, including the roles of the three-nucleon forces.

Furthermore, they needed to perform the corresponding simulations for nitrogen-14, the daughter nucleus of the carbon-14 decay. And, they needed to figure out how the computer code could be scaled up for use on the Jaguar petascale supercomputer. “It was six months of work pressed into three months of time,” Maris says.

Now they say there are more puzzles to solve: “Everybody now knows about these three-nucleon forces,” Vary says. “But what about four-nucleon forces? This does open the door for more study.”

The research was supported by contracts and grants from the U.S. Department of Energy Office of Science.

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