After the fission of an atomic nucleus, why are the fragments in a spin state? This question, which is important for improving the design of nuclear energy facilities, has been plaguing scientists for more than four decades. A recently published study says the answer has finally been found through experimentation – and it’s just like when a leather band is pulled.
It is generally understood that for an object to start spinning, it must have an initial force to make sense. So scientists have been puzzled as to how all parts of a fissioned atomic nucleus have the angular momentum of spin.
It seems as strange as making something out of nothing,” says Jonathan Wilson, a nuclear physicist at the University of Paris-Saclay (French: Université Paris-Saclay), the study’s lead author. Nature is playing magic with us. An object that’s not in spin splits and both parts are in spin.”
Previous studies have found that nuclear fission has a process in which protons within the nucleus squeeze each other making the nucleus unstable, plus the protons are all positively charged particles and they naturally repel each other, causing the nucleus to be stretched and split into two parts with a narrower bottleneck zone in between. When the nucleus is finally stretched, the two parts break away from the bottleneck and separate quickly.
In the past few decades, there have been two main types of explanations for this in the scientific community. One group believes that this element of spin arises before fission, due to the bending, twisting and tilting of particles within the nucleus, due to the thermal excitation of the particles, quantum fluctuations or a combination of both; the other group of explanations believes that this occurs after the nucleus fission, due to the interaction forces between the protons within the fragment causing the fragment to spin.
The study, published Feb. 24 in the journal Nature, concludes that this spin occurs after fission.
George Bertsch, a nuclear physicist at the University of Washington who was not involved in the study, said, “This is remarkable new data and an important advance in our understanding of the nuclear fission process.”
This study analyzes experiments on a variety of unstable isotopes, including thorium 232, uranium 238 and californium 252, focusing on the gamma rays they radiate after fission. The gamma rays carry within them the spin information of the fissioned fragments.
If the spins were already present before the fission, then the spins of the two parts should be equivalent, but in opposite directions, Wilson said. “But that’s not what we observed.” The researchers observed that each part each had its own rotation, independent of the other part. They observed a variety of isotopes where this was the case.
The researchers estimate that when the nucleus is stretched and broken, the fragments become teardrop-like and also have surface tension, tending to become spherical. And the energy released by this process causes the fragments to heat up and spin, somewhat like when a rubber band is pulled and the fragments bounce around in disorder.
Wilson admitted that their theory is also a simplified scenario that “explains 85 percent of the different scenarios, but a more refined theory is needed to make more accurate predictions.”
The researchers say the study not only answers a decades-old mystery, but more importantly, helps scientists better design nuclear reactor facilities in the future.
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