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.