Scientists led by Kelly Chipps of the Department of Energy’s Oak Ridge National Laboratory have successfully replicated a nuclear reaction that occurs on the surface of a neutron star. By using a unique gas jet target system, they have enhanced understanding of nuclear reactions that lead to the cre
that is dense, pure and localized within a couple millimeters. JENSA will also provide the primary target for the Separator for Capture Reactions, or SECAR, a detector system at FRIB that allows experimental nuclear astrophysicists to directly measure the reactions that power exploding stars. Co-author Michael Smith of ORNL and Chipps are members of SECAR’s project team.
High-resolution charged-particle detectors surrounding the gas jet precisely measured energies and angles of the proton reaction products. The measurement took advantage of detectors and electronics developed at ORNL under the leadership of nuclear physicist Steven Pain. Accounting for the conservation of energy and momentum, the physicists back-calculated to discover the dynamics of the reaction.
“Because neutron stars are so weird, they are a useful naturally occurring laboratory to test how neutron matter behaves under extreme conditions,” Chipps said. “Because the neutron star is superdense, its huge gravity can pull hydrogen and helium over from a companion star. As this material falls to the surface, the density and temperature grow so high that a thermonuclear explosion can occur that can propagate across the surface,” Chipps said. Thermonuclear runaway transforms nuclei into heavier elements. “The reaction sequence can produce dozens of elements.
“We’re testing the transition between the statistical model being valid or invalid,” Chipps said. “We want to understand where that transition happens. Because Hauser-Feshbach is a statistical formalism — it relies on having a large number of energy levels so effects over each individual level are averaged out — we’re looking for where that assumption starts to break down.
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