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Overview of Nuclear Physics Concepts

May 3, 2025

AQA Physics A-level - Section 8: Nuclear Physics

3.8.1 Radioactivity

3.8.1.1 - Rutherford Scattering

  • Rutherford's Experiment:
    • Disproved Thomson's plum pudding model.
    • Demonstrated existence of a small, dense, positively charged nucleus.
    • Used alpha particles, gold foil, and a fluorescent screen.
  • Observations:
    • Most alpha particles passed through without deflection: atom mostly empty space.
    • Some particles deflected at large angles: presence of dense, positively charged nucleus.

3.8.1.2 - Types of Radiation: Alpha, Beta, Gamma

  • Alpha Radiation (α):
    • Range: 2-10 cm in air, ionizing, stopped by paper.
  • Beta Radiation (β):
    • Range: ~1 m in air, weak ionization, stopped by aluminum foil (3 mm).
  • Gamma Radiation (γ):
    • Infinite range, very weak ionization, stopped by several meters of concrete or several inches of lead.
  • Applications in Industry:
    • Monitoring thickness in materials like aluminum foil, paper, and steel sheets.
  • Medical Use of Gamma Radiation:
    • As detectors, to sterilize equipment, in radiation therapy.
    • Follow inverse square law: intensity decreases with distance.

3.8.1.3 - Radioactive Decay

  • Nature of Decay:
    • Random process.
    • Described by decay constant and exponential decay equations.
  • Half-life (T 1/2):
    • Time taken for half of the nuclei to decay.
    • Can be determined graphically.
  • Applications:
    • Dating objects (e.g., carbon-14), medical diagnosis (e.g., Technetium-99m).
  • Safety in Handling:
    • Use tongs, store in lead-lined containers, measure background radiation.

3.8.1.4 - Nuclear Instability

  • Causes of Instability:
    1. Too many neutrons: beta-minus decay.
    2. Too many protons: beta-plus decay or electron capture.
    3. Too many nucleons: alpha decay.
    4. Excess energy: gamma emission.
  • Stability and Neutron-Proton Ratio:
    • Beyond 20 neutrons/protons, more neutrons are needed for stability.

3.8.1.4 - Nuclear Radius

  • Methods of Estimation:
    • Distance of closest approach (overestimates).
    • Electron diffraction (accurate measurement).
  • Nuclear Density:
    • Constant across different nuclei.
    • Atoms mostly empty space.

3.8.1.6 - Mass and Energy

  • Mass Defect and Binding Energy:
    • Mass of nucleus < mass of constituent nucleons.
    • Binding energy is energy required to separate nucleus into nucleons.
  • Nuclear Reactions:
    • Fission: splitting large nucleus, energy released.
    • Fusion: combining small nuclei, more energy released.
  • Binding Energy Graph:
    • Shows potential for fission (large nuclei) or fusion (small nuclei).

3.8.1.7 - Induced Fission

  • Chain Reaction:
    • Initiated by thermal neutrons.
    • Critical mass needed to maintain chain reaction.
  • Nuclear Reactor Components:
    • Moderator: slows neutrons.
    • Control rods: absorb neutrons.
    • Coolant: absorbs heat, generates steam.

3.8.1.8 - Safety Aspects

  • Fuel and Waste Management:
    • Use enriched uranium.
    • Handle waste with care (cooling ponds, vitrification).
  • Risk vs. Benefit:
    • Nuclear power: low fuel needs, no greenhouse gases, but produces radioactive waste.
    • Safety measures crucial to mitigate risks.