The Greek term “atom” means “indivisible.” However, don’t be fooled by the name.
The first completely microscopic description of the instant an atom splits in two has been offered by a simulation created by US theoretical physicists, offering new insights into an intense occurrence that came to define a new era in science and technology.
When physicists Otto Hahn, Lise Meitner, and Fritz Strassmann demonstrated in 1938 how uranium nuclei split in two when exposed to neutrons, we learned just how misleading that one little phrase is.
Nuclear fission is slow to reveal its secrets decades after it was first used in power, medicine, war, and scientific research.
The nucleus of a big atom is a frenzied storm of quantum activity that goes beyond the crude ideas of protons and neutrons clumped together like gumballs in a dispenser.
It’s difficult enough to comprehend the behavior and interactions of individual nucleons sitting quietly by themselves, let alone those going through major changes.
The fission process is broken down into four parts by theoretical physicists from the University of Washington (UW) and Los Alamos National Laboratory to make it a little easier to understand.
The injection of a slow-moving neutron causes the nucleus to bulge and reorganize itself in what is known as a saddle point for the first 10–14 seconds (give or take), giving the atom the appearance of a little peanut shell.
A far faster transition, known as saddle-to-scission, occurs shortly after, during which the pieces of the fission process are formed. About 5×10-21 seconds pass during this.
Step three transforms in a relative blink of 10–22 seconds, making it even faster. The nucleus formally disintegrates during a neck rupture, also known as a scission.
After a short delay, the fission pieces pull themselves into shape and speed away in the last step, which takes a sluggish 10–18 seconds to unfold. This releases neutrons and gamma rays and may trigger other decay processes.
Numerous theories explain the exact movement of subatomic particles from peanut to pop, but experimental findings frequently defy fundamental physics assumptions or conflict with “microscopic” models of the interactions between individual protons and neutrons.
The quantum many-body simulation, which is based on a framework created by lead author Aurel Bulgac, a physicist at the University of Washington, is the most realistic representation to date of what to anticipate at the precise moment of scission, when the bridge connecting the two halves of a massive atomic nucleus pinches in and splits.
The US Department of Energy’s Oak Ridge National Laboratory’s supercomputer was heavily used for computations on uranium-238, plutonium-240, and californium-252 under various initial conditions.
“This is probably the most precise and most carefully obtained theoretical description of neck rupture, without any assumptions and simplification,” Bulgac states.
We have a pretty specific prediction that hasn’t been made yet. “Let’s assume that this is happening, and if it’s happening, then this probably is going to be seen” was the foundation of all previous hypotheses. That’s not what we did. All we did was enter the highly precise equations of motion that have been known for many decades in nuclear physics along with quantum mechanics.
A few surprises in the fission process were discovered by the simulation. The team’s model revealed a distinct ‘wrinkle’ in the density of subatomic particles that appeared before the scission point, in contrast to other models that expected a liberal dusting of quantum randomness in the neck rupture process.
Additionally, it appeared that the two types of nucleons divided at different times, with the proton neck breaking before the neutron neck.
Importantly, the model was able to predict the energy, angular distribution, and even escape routes of the very energetic neutrons, confirming controversial theories that they were released during the scission stage.
“Most experiments look for them in the direction of the motion of the fission fragments, and they couldn’t distinguish the scission neutrons there because most of them were thermal neutrons emitted by the hot fragments,” adds Bulgac.
Predictions in hand, the next stage is to determine if these new discoveries about the ‘indivisible’ atom’s splitting in two are supported by experimentation.
This research was published in Physical Review Letters.