Up To Date Research
By exploring carbon-14, researchers seek to understand what binds the nuclei of atoms.
For much of the public, the term "carbon-14" is among the most frequently mentioned scientific phrases heard in the popular media. The predictable decay of carbon-14 nuclei has long been used as a scientific tool for dating carbon-based materials in fields as diverse as history and archeology, yet the actual science involved with describing the physical properties of carbon-14 itself is less well understood.
When, in 1994, researchers discovered the Chauvet Cave in southern France filled with Stone Age artwork, carbon-14 indicated the charcoal drawings on the cave walls were approximately 31,000 years old. Six years earlier, when scientists examined the Shroud of Turin, results of carbon-14 analysis indicated the relic, a linen cloth believed by many to have been placed over the body of Jesus at the time of his burial, was more likely woven in the Middle Ages, more than a millennium later.
For the past half-century, carbon-14 has enabled scientists to dramatically improve the dating of both the living and inanimate elements of human history, including skeletons, ruins or anything that was once part of a plant or animal. By existing in all living things and decaying at a steady rate, carbon-14 gives researchers the ability to determine with reasonable accuracy the age of a long-abandoned community, tool or other artifact. And because, for reasons not yet understood, carbon-14 decays far more slowly than most isotopes in its weight class, the process provides researchers with the confidence to date items as far back as 60,000 years.
At ORNL a team led by David Dean is using the unprecedented computing power of the laboratory's petascale supercomputer to examine the carbon-14 nucleus. The team, which includes Hai Ah Nam of ORNL, James Vary and Pieter Maris of Iowa State University and Petr Navratil and Erich Ormand of Lawrence Livermore National Laboratory, hopes to explain carbon-14's long half-life and advance understanding of what holds all nuclei together.
"Carbon-14 is interesting to us because the physics says it should decay quickly, yet the measured half-life is much longer than expected," explained Nam, a physicist with ORNL's National Center for Computational Sciences. "The existing theoretical models used to describe light nuclei such as lithium, with six particles, or boron, with ten, have produced good results. But carbon-14, also a light nucleus, has been elusive. The models do not yield a value that matches what we measure experimentally, which means we are not capturing all of the physics."
Parsing the nuclear attraction
An isotope's half-life is the time required for one-half the atoms in a sample to decay. For most light isotopes the half-life is typically minutes or even seconds. Carbon-14, with a half-life of nearly 6,000 years, is an anomaly. A simulation that reveals why the half-life of this unique isotope is so long also has the potential to illuminate the half-lives of all isotopes, both long and short, thus providing a better understanding of how the observable matter in the universe is bound together.
The task is especially challenging because some of the precise properties that bind an atom's nucleus are not fully understood. Scientists have long known that the nucleus is made up of protons and neutrons, known generically as nucleons. More recently, they have determined that the nucleons are made up of even smaller particles, known as quarks and gluons, held together by the "strong force." To nuclear scientists, the holy grail of nuclear physics would be represented by a theoretical description of the properties of all nuclei, including the stable and unstable, mundane and exotic, large and small.
Dean and his colleagues have been awarded an allocation of 30 million processor hours on ORNL's supercomputer to dissect the secrets of carbon-14 with an application known as Many Fermion Dynamics—nuclear, created by Vary at Iowa State. Dean's team will be using nearly 150,000 of the supercomputer's more than 180,000 computing cores on the project. The application is ready to scale to even more cores as they become available.
The team is using an approach known as the no-core shell model to describe the nucleus. Analogous to the atomic shell model that predicts how many electrons can be found at any given orbit, the nuclear shell model defines the number of nucleons that can be found at a given energy level. Generally speaking, the nucleons gather at the lowest available energy level until the addition of more would violate the Pauli exclusion principle, which states that no two particles can be in the same quantum state. At that point nucleons occupy the next higher energy level. The force between nucleons complicates this picture and creates a computational problem of enormous complexity for researchers.
Using the power of the petascale
The unprecedented computational power available at Oak Ridge enables the team to depart from other nuclear structure studies in a variety of respects. The project is taking a microscopic look at the nucleus, working with its smallest known constituents. Nuclear models have been moving in this direction for seven decades, from the liquid drop model of Niels Bohr, which treated the nucleus as a single drop of nuclear fluid, to later models that examined the protons and neutrons separately. The ORNL team is able to probe even deeper. Utilizing an ab initio, or first principles, approach, they are working from the strong-force interactions of the quarks and gluons within each nucleon. In addition, Dean's team has adopted a "no-core" strategy that incorporates all 14 nucleons and includes more energy levels in the model. The simulations also extend beyond two-body forces, which include the interaction of every nucleon with every other nucleon two at a time, to incorporate three-body forces.
"Previously we could only consider two-nucleon interactions because the number of combinations needed to describe all the different interactions is vast, even for only two particles at a time," Nam explained. "And while two-particle interactions are the dominant way that these particles interact, there are some nuclear phenomena, like the half-life of carbon-14, that cannot be described using only a two-nucleon interaction, meaning three-particle interactions or higher are also significant.
"Our project is probing whether these two approaches, using the ab initio methods and the higher number of interactions, will describe more accurately why carbon-14 has such a long half-life and help explain how all nuclei are put together."
Calculations at this scale are now possible at Oak Ridge using a supercomputer capable of 2.3 thousand trillion calculations a second—making it the world's fastest scientific supercomputer. Also critical are the machine's 362 terabytes of memory, three times more memory than other system. Prior to the system's installation in 2008, such a simulation of the carbon-14 nucleus working from its smallest known constituents would have been unthinkable.
"These types of calculations for carbon-14 were previously not possible because the research required a memory-intensive calculation," explained Nam. "Accounting for the three-nucleon force amounts to storing tens of trillions of elements, or hundreds of terabytes of information."
By making use of such extraordinary computing power, the ORNL team hopes to move a little closer to an understanding of the atom's nucleus. If they are successful, carbon-14 will become an even bigger star in the scientific galaxy.
Web site provided by Oak Ridge National Laboratory's Communications and External Relations