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Nuclear Structure and Astrophysics

The Nuclear Structure and Astrophysics Group performs pioneering measurements with beams of unstable nuclei to determine the mechanism of how stars explode and the accompanying cosmic creation and dispersion of the elements of our world, including those necessary for life. We carry out our measurements at accelerator laboratories throughout the U.S. and internationally, with a scientific focus on understanding nova explosions, supernova explosions, X-ray bursts, and neutron-star mergers. We develop advanced detector and targetry systems that we use in our experiments. Our Group also performs synergistic data evaluations and cosmic element synthesis simulations to fully explore these fascinating astrophysical systems. Our focus on measurements with unstable nuclei is fully aligned with the latest Long Range Plan for U.S. Nuclear Science, and has applications in a variety of areas including homeland security, nuclear non-proliferation, and medical isotopes.

Research

Our Group members make direct measurements of thermonuclear capture reactions on proton-rich unstable nuclei that drive some stars to undergo violent nova explosions or X-ray bursts. A significant result in this work was our utilization of the Daresbury Recoil Separator at ORNL to measure proton capture on radioactive 17F. Our measurement dramatically improved predictions of the cosmic production of the long-lived radionuclide 18F that is used as a diagnostic of the nova explosion mechanism. Our next measurements of this type will involve the Separator for Capture Reactions SECAR that we are helping to construct at the Facility for Rare Isotope Beams (FRIB) at Michigan State University.

We also use beams of neutron-rich unstable nuclei to determine the rates of reactions that produce the heaviest nuclei in neutron-star mergers and core-collapse supernovae. Our recent accomplishment in this effort is the experimental determination of the neutron capture rates on neutron-rich exotic tin isotopes (124,126,128,130,132Sn), the first such systematic data set on unstable nuclei.

To carry out this work, we develop advanced detector, targetry, and data acquisition systems. A prime example is our development of the Jet Experiments for Nuclear Structure and Astrophysics (JENSA) gas jet target system that produces the highest-density helium jet for accelerator experiments in the world. JENSA was recently used with a radioactive 34Ar beam for a measurement of the 34Ar + alpha —> 37K + p reaction that is important in understanding the synthesis of elements in X-ray bursts. Other systems we have developed include the SuperORRUBA array of silicon strip detectors, the VANDLE array of neutron detectors, and the SABRE and ODeSA neutron detector arrays.