How scientists reproduced the nuclear magic of neutron stars

A group of scientists from Oak Ridge National Laboratory has recreated a major nuclear reaction that takes place on the surface of a neutron star that consumes the mass of a companion star. The team, working in collaboration with nine institutions from three countries, used a unique jet gas targeting system to mimic the reaction and thus improve our understanding of the stellar processes and formation of diverse nuclear isotopes. This experiment provides insight into the process of nucleosynthesis on neutron stars, in which hydrogen and helium are pulled from a nearby star by the star’s massive gravity, triggering explosions that form new elements. Credit: Jacquelyn DeMink/ORNL, US Department of Energy

Chipps heads the Jet Experiments in Nuclear Structure and Astrophysics, or JENSA, which has collaborators from nine institutions in three countries. The team uses a unique gas jet target system, which produces the world’s highest-density helium jet for accelerator experiments, to understand nuclear reactions that proceed with the same physics on Earth as in outer space.

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.

Built at ORNL and now at the Facility for Rare Isotope Beams, a DOE Office of Science user facility that MSU operates, the JENSA system provides a target of lightweight gas 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.

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.

The lab experiment improves understanding of nuclear reactions that occur when material falls onto the surface of an important subset of neutron stars. These stars are born when a massive star runs out of fuel and collapses into a sphere about as wide as a city such as Atlanta, Georgia. Then gravity squeezes fundamental particles as close together as they can get, creating the densest matter we can directly observe. One teaspoon of neutron star would weigh as much as a mountain. Neutron-packed stars rotate faster than blender blades and make the universe’s strongest magnets. They have solid crusts surrounding liquid cores containing material shaped like spaghetti or lasagna noodles, earning them the nickname “nuclear pasta.”

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

In this animation, a powerful neutron star, right, is being fed by a companion star. Nuclear reactions on the surface of a neutron star can ignite again, producing a complex mixture of reactants. Credit: Jacquelyn DeMink/ORNL, US Department of Energy

Achieving this understanding requires teamwork. Astronomers observe the star and collect data. Theorists are trying to understand the physics inside a star. Nuclear physicists measure nuclear reactions in the laboratory and test them against models and simulations. This analysis reduces the large uncertainties caused by the paucity of empirical data. “When you put all of those things together, you really start to understand what’s going on,” Chibs said.

“Because the neutron star is so dense, its massive gravity can pull hydrogen and helium from the companion star. As this material falls to the surface, the density and temperature increase so much that a thermonuclear explosion can occur that can propagate across the surface,” Chips said. A thermonuclear runaway converts nuclei into heavier elements. “The reaction sequence can produce dozens of elements.”

The surface explosions do not destroy the neutron star, which goes right back to what it was doing before: feeding its companion and exploding. Repeated eruptions pull crustal material into the mix, creating a strange combination in which heavy elements formed during previous eruptions react with lightweight hydrogen and helium.

Theoretical models predict which elements form. Typically, scientists analyze the interaction measured by the JENSA team using a statistical theoretical model called the Hauser-Feshbach formalism, which posits that a continuum of excited energy levels of the nucleus can be involved in the interaction. Other models instead assume that only one energy level is involved.

“We’re testing the transition between the statistical model being valid or invalid,” Chips said. We want to understand where this transition is taking place. Because Hauser-Feshbach is a statistical formalist—it relies on having a large number of energy levels so that the effects are averaged at each individual level—we look for where this assumption starts to break down. For nuclei such as magnesium 22 and argon 34, there is an expectation that the nucleus does not have sufficient levels for this averaging approach to be correct. We wanted to test that.”

A question remained as to whether the statistical model would be valid for such interactions that occur in stars rather than in terrestrial laboratories. “Our result shows that the statistical model is valid for this specific interaction, and this removes a huge amount of uncertainty from our understanding of neutron stars,” said Chips. “This means that we now have a better understanding of how these nuclear reactions proceed.”

Next, the researchers will attempt to improve the statistical model by further testing its limitations. a past paper He discovered atomic mass 22, which is a magnesium nucleus, and found that the model was incorrect by a factor of about 10. The current paper led by ORNL, which examined 12 atomic mass units above this, found that the model correctly predicted reaction rates.

“Somewhere in between [atomic] Block 20 and 30, this transition between where the statistical model is correct and where it is not,” said Chips. “The next thing is to look for feedback in the middle of that range to see where that transition occurs.” Chips and her collaborators at JENSA began this endeavor.

The title of the paper is “The First Direct Measurement of ³⁴p (α, p)³⁷Cross-section of the K reaction of burning mixed hydrogen and helium in accretion neutron stars”.

Reference: “First direct measurement constraining the reaction cross-section of 34Ar(α,p)37K of mixed hydrogen-helium burning in accretion neutron stars” by J. Browne et al. (JENSA Cooperative), May 22, 2023, Available Here. Physical review letters.
DOI: 10.1103/PhysRevLett.130.212701

The Department of Energy’s Office of Science, the National Science Foundation, and the ORNL Laboratory Research and Development Program supported the work.