Mississippi physicist creates neutron star reaction in lab

For years, physicists have wondered if one unstable form of copper might act like a traffic jam in the universe’s most violent explosions.

This question is important because these explosions, called Type IX-ray bursts, are part of the cosmic machinery that helps build heavier elements. In the early universe, hydrogen and helium were dominant. Much of what came into being later, such as oxygen in the air and iron deep in the Earth, had to be forged in stars and stellar explosions.

Now, a team led by Mississippi State physicist Jaspreet Randhawa has directly measured the key nuclear reactions involved in that process. The results suggest that the suspected slowdown is much weaker than scientists feared. Therefore, heavy elements have a clear path to form during an explosion in a neutron star.

“The universe began almost entirely with hydrogen and helium,” Randhawa said. “All heavy elements, from the oxygen we breathe to the iron in Earth’s core, were produced in subsequent stars and stellar explosions. By determining how stellar explosions build heavy elements, scientists can better understand how the elements that form planets and support life are distributed across the universe.”

This study represents the first direct laboratory measurement of this specific reaction under conditions relevant to the problem. Randhawa’s graduate student, Muhammad Asif Zubair, also participated in the study.

Neutron stars are what remains after a massive star explodes. They are only the size of a city, but can pack more mass than the Sun. In some binary star systems, the neutron star pulls material from its companion star. The stolen material accumulates under overwhelming pressure and searing heat, and a burst of X-rays ignites the surface.

Inside these explosions, nuclear reactions can proceed rapidly, producing heavier nuclei. Scientists have long suspected that one step in this chain involving copper-59 traps the flow of material in a repeating loop called the NiCu cycle.

The problem boils down to a competition between two possible reactions. One route allows the chain to continue moving toward heavier elements. The other sends the material back to Nickel-56, effectively recycling it and undermining the large-scale elemental construction process.

If this recycling pathway were to prevail, it could stall the production of heavy materials from X-ray bursts.

Copper 59 made this question particularly difficult to answer. It decays within two minutes, leaving only a short time to measure what it does before it creates, accelerates, and disappears.

“We wanted to know whether nature has built-in obstacles that prevent the formation of heavy elements during X-ray bursts on the surface of neutron stars,” Randhawa said. “Our measurements show that this obstacle is much weaker than expected, meaning the process of building heavier elements can continue.”

To make the measurements, the research team conducted an experiment at TRIUMF in Canada. TRIUMF is one of the few places that can produce beams of copper-59 in useful quantities.

The researchers created a radioactive beam of copper-59, accelerated it before the isotope decayed, and fired it at a frozen hydrogen target. This setup used a windowless solid hydrogen target cooled to approximately 4 Kelvin. In addition, downstream particle detectors classified reaction products, including alpha particles and protons. Separate measurements tracked the target thickness and helped remove background from the silver foil lining.

The research team collected data at two center-of-mass energies: 4.0 ± 0.4 MeV and 4.68 ± 0.25 MeV. Total cross-sectional areas of 0.28 ± 0.06 and 0.85 ± 0.21 millibar were extracted from these runs.

These values ​​are important because they allow researchers to calculate the rate of the copper 59 proton-alpha reaction with much tighter limits than previously possible.

Previous studies have had to rely heavily on theory. The reaction library treated uncertainties as huge, sometimes as much as 100 times higher or lower. The new study reduces that uncertainty by about a factor of two.

This is a major change for a problem that has remained one of nuclear physics’ most stubborn uncertainties in X-ray burst modeling.

The research team found that the newly recommended response rate was about one-half lower than predicted by commonly used statistical models. Still, the data cap doesn’t completely eliminate that old prediction.

Still, the picture becomes clearer when the new results are compared with available estimates of competing proton-gamma reactions. Across the temperatures most relevant to X-ray bursts, the recycling strength of the NiCu cycle appears to be negligible, less than 5%.

In layman’s terms, the bottlenecks of concern are not very blocked.

It has implications beyond nuclear bookkeeping. The energy released by these reactions helps shape the X-ray burst light curve, the pattern of brightness that astronomers observe with telescopes. These light curves are used to test models of neutron stars, including efforts to determine properties such as compactness and mass-radius relationships.

To see what changes the new measurements make, the researchers ran a one-zone X-ray burst model using ignition conditions adjusted for the well-studied burster GS1826-24. When the newly measured reaction rate was varied within a tighter uncertainty range, the burst light curve did not change.

This result removes one major source of ambiguity from future comparisons of astrophysical models and observations.

It will also make more accurate predictions about the “ash” that remains after the outburst, the material that becomes part of the neutron star’s crust. Previous sensitivity studies suggested that this large variation in reaction rates could change the final composition of the ash over several mass numbers. This experiment narrows that uncertainty. However, the researchers note that a complete multizone calculation is beyond the scope of the current study.

This is not an outcome that will change the night sky overnight.

It is to reduce the ambiguity of the physical phenomena underlying the distant explosion. The more scientists understand which reactions accelerate, which stall, and which barely matter, the more accurately they will be able to interpret what space telescopes see when a neutron star flashes.

This is important for two related reasons. First, X-ray bursts are one of the few places researchers can test models of matter under extreme conditions. Second, those internal reactions are part of the broader story of where heavy elements come from.

This measurement does not answer all open questions. This does not fully resolve all reaction rates in the chain, nor does it replace the need for more detailed multizone simulations. However, this closes out the long-held possibility that a strong NiCu cycle was blocking the way to heavier elements during these bursts.

That seems much less likely now.

The new measurements have given astrophysicists more certainty for a reaction with very large uncertainties.

This improves X-ray burst models, reduces one important source of error when compared to telescope data, and should allow researchers to make more reliable predictions about how a neutron star’s crust builds over time.

It also strengthens the argument that element formation in these stellar explosions may continue to migrate beyond the copper region, rather than being trapped in a recycling loop.