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Finding Sterile Neutrinos
The existence of theoretical particles called “sterile neutrinos” could offer a deeper understanding of dark matter, the strange material that permeates the universe and accounts for 85 percent of its total mass. Livermore scientist Stephan Friedrich led a team from Lawrence Livermore National Laboratory and the Colorado School of Mines in an experiment funded by the Laboratory Directed Research and Development (LDRD) Program demonstrating the power of using nuclear decay in high-rate quantum sensors in the search for sterile neutrinos. The findings are the first measurements of their kind and appeared in the January 13, 2021, issue of Physical Review Letters.
In the experiment, nicknamed the “BeEST” (Beryllium Electron-capture with Superconducting Tunnel junctions), researchers implanted radioactive beryllium-7 atoms into superconducting sensors developed at Livermore. A process called electron capture—in which an electron in an atom’s inner shell is drawn into the nucleus and combines with a proton—decayed the beryllium-7, forming lithium-7 and a neutrino. The neutrino then escaped, making it undetectable, but the reduced recoil energy or byproduct of the lithium-7 produced a measurable signal of the neutrino mass.
The team performed simulations on Laboratory supercomputers to gain confidence in the detection of sterile neutrinos and understand the materials effects in the detector. Friedrich says, “This research lays the groundwork for more intensive searches for these new particles using large arrays of sensors with new superconducting materials.”
Contact: Stephan Friedrich (925) 423-1527 (friedrich1@llnl.gov).
Doubling Laser-Produced Antimatter
The ability to create numerous positrons in a laboratory setting opens new doors to antimatter research, including an understanding of the physics underlying astrophysical phenomena such as black-hole accretion and gamma-ray bursts. Livermore physicists used the high-intensity OMEGA Extended Performance (EP) laser at the University of Rochester’s Laboratory for Laser Energetics to shoot through a gold target with microstructures, called a silicon microwire array, and produced high-energy electrons, generating electron–positron pairs. The research results appeared in the March 2, 2021, issue of Applied Physics Letters.
Prior to the physical experiment, particle-in-cell simulations optimized the spacing and length of the micro-structures added to the typical gold target. These highly ordered silicon microwire arrays faced the OMEGA EP laser pulse and guided the relativistic electron beam along a structured surface to facilitate a more direct laser acceleration. The laser irradiated the gold target’s microstructures—much like assembled Legos, but only 1 millimeter in size. The laser–plasma interaction generated relativistic electrons and transported them through the material, making high-energy photons and spontaneously producing antimatter pairs in response to the laser energy transforming into mass.
Previous research, using flat, unstructured targets, produced around 100 billion particles of antimatter. These new experiments doubled the result, increasing antimatter production by 100 percent. “Adding front surface microstructures to the typical gold target constitutes a cost-effective approach to substantially increase the positron yield while keeping the same laser conditions, putting us one step closer toward using laser-generated positron sources for a variety of applications,” says Sheng Jiang, the lead author of the paper.
Contact: Sheng Jiang (925) 424-2905 (jiang8@llnl.gov).
Clarifying Ignition Performance
In inertial confinement fusion experiments at Lawrence Livermore’s National Ignition Facility (NIF), a spherical shell of deuterium–tritium fuel is imploded to reach the conditions needed for fusion, self-heating, and eventual ignition. Livermore researchers partnered with the University of Rochester’s Laboratory for Laser Energetics and Los Alamos National Laboratory to develop a compression-scaling model benchmarked to 1D implosion simulations across relevant implosion designs. Ultimately, their results, featured in the April 13, 2021, issue of Physics of Plasmas, could lead to improved ignition efficacy.
The team developed an isobaric and isentropic compression scaling model incorporating sensitivity to key parameters involving pressure, implosion velocity, fuel aspect ratio, and mass ratio and compared the model to compressibility trends across NIF implosion data for three ablators—plastic, carbon, and beryllium. Researchers found the strength of the first shock is the dominant contributor setting maximum fuel convergence and observed additional sensitivities to successive shock strengths and fuel aspect ratios that improve the agreement between the expected and measured compression for carbon and beryllium designs.
The best compression levels followed the expectations of the model with the exception of high-energy-density carbon shells that exhibited a lower level of compression independent of the laser drive conditions. The paper’s lead author, Livermore’s Otto Landen says, “Understanding compression trends as we varied laser and capsule parameters motivates further research in improving compression without necessarily demanding higher laser energy.”
Contact: Otto Landen (925) 424-5581 (landen1@llnl.gov).
