# Nuclear Physics The nucleus is a hundred-thousandth the size of its atom yet holds nearly all its mass, bound by a force that overwhelms the electric repulsion of its packed protons. Nuclear physics asks what holds it together, and how it comes apart. _Figure 001 — Binding energy per nucleon: fusion climbs the left slope, fission descends the right, both toward the iron peak. _ _Figure 002 — Radioactive decay is exponential — the population halves over each successive half-life. _ One curve organizes the whole subject: the binding energy per nucleon. It rises steeply through the light elements, crests at iron — the most tightly bound matter there is — and falls away through the heavy ones. Everything downhill of that peak releases energy. Stability is never guaranteed. An unbalanced nucleus is _radioactive_: it transmutes toward the valley of stability at a fixed probability per unit time, so a population decays exponentially and each isotope carries its own **half-life**, from fractions of a second to billions of years. _Figure 003 — The three decay modes: an alpha cluster, a beta electron, and a gamma photon leaving one nucleus. _ Decay comes in three modes. **Alpha** ejects a helium-4 cluster; **beta** converts a neutron to a proton and emits an electron; **gamma** sheds pure energy as a photon, dropping the nucleus to a lower state without changing what it is. Firing particles at nuclei drives _reactions_ — the controlled transmutations that build new isotopes and reveal nuclear structure, each governed by the same conservation laws of energy, momentum, and charge. _Figure 004 — A fission chain reaction branches: each split releases neutrons that split more nuclei. _ Split a heavy nucleus and the fragments are more tightly bound than the whole — **fission** releases energy, plus spare neutrons that can split more nuclei. When each fission provokes more than one further fission, the chain reaction sustains itself, tamed in a reactor or unleashed in a bomb. _Figure 005 — Fusion joins two light nuclei into a more tightly bound one, releasing the difference. _ Run the curve uphill from the other end and light nuclei **fuse** into heavier, more tightly bound ones — the process that powers the stars and forged the elements, if the reactants can first be forced through their mutual repulsion. From sizes and masses through the nuclear force, the decay laws, reactions, fission, and fusion, the subject is one long reading of a single curve — and of the enormous energies stored in the gap between where a nucleus sits and where it would rather be. --- ## Contents ### 1. Nuclear Properties 1. [Nuclear Composition and Ground-State Properties](/nuclear-physics/nuclear-properties/nuclear-constituents-nuclide-chart) 2. [Nuclear Size, Shape, and Charge Distributions](/nuclear-physics/nuclear-properties/nuclear-size-charge-distributions) 3. [Nuclear Masses, Mass Excess, and Separation Energies](/nuclear-physics/nuclear-properties/nuclear-masses-binding-energy) 4. [The Semi-Empirical Mass Formula and the Valley of Stability](/nuclear-physics/nuclear-properties/semi-empirical-mass-formula) 5. [Nuclear Spin, Magnetic Dipole, and Electric Quadrupole Moments](/nuclear-physics/nuclear-properties/nuclear-moments-multipoles) ### 2. The Nuclear Force 1. [The Nuclear Force and the Shell Model](/nuclear-physics/nuclear-force-deuteron/nuclear-force-shell-overview) 2. [The Deuteron and the Tensor Force](/nuclear-physics/nuclear-force-deuteron/the-deuteron) 3. [Nucleon-Nucleon Scattering and the Interaction's Structure](/nuclear-physics/nuclear-force-deuteron/nucleon-nucleon-scattering) 4. [Meson Exchange, the Yukawa Potential, and Isospin](/nuclear-physics/nuclear-force-deuteron/meson-theory-isospin) ### 3. Nuclear Models 1. [The Fermi Gas Model](/nuclear-physics/nuclear-models/fermi-gas-model) 2. [The Liquid-Drop Model and Collective Deformation](/nuclear-physics/nuclear-models/liquid-drop-collective-coordinates) 3. [The Shell Model: Single-Particle States and Spin-Orbit Coupling](/nuclear-physics/nuclear-models/shell-model-single-particle) 4. [The Collective Model: Rotations, Vibrations, and Deformed Nuclei](/nuclear-physics/nuclear-models/collective-model-rotations-vibrations) ### 4. Radioactive Decay 1. [Radioactivity and Decay Modes](/nuclear-physics/radioactive-decay/decay-law-modes) 2. [Serial Decay, the Bateman Equations, and Radioactive Equilibrium](/nuclear-physics/radioactive-decay/decay-kinetics-equilibrium) ### 5. Alpha Decay 1. [Alpha Decay and the Gamow Theory of Tunneling](/nuclear-physics/alpha-decay/alpha-decay-gamow-theory) 2. [Fine Structure, Angular Momentum, and Hindrance Factors](/nuclear-physics/alpha-decay/alpha-fine-structure-hindrance) ### 6. Beta Decay and the Weak Interaction 1. [Beta Decay Energetics and the Neutrino](/nuclear-physics/beta-decay/beta-decay-energetics-neutrino) 2. [Fermi's Theory: Kurie Plots and ft Values](/nuclear-physics/beta-decay/fermi-theory-beta-decay) 3. [The Weak Interaction and Parity Violation](/nuclear-physics/beta-decay/weak-interaction-parity-violation) 4. [Double Beta Decay and Neutrino Mass](/nuclear-physics/beta-decay/double-beta-decay-neutrino-mass) ### 7. Gamma Decay 1. [Multipole Radiation and Selection Rules](/nuclear-physics/gamma-decay/gamma-multipole-radiation) 2. [Internal Conversion and Isomers](/nuclear-physics/gamma-decay/internal-conversion-isomers) 3. [Angular Correlations and the Mössbauer Effect](/nuclear-physics/gamma-decay/angular-correlations-mossbauer) ### 8. Nuclear Reactions 1. [Nuclear Reactions, Fission, and Fusion](/nuclear-physics/nuclear-reactions/reaction-kinematics-cross-sections) 2. [The Compound Nucleus and Resonance Reactions](/nuclear-physics/nuclear-reactions/compound-nucleus-resonances) 3. [Direct Reactions and the Optical Model](/nuclear-physics/nuclear-reactions/direct-reactions-optical-model) ### 9. Nuclear Fission 1. [The Fission Barrier and Fragment Energetics](/nuclear-physics/fission/fission-barrier-dynamics) 2. [Chain Reactions and Reactor Physics](/nuclear-physics/fission/chain-reactions-reactor-physics) ### 10. Fusion and Nucleosynthesis 1. [Fusion Reactions and Confinement](/nuclear-physics/fusion-nucleosynthesis/fusion-reactions-confinement) 2. [Stellar Nucleosynthesis](/nuclear-physics/fusion-nucleosynthesis/stellar-nucleosynthesis) 3. [Big-Bang Nucleosynthesis](/nuclear-physics/fusion-nucleosynthesis/big-bang-nucleosynthesis) ### 11. Radiation and Applications 1. [Stopping Power and the Range of Charged Particles](/nuclear-physics/radiation-matter-applications/charged-particle-stopping-power) 2. [Interactions of Photons and Neutrons](/nuclear-physics/radiation-matter-applications/photon-neutron-interactions) 3. [Radiation Detectors and Nuclear Spectroscopy](/nuclear-physics/radiation-matter-applications/radiation-detectors) 4. [Dosimetry, Radiation Biology, and Protection](/nuclear-physics/radiation-matter-applications/dosimetry-radiation-biology) 5. [Applications — Dating, Analysis, and Nuclear Medicine](/nuclear-physics/radiation-matter-applications/nuclear-applications-dating-medicine)