The physic of Nuclear Fission
Unveiling the Forces and Energy Behind Nuclear Fission: Exploring the Science That Powers Our World
This article was written for the Comprendre newsletter: subscribe to get the next article and more!
Last week, we delved into the intriguing history of Nuclear Fission, a groundbreaking discovery unveiled to the world in 1939, thanks to the brilliant minds of scientists like Lise Meitner, often credited with theorizing fission. If you haven’t had the chance to read it yet, you can find last week’s article on our website.
Today, we plunge into the fascinating realm of the physics that governs this incredible process. Picture a world where the smallest particles and the most immense energies intertwine to reveal one of the most profound phenomena in science. What mechanisms lie behind the extraordinary process of Nuclear Fission?
Join us as we unravel the physics behind nuclear fission, where understanding the fundamental forces at play leads to a deeper appreciation of the monumental impact this discovery has had.
Forces and energy, the Science behind Nuclear Fission.
To theorize nuclear fission, Meitner employed a highly effective visualization technique (learn more about it here). She envisioned the atom as a water droplet. When a neutron strikes the atom, the water droplet elongates and then constricts at its center, ultimately splitting into two smaller drops. As simple as it may appear, this idea conceals the entire process of nuclear fission. Let’s delve into the process step by step so that we can comprehend the forces at play, beginning with our understanding of the water droplet itself: the atom.
The structure of the atom has long been a subject of wonder and theoretical exploration. From the initial theory proposed by John Dalton in 1803, envisioning it as a solid sphere, to the latest quantum model by Erwin Schrödinger in 1926, our understanding of the atom underwent significant transformations throughout the 19th century. It wasn’t until the early 20th century that we began to gain a more profound insight into its underlying structure, thanks to the pioneering work of scientists like Ernest Rutherford and Niels Bohr. They laid the foundational concepts: an atom consists of a central mass, the nucleus, around which small particles called electrons “orbit”. This groundwork paved the way for the theory of nuclear fission and Meitner’s water droplet model, in which the droplet primarily represents the nucleus, the core of the atom.
Physicists later discovered that the nucleus itself is composed of two fundamental elements: protons and neutrons. These constituents are held together by one of the most potent forces in the universe: the strong nuclear force. Operating on the minuscule scale, this force exerts a strength that is approximately $10^{38}$ (a 10 followed by 38 zeros) times more powerful than gravity, the force that keeps us anchored to the Earth. It ensures the nucleus’s structural integrity under all circumstances, rendering it incredibly robust. Meitner’s water droplet analogy now transforms into an impenetrable fortress, akin to the legendary city of Troy.
Yet history teaches us that Troy was not an impenetrable fortress, as isn’t the nuclei. The question then becomes: how to break the nuclei, therefore initializing the process of nuclear fission.
Science is some time of beautiful complexity, other times of simple brutality. In this case, the solution leaned toward the latter. To break the nucleus, a stronger force must be applied, one that destabilizes the atom’s integrity.
Given the formidable strength of the strong nuclear force, this task seemed insurmountable. Breaking the nucleus would require an immense force. However, another intriguing scientific observation came into play. Through experimentation, physicists realized that the larger an atom is, the less stable it becomes, or, in scientific terms, the more fissile it is. Alternatively, the bigger and more unstable the atom, the less force is needed to break it apart.
Some materials are incredibly fissile, requiring only a gentle nudge from a small particle like a neutron to kickstart the fission process. One well-known example is uranium, particularly its less stable variant, uranium-235, often referred to as a fissile isotope. Using those fissile materials scientists succeed in initiating the process. Now fission itself, the splitting of the atom, happens.
What is about to unfold can be measured in attoseconds ($10^{-18}$ seconds), yet it is an event of astounding intricacy. As the atom undergoes fission, its protons and neutrons begin the intricate dance of regrouping into two distinct nuclei, but not all participate; a handful of neutrons embark on a journey into the unknown (keep them in mind, for they will play a pivotal role later). The newly formed nuclei, nearly identical yet distinct, experience an intense mutual repulsion akin to the force exerted by two similar magnets — an effect known as Coulomb repulsion. This repulsion is a direct consequence of the extraordinary energy involved in the process.
Simultaneously, due to the release of such high energy, a cascade of electromagnetic radiation is unleashed, including the notorious gamma rays. These gamma rays carry away a portion of the released energy as high-frequency electromagnetic radiation, adding to the spectacle of the fission event.
But the story doesn’t end here. Within these newly formed nuclei, certain particles are not content to stay as they are. Some neutrons, crucial players in the unfolding drama, undergo a transformation known as beta decay. In this process, a neutron is converted into a proton by emitting an electron and an elusive companion — [the antineutrino](https://neutrinos.fnal.gov/types/antineutrinos/#:~:text=An antineutrino is the antiparticle,kind of charge%3A lepton number.). The antineutrino, a nearly massless and chargeless particle, escapes from the scene, barely interacting with its surroundings. This process not only alters the composition of the nuclei but also plays a pivotal role in the intricate balance of particle interactions within the aftermath of nuclear fission. The journey of these particles, spanning mere attoseconds yet underpinning the fundamental processes of our universe, is a testament to the marvels of particle physics and nuclear science.
Nuclear fission has taken place; our water droplet has now split into two, giving rise to a myriad of intriguing phenomena along the way. Yet, a fundamental question remains: What do we do with this transformative event? How can we harness its power for the betterment of humanity, or, as history would show us, for its diminishment?
In the upcoming and final chapter of this series on Nuclear Fission, we will delve into the theme of transformation. We will examine the immense energy involved and the ingenious solutions that scientists have devised to harness it effectively.
