Scientists have been perplexed by a subatomic riddle for more than 40 years: why can pieces of splitting atomic nuclei emerge turning from the wreckage? According to experts, these baffling gyrations may now be explained by an action similar to that which occurs when a rubber band is snapped.
Consider a big stack of coins to understand why this spinning is perplexing. It would be expected if this shaky structure collapsed. However, you wouldn’t anticipate all of the coins to start spinning as soon as they struck the ground after this stack fell.
Atomic nuclei with a lot of protons and neutrons are unstable, like a towering stack of pennies. Such hefty nuclei, rather than collapsing, are prone to splitting, a phenomenon known as nuclear fission. The shards that arise spin, which can be particularly perplexing if the split’s nuclei were not spinning themselves. Just as you wouldn’t expect an item to start moving on its own without some external force acting on it, a body starting to turn without an initial torque would appear to be supernatural, defying the rule of conservation of angular momentum.
According to research lead author Jonathan Wilson, a nuclear physicist at the Université Paris-Irene Saclay’s Joliot-Curie Laboratory in Orsay, France, this “makes it appear like something was made from nothing.” “Nature plays a cruel joke on us. We start with a non-spinning item, and after dividing it apart, both portions spin. Angular momentum, on the other hand, must be conserved.”
Fission is thought to start when a nucleus becomes unstable due to jostling between the protons, which naturally resist each other since they are positively charged. The embryonic pieces develop a neck between them as the nucleus lengthens. When the nucleus eventually disintegrates, these parts rapidly separate, and the neck breaks, a process known as scission.
According to Wilson, scientists have proposed a dozen or more different explanations for this spinning throughout the decades. The bending, wiggling, tilting, and twisting of the particles that make up the nucleus before the split, movements coming from thermal excitations, quantum fluctuations, or both, are one class of explanations. Another school of thought holds that spin develops after scission due to repulsion between protons in pieces. However, according to Wilson, “the outcomes of the trials looking into this all opposed each other.”
Wilson and his colleagues have now proven definitely that this spinning occurs after the break, according to data published in Nature on February 24. Nuclear physicist George Bertsch of the University of Washington in Seattle, who was not involved in the work, adds, “This is excellent new data.” “It’s a significant step forward in our knowledge of nuclear fission.”
The scientists looked at nuclei formed by the fission of various unstable elemental isotopes, including thorium-232, uranium-238, and californium-252. They concentrated on the gamma rays produced by nuclear fission, which stored information on the spin of the pieces that resulted.
If the spinning was caused by factors other than scission, the pieces should have equal and opposing spins. Wilson, however, claims that “this is not what we observe.” Instead, it appears that each piece spins independently of its companion, a finding that was consistent across all batches of nuclei studied, regardless of isotopes.
The researchers believe that as a nucleus lengthens and separates, the leftovers resemble teardrops at first. According to Wilson, these pieces have a property similar to surface tension that causes them to lower their surface area by adopting more stable spherical geometries, identical to how bubbles do. The remains heat up and spin due to the release of this energy, similar to how expanding a rubber band to the point of snapping results in a chaotic, elastic flailing of shards.
Wilson adds that this situation is exacerbated by the fact that each piece of nuclear debris isn’t just a uniform piece of rubber but resembles a bag of buzzing bees since its components are all moving and often colliding. “They’re like two little swarms that split off and go their separate ways,” he explains.
Overall, “these findings provide good support for hypothesis that the morphologies of nuclei at the time of separation affect their energy and the attributes of the fragments,” according to Bertsch. “This is critical for making fission theory more predictive and allowing us to describe how it can create elements reliably.”
Previous examinations of fissioning atoms, according to Wilson, failed to establish the sources of these gyrations because they lacked the advantages of modern, ultrahigh-resolution detectors and computationally demanding data-analysis tools.
Previous research has focused on the unconventional structures of “extreme” superheavy neutron-rich nuclei to determine how standard nuclear theory might account for such outliers. Much of the previous research avoided gathering and analyzing the massive amount of additional data required to analyze how the nuclear bits spun. Still, this current study was specifically designed to do so, he continues. The most unexpected aspect of the measurement, according to Bertsch, is that it could be done at all with such unambiguous findings.
Wilson warns that further research is needed to understand how scission causes spin entirely. He admits that his thesis is “simplistic.” “It can explain roughly 85% of the fluctuations in a spin as a function of mass that we experience, but a more complex theory could undoubtedly make more precise predictions. We’re not claiming anything more than that; it’s just a beginning point.” Other scientists at the European Commission’s Joint Research Center site in Geel, Belgium, have now validated the findings using a different approach, he says, and those results should be published soon.
These results may not only help scientists build better nuclear reactors in the future, but they may also help them answer a decades-old riddle. They might, for example, give insights into the nature of the gamma rays released by spinning nuclear pieces during fission, which may heat reactor cores and materials nearby. These thermal effects are still unknown, particularly regarding how they change between different types of nuclear power plants.
“There is up to a 30% difference between the models and the real data on these heating effects,” Wilson explains. “Our findings are only a fraction of the complete picture that should be considered when modeling future reactors, but a whole picture is required.”
These subatomic angular momentum investigations may also aid scientists in determining which superheavy elements and other unusual atomic nuclei they can make synthesis to shed more light on the still-obscure depths of nuclear structure. According to Wilson, there are around 7,000 nuclei that can potentially exist, but only 4,000 can be reached in the laboratory. “Knowing more about how fission pieces create spin can help us figure out what nuclear states we can access.”
Future studies may, for example, look at what happens when light or charged particles hit nuclei, causing them to fission. Wilson claims that the incoming energy might cause pre-scission effects on the spinning of the resulting fragments in such instances.
Wilson explains, “Even though fission was found 80 years ago, it’s so intricate that we’re still getting intriguing outcomes today.” “The narrative of fission is far from over—there are undoubtedly more tests to be done.”