Researchers at the Oak Ridge National Laboratory, under the direction of Kelly Chipps, have created a characteristic nuclear reaction that takes place on the surface of a neutron star that is consuming mass from a companion star. Their accomplishment advances knowledge of stellar mechanisms that produce a range of nuclear isotopes.
“Neutron stars are really fascinating from the points of view of both nuclear physics and astrophysics,” said ORNL nuclear astrophysicist Kelly Chipps, who led a study published in Physical Review Letters. “A deeper understanding of their dynamics may help reveal the cosmic recipes of elements in everything from people to planets.”
Chipps heads the Jet Experiments in Nuclear Structure and Astrophysics, or JENSA, which has collaborators from nine institutions in three countries. For accelerator experiments, the team uses a special gas jet target system that generates the highest-density helium jet in the world to better understand nuclear reactions that follow the same physics on Earth as they do in space.
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 can draw hydrogen and helium from a neighboring star due to its powerful gravitational pull. As the material builds up on the neutron star’s surface, it repeatedly explodes, producing 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 advances knowledge of the stellar dynamics responsible for isotope generation and directly constrains the theoretical model normally employed to forecast element creation.
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.
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. Thermonuclear runaway transforms nuclei into heavier elements. The reaction sequence can produce dozens of elements.
Kelly Chipps
The gas jet’s surrounding high-resolution charged-particle detectors accurately detected the energy and angles of the proton reaction products. Under the direction of nuclear physicist Steven Pain, ORNL built the detectors and electronics used in the measurement. The scientists used back calculations to determine the reaction’s kinetics while taking into account the conservation of energy and momentum.
“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 laboratory experiment advances knowledge of nuclear events that take place when particles hit the surface of a significant subset of neutron stars. When a large star runs out of fuel and collapses into a sphere approximately the size of a city like Atlanta, Georgia, these stars are created. The densest matter we can directly detect is created when gravity forces elementary particles as near together as possible.
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.
Achieving that understanding takes teamwork. Data is gathered by astronomers as they study the star. The physics of the star is being studied by theorists. In the laboratory, nuclear physicists monitor nuclear processes and compare them to simulations and models. Large uncertainties brought on by a lack of experimental data are reduced by this analysis.
“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. A peculiar composition is produced when heavy components created during earlier explosions react with lightweight hydrogen and helium as a result of repeated explosions drawing crust material into the mixture.
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.”
Next, the researchers will try to improve the statistical model by further testing its limits. A past paper explored atomic mass 22, a magnesium nucleus, and found the model incorrect by almost a factor of 10. The current ORNL-led paper, probing 12 atomic mass units above this, found that the model correctly predicted reaction rates.
“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.
The title of the paper is “First direct measurement of the 34Ar(α,p)37K reaction cross section for mixed hydrogen and helium burning in accreting neutron stars.”
DOE’s Office of Science, the National Science Foundation, and ORNL’s Laboratory Directed Research and Development program supported the work.
