Mar 12 2019
A half-a-decade-old puzzle has been solved by an international collaboration, including researchers at the Department of Energy’s Oak Ridge National Laboratory, explaining the reason for beta decays of atomic nuclei being slower compared to what is anticipated based on the beta decays of free neutrons.
First-principles calculations show that strong correlations and interactions between two nucleons slow down beta decays in atomic nuclei compared to what’s expected from the beta decays of free neutrons. This impacts the synthesis of heavy elements and the search for neutrinoless double beta decay. (Image credit: Andy Sproles/Oak Ridge National Laboratory, U.S. Department of Energy)
The study outcomes, reported in Nature Physics, bridge a long-time gap in physicists’ knowledge of beta decay, a vital process used by stars to form heavier elements, and reiterate the need to take subtle effects—or more realistic physics—into account while predicting specific nuclear processes.
For decades, scientists have lacked a first-principles understanding of nuclear beta decay, in which protons convert into neutrons, or vice versa, to form other elements. Our team demonstrated that theoretical models and computation have progressed to the point where it is possible to calculate some decay properties with enough precision to allow for direct comparison to experiment.
Gaute Hagen, Study Lead, Staff Scientist, ORNL.
In order to solve the puzzle, the researchers simulated the decay of tin-100 into indium-100, a neighboring element on the periodic table. Although both the elements share the same nucleons numbers (protons and neutrons), tin-100 has 50 protons and indium-100 has 49 protons.
In order to accurately calculate the beta decay, the researchers not just had to precisely simulate the structure of the mother and daughter nuclei but also take the correlated interactions between two nucleons at the time of the transition into account. This supplementary treatment posed an acute computational difficulty owing to the combination of strong nuclear correlations and interactions between the decaying nucleons.
Previously, nuclear physicists tried to solve this puzzle by incorporating a fundamental constant to account for the observed beta-decay rates of neutrons within and outside the nucleus—a process called “quenching.” However, using machines such as the ORNL’s Titan supercomputer, Hagen’s group showed that this mathematical crutch is not needed anymore.
Nobody really understood why this quenching factor worked. It just did. We found that it could largely be explained by including two nucleons in the decay—for example, two protons decaying into a proton and a neutron, or a proton and a neutron decaying into two neutrons.
Gustav Jansen, Computational Scientist, ORNL.
The group, including collaborators from Lawrence Livermore National Laboratory, University of Tennessee, University of Washington, TRIUMF (Canada), and Technical University Darmstadt (Germany), carried out an extensive analysis of beta decays from light to medium-heavy nuclei up to tin-100.
The accomplishment offers nuclear physicists more confidence in their search for solutions to certain extremely puzzling mysteries associated with the formation of matter in the universe. Apart from routine beta decay, researchers are seeking to calculate neutrinoless double beta decay, a theorized form of nuclear decay that, if observed, would investigate significant new physics and help to ascertain the mass of the neutrino.
Tin to Indium
Several elements include isotopes that decay over longer time periods. For instance, the half-life of carbon-14, the nucleus used for carbon dating, is 5730 years. However, other nuclei occur only for fractions of a second prior to emitting particles in an attempt to stabilize.
During neutron beta decay, an anti-neutrino and an electron are ejected. During the transformation of tin-100 into indium-100, the nucleus experiences beta-plus decay, ejecting a positron and a neutrino while converting a proton into a neutron.
Since tin-100 has equal number of protons and neutrons, it presents an abnormally high rate of beta decay, providing the ORNL researchers with a strong signal that can be used to verify the results. Moreover, the tin-100 nucleus is “doubly magic,” that is, the nucleons occupy defined shells within the nucleus, making it strongly bound and comparatively simple in structure. The ORNL researchers’ NUCCOR code, programmed to solve the nuclear many-body problem, is an excellent code for describing doubly magic nuclei up and down the nuclear chart.
A doubly magic nucleus like tin-100 isn’t as complicated as many other nuclei. This means we can reliably compute it using our coupled cluster method, which calculates properties of large nuclei by accounting for forces between the individual nucleons.
Thomas Papenbrock, Researcher, University of Tennessee and ORNL.
However, for modeling beta decay, the researchers also had to compute the structure of indium-100, a more intricate nucleus compared to the doubly magic tin-100. This necessitated a highly precise treatment of the strong correlations between the nucleons. Hagen’s group borrowed concepts from quantum chemistry, which addresses electrons as waves, to successfully develop methods for modeling these processes.
In our case we are dealing with nucleons instead of electrons, but the quantum chemistry concepts have helped us branch out from doubly magic nuclei and expand into these open-shell regions.
Titus Morris, Physicist, ORNL.
Guiding Experiment
With Hagen’s group revealing its on par understanding of beta decay with this experiment, it is now seeking to take advantage of innovative supercomputers such as ORNL’s Summit, the world’s most robust, to guide existing and future experiments.
At present researchers are using Summit to simulate the way in which calcium-48, another doubly magic nucleus, would experience neutrinoless double beta decay—a process through which two neutrons beta decay into protons, but without the ejection of any neutrinos. The outcomes could help experimentalists in selecting an ideal detector material for the prospective discovery of this rare phenomenon.
Currently, calculations using different nuclear models of neutrinoless double beta decay may differ by as much as a factor of six. Our goal is to provide a benchmark for other models and theories.
Gaute Hagen, Study Lead, Staff Scientist, ORNL.
The DOE Office of Science supported this study.