From Lawrence Livermore National Laboratory and ORNL: “Solving a 50-year-old beta decay puzzle with advanced nuclear model simulations”

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Oak Ridge National Laboratory

From Lawrence Livermore National Laboratory

March 19, 2019

Anne M Stark
stark8@llnl.gov
925-422-9799

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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 by Andy Sproles/Oak Ridge National Laboratory.

For the first time, an international team including scientists at Lawrence Livermore National Laboratory (LLNL) has found the answer to a 50-year-old puzzle that explains why beta decays of atomic nuclei are slower than expected.

The findings fill a long-standing gap in physicists’ understanding of beta decay (converting a neutron into a proton and vice versa), a key process stars use to create heavier elements. The research appeared in the March 11 edition of the journal Nature Physics.

Using advanced nuclear model simulations, the team, including LLNL nuclear physicists Kyle Wendt (a Lawrence fellow), Sofia Quaglioni and twice-summer intern Peter Gysbers (UBC/TRIUMF), found their results to be consistent with experimental data showing that beta decays of atomic nuclei are slower than what is expected, based on the beta decays of free neutrons.

“For decades, physicists couldn’t quite explain nuclear beta decay, when protons inside atomic nuclei convert into neutrons or vice versa, forming the nuclei of other elements,” Wendt said. “Combining modern theoretical tools with advanced computation, we demonstrate it is possible to reconcile, for a considerable number of nuclei, this long-standing discrepancy between experimental measurements and theoretical calculations.”

Historically, calculations of beta decay rates have been much faster than what is seen experimentally. Nuclear physicists have worked around this discrepancy by artificially scaling the interaction of single nucleons with the electroweak force, a process referred to as “quenching.” This allowed physicists to describe beta decay rates, but not predict them. While nuclei near each other in mass would have similar quenching factors, the factors could differ dramatically for nuclei well separated in mass.

Predictive calculations of beta decay require not just accurate calculations of the structure of both the mother and daughter nuclei, but also of how nucleons (both individually and as correlated pairs) couple to the electroweak force that drives beta decay. These pairwise interactions of nucleons with the weak force represented an extreme computational hurdle due to the strong nuclear correlations in nuclei.

The team simulated beta decays from light to heavy nuclei, up to tin-100 decaying into indium-100, demonstrating their approach works consistently across the nuclei where ab initio calculations are possible. This sets the path toward accurate predictions of beta decay rates for unstable nuclei in violent astrophysical environments, such as supernova explosions or neutron star mergers that are responsible for producing most elements heavier than iron.

“The methodology in this work also may hold the key to accurate predictions of the elusive neutrinoless double-beta decay, a process that if seen would revolutionize our understanding of particle physics,” Quaglioni said.

Other institutions include Oak Ridge National Laboratory, TRIUMF and the Technische Universität Darmstadt Germany.


Technische Universität Darmstadt campus

Technische Universität Darmstadt

The work was funded by the Laboratory Directed Research and Development Program.

See the full article here .


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LLNL Campus

Operated by Lawrence Livermore National Security, LLC, for the National Nuclear Security Administration
Lawrence Livermore National Laboratory (LLNL) is an American federal research facility in Livermore, California, United States, founded by the University of California, Berkeley in 1952. A Federally Funded Research and Development Center (FFRDC), it is primarily funded by the U.S. Department of Energy (DOE) and managed and operated by Lawrence Livermore National Security, LLC (LLNS), a partnership of the University of California, Bechtel, BWX Technologies, AECOM, and Battelle Memorial Institute in affiliation with the Texas A&M University System. In 2012, the laboratory had the synthetic chemical element livermorium named after it.

LLNL is self-described as “a premier research and development institution for science and technology applied to national security.” Its principal responsibility is ensuring the safety, security and reliability of the nation’s nuclear weapons through the application of advanced science, engineering and technology. The Laboratory also applies its special expertise and multidisciplinary capabilities to preventing the proliferation and use of weapons of mass destruction, bolstering homeland security and solving other nationally important problems, including energy and environmental security, basic science and economic competitiveness.

The Laboratory is located on a one-square-mile (2.6 km2) site at the eastern edge of Livermore. It also operates a 7,000 acres (28 km2) remote experimental test site, called Site 300, situated about 15 miles (24 km) southeast of the main lab site. LLNL has an annual budget of about $1.5 billion and a staff of roughly 5,800 employees.

LLNL was established in 1952 as the University of California Radiation Laboratory at Livermore, an offshoot of the existing UC Radiation Laboratory at Berkeley. It was intended to spur innovation and provide competition to the nuclear weapon design laboratory at Los Alamos in New Mexico, home of the Manhattan Project that developed the first atomic weapons. Edward Teller and Ernest Lawrence,[2] director of the Radiation Laboratory at Berkeley, are regarded as the co-founders of the Livermore facility.

The new laboratory was sited at a former naval air station of World War II. It was already home to several UC Radiation Laboratory projects that were too large for its location in the Berkeley Hills above the UC campus, including one of the first experiments in the magnetic approach to confined thermonuclear reactions (i.e. fusion). About half an hour southeast of Berkeley, the Livermore site provided much greater security for classified projects than an urban university campus.

Lawrence tapped 32-year-old Herbert York, a former graduate student of his, to run Livermore. Under York, the Lab had four main programs: Project Sherwood (the magnetic-fusion program), Project Whitney (the weapons-design program), diagnostic weapon experiments (both for the Los Alamos and Livermore laboratories), and a basic physics program. York and the new lab embraced the Lawrence “big science” approach, tackling challenging projects with physicists, chemists, engineers, and computational scientists working together in multidisciplinary teams. Lawrence died in August 1958 and shortly after, the university’s board of regents named both laboratories for him, as the Lawrence Radiation Laboratory.

Historically, the Berkeley and Livermore laboratories have had very close relationships on research projects, business operations, and staff. The Livermore Lab was established initially as a branch of the Berkeley laboratory. The Livermore lab was not officially severed administratively from the Berkeley lab until 1971. To this day, in official planning documents and records, Lawrence Berkeley National Laboratory is designated as Site 100, Lawrence Livermore National Lab as Site 200, and LLNL’s remote test location as Site 300.[3]

The laboratory was renamed Lawrence Livermore Laboratory (LLL) in 1971. On October 1, 2007 LLNS assumed management of LLNL from the University of California, which had exclusively managed and operated the Laboratory since its inception 55 years before. The laboratory was honored in 2012 by having the synthetic chemical element livermorium named after it. The LLNS takeover of the laboratory has been controversial. In May 2013, an Alameda County jury awarded over $2.7 million to five former laboratory employees who were among 430 employees LLNS laid off during 2008.[4] The jury found that LLNS breached a contractual obligation to terminate the employees only for “reasonable cause.”[5] The five plaintiffs also have pending age discrimination claims against LLNS, which will be heard by a different jury in a separate trial.[6] There are 125 co-plaintiffs awaiting trial on similar claims against LLNS.[7] The May 2008 layoff was the first layoff at the laboratory in nearly 40 years.[6]

On March 14, 2011, the City of Livermore officially expanded the city’s boundaries to annex LLNL and move it within the city limits. The unanimous vote by the Livermore city council expanded Livermore’s southeastern boundaries to cover 15 land parcels covering 1,057 acres (4.28 km2) that comprise the LLNL site. The site was formerly an unincorporated area of Alameda County. The LLNL campus continues to be owned by the federal government.

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