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Elemental Alchemy: How Scientists Replicated the Nuclear Magic of Neutron Stars

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A group of scientists from Oak Ridge National Laboratory has recreated a key nuclear reaction that happens on the surface of a neutron star consuming mass from a companion star. The team, working in collaboration with nine institutions from three countries, used a unique gas jet target system to mimic the reaction and thereby improved our understanding of stellar processes and the formation of diverse nuclear isotopes. This experiment provides insights into the nucleosynthesis process on neutron stars, where hydrogen and helium from a nearby star are drawn in by the star’s immense gravity, leading to explosions that form new elements. Credit: Jacquelyn DeMink/ORNL, U.S. Dept. of Energy

For spectroscopy of light elements leaving the target during nuclear reactions, JENSA lead scientist Kelly Chipps of ORNL uses high-resolution detectors. Credit: Erin O’Donnell/Facility for Rare Isotope Beams.

The process of nucleosynthesis creates new atomic nuclei. One element can turn into another when protons or neutrons are captured, exchanged or expelled.

A neutron star has an immense gravitational pull that can capture hydrogen and helium from a nearby star. The material amasses on the neutron star surface until it ignites in repeated explosions that create new chemical elements.

Many nuclear reactions powering the explosions remain unstudied. Now, JENSA collaborators have produced one of these signature nuclear reactions in a lab at Michigan State University. It directly constrains the theoretical model typically used to predict element formation and improves understanding of the stellar dynamics that generate isotopes.

For the current experiment, the scientists struck a target of alpha particles (helium-4 nuclei) with a beam of argon-34. (The number after an isotope indicates its total number of protons and neutrons.) The result of that fusion produced calcium-38 nuclei, which have 20 protons and 18 neutrons. Because those nuclei were excited, they ejected protons and ended up as potassium-37 nuclei.

ORNL researchers Michael Smith, Steven Pain, and Kelly Chipps use JENSA, a unique gas jet system, for laboratory studies of nuclear reactions that also occur in neutron stars in binary systems. Credit: Steven Pain/ORNL, U.S. Dept. of Energy

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.

“Not only do we know how many reactions occurred, but also we know the specific energy that the final potassium-37nucleus ended up in, which is one of the components predicted by the theoretical model,” Chipps said.

“Because neutron stars are so weird, they are a useful naturally occurring laboratory to test how neutron matter behaves under extreme conditions,” Chipps said.

Achieving that understanding takes teamwork. Astronomers observe the star and collect data. Theoreticians try to understand physics inside the star. Nuclear physicists measure nuclear reactions in the lab and test them against models and simulations. That analysis reduces large uncertainties resulting from a dearth of experimental data. “When you put all of those things together, you really start to understand what’s happening,” 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.”

Surface explosions do not destroy the neutron star, which goes right back to what it was doing before: feeding off its companion and exploding. Repeated explosions pull crust material into the mix, creating a bizarre composition in which heavy elements formed during previous explosions react with lightweight hydrogen and helium.

Theoretical models predict which elements form. Scientists typically analyze the reaction that the JENSA team measured using a statistical theoretical model called the Hauser-Feshbach formalism, which assumes that a continuum of excited energy levels of a nucleus can participate in a reaction. Other models instead assume that only a single energy level participates.

“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. For nuclei like magnesium-22 and argon-34, there’s an expectation that the nucleus doesn’t have enough levels for this averaging approach to be valid. We wanted to test that.”

A question remained about whether the statistical model was valid for such reactions taking place in stars rather than earthly laboratories. “Our result has shown that the statistical model is valid for this particular reaction, and that removes a tremendous uncertainty from our understanding of neutron stars,” Chipps said. “It means that we now have a better grasp of how those nuclear reactions are proceeding.”

“Somewhere between [atomic] mass 20 and 30, this transition between where the statistical model is valid and where it’s not valid is taking place,” Chipps said. “The next thing is to look for reactions in the middle of that range to see where this transition is occurring.” Chipps and her JENSA collaborators have begun that endeavor.

DOE’s Office of Science, the National Science Foundation and ORNL’s Laboratory Directed Research and Development program supported the work.

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