Neutron star surface explosions explained by element creation in lab

A nuclear reaction experiment provides a glimpse at what is going on when neutron stars consume nearby companion stars.

Neutron stars are among the densest objects in the universe, reaching several times the mass of our Sun, but little more than 20 kilometres in diameter. One teaspoon of a neutron star’s material would weigh as much as a mountain. Their immense gravitational pull will suck in material from other cosmic objects that venture too close.

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This process can strip hydrogen and helium from a nearby star, which then amasses on the surface of the neutron star. Repeated ignitions of the matter in explosions creates new chemical elements.

Called nucleosynthesis, this process involves the exchange between atomic nuclei of their protons and neutrons to make new elements.

“Neutron stars are really fascinating from the points of view of both nuclear physics and astrophysics,” says lead researcher nuclear astrophysicist Kelly Chipps, from the US Department of Energy’s Oak Ridge National Laboratory (ORNL) in Tennessee. “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 (JENSA) which uses the world’s highest-density helium jet for accelerator experiments. The team specialises in looking at nuclear reactions on Earth to understand processes throughout the cosmos.

JENSA scientists produced a signature nuclear reaction that would occur on the surface of a neutron star at a lab at Michigan State University.

The experiment involved shooting a beam of argon-34 at a target made up of helium-4 (the numbers following the isotopes refers to the total number of protons and neutrons in the atomic nucleus). The fusion process resulted in the production of calcium-38 nuclei with 20 protons and 18 neutrons.

The calcium-38 nuclei then ejected protons to end up as potassium-37 nuclei.

By measuring the energies and angles of the particles produced, the physicists were able to determine 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-37 nucleus ended up in, which is one of the components predicted by the theoretical model,” Chipps explains.

Read more: Neutron stars collide and create a neutron star heavier than previously thought possible

The test verified a statistical 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.

“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.”

The research is published in Physical Review Letters.

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