Calcium-48's 'neutron skin' thinner than previously thought

"We built a nucleus from scratch from its basic constituents--protons and neutrons," said Hagen, a theoretical physicist who initiated and led the project with an award from DOE's Office of Science Early Career Research Program. "To solve this strongly correlated system of 48 nucleons is far from trivial, because it is a complicated quantum mechanical many-body problem. Many things had to come together--accurate nuclear forces, sophisticated computational algorithms and a powerful tool such as Titan at ORNL--to achieve these results."

While the distribution of electric charge inside the atomic nucleus is well known from experiments involving electron scattering, the distribution of the neutrons, which have no electric charge, is difficult to measure. In the nucleus of calcium-48, which has eight more neutrons than protons, the neutron distribution extends beyond the charge distribution and thereby sets the actual size of this nucleus. "This is the first really reliable calculation of such a massive nucleus from first principles," said Hagen. "We reproduced basic observables for the first time by building this nucleus from scratch. We've answered a basic question--what is the size of the atomic nucleus?"

The ORNL-led team calculated radii, binding energies and dipole polarizabilities for helium-4, oxygen-16 and calcium-40 and accurately reproduced measurements of these isotopes. "That was a quality check of our calculation," Hagen said. "It gives us confidence about our predictions for calcium-48."

The team, which included partners from the University of Tennessee, Michigan State University, Chalmers University of Technology (Sweden), TRIUMF (Canada), Hebrew University (Israel), Technical University Darmstadt (Germany), University of Oslo (Norway) and University of Trento (Italy), turned to Titan, a Cray XK7 system at Oak Ridge Leadership Computing Facility, a DOE Office of Science User facility at ORNL. The project ran the nuclear structure code NUCCOR (Nuclear Coupled Cluster at Oak Ridge), for about 15 million CPU hours, allocated through the Innovative and Novel Impact on Theory and Experiment program.

Researchers at Jefferson Lab are preparing to measure the neutron radius of calcium-48. A team of physicists, the Darmstadt-Osaka Collaboration, has already measured its dipole polarizability and is analyzing the results. The findings could validate the work of Hagen's team and constrain future theoretical models.