Nuclear fission has powered our world and medical advances for decades, yet some of its secrets have remained elusive.
One of the biggest puzzles? What exactly happens when the nucleus of an atom splits at the “neck break” point.
Aurel Bulgac, a professor of physics at the University of Washington, has delved into this question. He and his team set out to simulate the intricate dance of particles during this critical moment of separation.
The supercomputer tackles the quantum realm
To meet this challenge, Bulgac’s team collaborated with researchers from Los Alamos National Laboratory (LANL).
They harnessed the enormous computing power of the Summit supercomputer at the Department of Energy’s Oak Ridge National Laboratory (ORNL).
This was the first fully microscopic, quantum many-body simulation of the neck rupture of a nucleus.
“This is probably the most accurate and careful theoretical description of neck snapping, without any assumptions and simplifications,” Bulgac noted. “We have a very specific prediction, which until now did not exist.”
Rules and dynamics of nuclear fission
For more than 80 years, scientists have proposed various models to explain the dynamics of separation. The classical theory of “fluid collapse” compared the nucleus to a charged sphere of fluid that separates and sends out particles when disturbed.
But Bulgac’s findings suggest a different story.
First, the team discovered that neck snapping is not random. Instead, where the core eventually splits is determined well in advance of the actual cutting.
Furthermore, they found that the proton neck breaks earlier than the neutron neck, meaning that the neutrons hold the neck together just before it breaks completely.
Perhaps most intriguingly, they confirmed the existence of shear neutrons. These are neutrons emitted just when the nucleus splits.
“There were people who said they exist, and others said they don’t exist. We see them, we don’t see them, and so on,” Bulgac noted.
Their simulation not only showed that shear neutrons are real, but also detailed where they go and how much energy they carry.
Nuclear fission and shear neutrons
The team’s simulation revealed that shear neutrons are emitted in two different ways. Some shoot to the side, perpendicular to the direction the fragmentation fragments are moving.
Others emerge from the “noses” of the split fragments due to the matter waves sent out during the neck snap.
“After we put everything together and ran the simulation, we discovered some things that were completely unexpected: neutrons coming off the side and then in the direction of the moving fragments,” Bulgac said.
“These kinds of distinct spectra are something that none of the previous models predicted. Moreover, they are of very high energy, so they must have very distinct properties from thermal neutrons.
A very “super” computer simulation.
Running such a detailed simulation was not easy. The team used the entire Summit supercomputer for nearly 1 million node hours.
Each run lasted about 15 hours and covered nuclei such as uranium-238, plutonium-240 and californium-252 under various conditions.
“One absolutely crucial tool was the use of supercomputers at Oak Ridge, starting with Jaguar and Titan, then Summit,” Bulgac explained. “Without them, all this would be impossible. Absolutely impossible.”
The mathematical framework behind the simulation is something Bulgac has been refining since the early 2000s.
Instead of tracking every single particle, his approach focuses on the overall distribution of particles in space, making the complex problem more manageable.
What’s next for nuclear fission?
The next big step is to test these predictions in the real world. Bulgac has already begun conversations with experimentalists eager to verify the findings.
“We used exact values for the parameters that describe this phenomenon – and nothing else. And these are the results we got”, he said. “Now, if they are not true, then there is a very big question: What is wrong with the theory?”
It is a key moment. If experiments confirm the simulation, it could reshape our understanding of nuclear fission. If not, it opens up new avenues of inquiry.
“At the moment, it’s very difficult to see what would be wrong, but we have to wait and see,” Bulgac said. “If you make a prediction, it is confirmed or not, and then you have to watch and see. That’s how physics works.”
Why does any of this matter?
Understanding the nuances of nuclear fission is not just an academic exercise. It has practical implications for nuclear power generation, medical applications and even our understanding of fundamental physics.
By shedding light on the fission process and the behavior of fission neutrons, Bulgac’s work could lead to more efficient nuclear reactors or new technologies that exploit fission.
Publication of the team’s paper in the journal Physical review papers has already sparked interest. Scientists around the world are eager to see where this research leads.
For those of us outside the lab, it’s a reminder of how much there is still to learn about the forces that shape our universe.
The new chapter in nuclear physics
As experiments are underway to test these findings, the scientific community watches with bated breath.
Whether confirming or challenging Bulgac’s simulation, the results will undoubtedly deepen our understanding of nuclear fission.
And perhaps, in the not-too-distant future, we will look back on this research as a turning point—a moment when we peeled back another layer of the atomic world and discovered something truly remarkable.
Who would have thought that after all these years, the simple act of splitting an atom could hold so many surprises?
The full study is published in the journal Physical review papers.
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