. Scientific Frontline: Study Reveals Diminished Role for Neutrons in Creation of Carbon in Stars

Monday, April 25, 2022

Study Reveals Diminished Role for Neutrons in Creation of Carbon in Stars

The Texas Active Target (TexAT) particle detector designed and built by Texas A&M physicists is the centerpiece in a collaborative experiment to test whether other particles — specifically stray neutrons — could be involved in creating carbon.
Credit: Rogachev Group

A Texas A&M University-led collaboration has yielded new insight into one of the universe’s most important primordial reactions that made all life on Earth possible.

The multi-institution team of nuclear scientists that also includes researchers from Washington University in St. Louis and Ohio University concluded in a recent study that the role neutrons play in the creation of carbon, considered the definitive building block of life, actually is much smaller than previously thought.

Their data, acquired during 2020 and analyzed within the past year, is detailed in a paper published in Nature Communications.

“These findings are extremely significant because it demonstrates that the rate at which stars burn helium together to form carbon is less sensitive to any neutrons in the stars, which were previously thought to speed up this process,” said Dr. Jack Bishop, an assistant research scientist at the Texas A&M Cyclotron Institute and a lead author of the paper. “Understanding the way that stars burn is extremely important in understanding the life and death of stars, as well as where the elements that make up our universe originate from and in what quantity.”

The experiment specifically focused on the formation of carbon-12, which is produced inside the cores of dying stars in a rare phenomenon called the triple-alpha process where three alpha particles (helium-4 nuclei) collide and fuse. While scientists have long known the highly excited state of carbon produced as a result of this process emits gamma rays to achieve a stable condition (regular carbon-12), they have suspected that other particles also play a role in that de-excitation, specifically neutrons, which carry no electrical charge and can remove extra energy.

Because the reaction requires four particles — a neutron and three alpha particles — to occur within an extremely small volume of space comparable to the size of an atomic nucleus and a tiny time interval equal to a 10th of a femtosecond, directly measuring this process as it occurs in stars is essentially impossible. The team instead measured the probability of its inverse, breaking apart the carbon-12 with neutrons in a time-reversed reaction that effectively reverts the carbon-12 back to its three alpha particles.

Until recently, the probability for this process was estimated only theoretically, says Texas A&M physicist Dr. Grigory Rogachev, an expert in low-energy experimental nuclear physics and a principal investigator for the project.

“We now have the direct measurement, which provides accurate data on the probability of this process,” said Rogachev, head of the Texas A&M Department of Physics and Astronomy and a Cyclotron Institute member. “It turned out that the theoretical predictions for the probability of this neutron upscattering on carbon were off by more than one order of magnitude. The experimentally measured value is much smaller.”

To prove this, the team utilized a testing apparatus called the Texas Active Target, or TexAT — a specialized particle detector known as an active-target time-projection chamber that recently was constructed at the Cyclotron Institute. By firing beams of high-energy neutrons into the detector, they were able to blast apart carbon nuclei in carbon dioxide gas and observe the breakup pattern for a variety of different energies in three-dimensional renderings. The propensity of carbon-12 to break apart into three alpha particles indeed was seen to be lower and therefore much less important than expected, based on the previous theoretical models.

Ohio University nuclear astrophysicist Dr. Carl Brune oversaw beam testing at his institution’s 17,400-square-foot John E. Edwards Accelerator Laboratory, which boasts a rare 4.5-million-volt tandem accelerator capable of discharging atomic particles to energies up to 14 percent of the speed of light.

“Here at Ohio University, we are proud that our unique neutron science capabilities — an intense and well-characterized neutron beam in this case — made a significant contribution to this outstanding new science result,” Brune said. “It is very satisfying to see a multi-institution collaboration like this be successful.”

Approximately 5,000 neutrons per second were fired into TexAT, with only about one in a million of them rendering a reverse triple-alpha process — roughly one occurrence every five minutes. Texas A&M Cyclotron Institute postdoctoral research associate Dr. Cody Parker, a key member of the collaboration with expertise in neutron detector development, assisted the Ohio team with optimizing the neutron beam for precise measurement.

“Performing neutron-induced reactions is a complicated effort and not something you can do every day, because you can’t simply make a neutron beam,” Parker said. “A lot of people were involved in making this possible. We’re grateful for their contributions and are pleased with the outcome.”

Washington University nuclear chemist and principal investigator Dr. Lee Sobotka first proposed the idea of utilizing a time-projection chamber to determine the influence of neutrons on the triple-alpha process to his co-collaborators in 2017. Time-projection chambers like TexAT typically are used to construct visual renderings of the trajectories of rare isotope beams and the products of nuclear reactions induced by those beams. Using one to explore nuclear reactions with neutrons as in this instance, however, is an undertaking that’s never before been done.

“This experiment married a very special tool — an active-target time-projection chamber — with a low-energy accelerator capable of producing near mono-energetic neutrons,” Sobotka said. “It’s a first-of-its-kind union that provided the answer to a question first posed 55 years ago on the making of the seed for heavy-element synthesis.”

The team’s paper, “Neutron-upscattering enhancement of the triple-alpha process,” can be viewed online along with related figures and captions. Their work was supported by the Department of Energy’s National Nuclear Security Administration and Office of Science, the National Science Foundation, the Science and Technology Facilities Council and the Center for Excellence in Nuclear Training and University-based Research (CENTAUR), a multi-institutional effort led by Texas A&M that seeks to support collaborative research efforts in the area of low-energy nuclear science.

Source/Credit: Texas A&M University


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