Part IV: Nuclear Reactions

"In science, there is only physics. Everything else is stamp collecting." — Ernest Rutherford (attributed)

If radioactive decay is what nuclei do on their own, nuclear reactions are what happens when we bring nuclei together. And when nuclei collide, the results range from gentle elastic scattering to the complete rearrangement of nuclear matter in fission and fusion — the processes that power nuclear reactors and stars.

In these five chapters, we develop nuclear reaction physics from first principles. Chapter 17 establishes the kinematics (conservation laws, Q-values, center-of-mass transformations) and the concept of the cross section — the fundamental quantity that describes how likely a reaction is. We introduce partial wave analysis and the optical model that treats the nucleus as a partially absorbing sphere.

Chapter 18 develops Bohr's compound nucleus model, in which projectile and target fuse into an excited intermediate state that "forgets" how it was formed. The Breit-Wigner resonance formula, derived here, is one of the most important results in nuclear physics — and one you will encounter again in particle physics, atomic physics, and electrical engineering.

Chapter 19 treats direct reactions — fast, peripheral processes where the projectile interacts with individual nucleons rather than the whole nucleus. Stripping reactions like (d,p) are the primary experimental tool for determining single-particle structure, and knockout reactions at radioactive beam facilities are revealing the structure of exotic nuclei that exist for mere milliseconds.

Chapters 20 and 21 cover the two reactions that most profoundly affect human civilization. Nuclear fission — the splitting of heavy nuclei — powers reactors and weapons, and its physics (the fission barrier, neutron multiplication, criticality) is developed from the liquid drop model. Nuclear fusion — the joining of light nuclei — powers every star in the universe, and the quest to achieve controlled fusion on Earth is one of the great scientific and engineering challenges of our time.

Chapters in Part IV: - Chapter 17: Nuclear Reaction Fundamentals — kinematics, Q-values, cross sections - Chapter 18: Compound Nucleus Reactions and Resonances - Chapter 19: Direct Reactions — stripping, pickup, and knockout - Chapter 20: Nuclear Fission — splitting the atom - Chapter 21: Nuclear Fusion — powering the stars and (maybe) the grid

Chapters in This Part