Topic 8 of 8 · Form 4
Nuclear physics
Protons and neutrons in the nucleus; nucleon number A and proton number Z; isotopes. Rutherford's alpha-scattering experiment showed a small, dense, positively charged nucleus.
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The atom and nucleus
Protons and neutrons in the nucleus; nucleon number A and proton number Z; isotopes. Rutherford's alpha-scattering experiment showed a small, dense, positively charged nucleus.
Radioactivity
- Alpha (helium nucleus, highly ionising, stopped by paper), beta (fast electron, stopped by a few mm of aluminium), gamma (electromagnetic radiation, reduced by thick lead or concrete).
- Decay equations: ²²⁶Ra → ²²²Rn + ⁴He (alpha); ¹⁴C → ¹⁴N + ⁰₋₁e (beta).
- Detection: Geiger–Müller tube and counter; background radiation.
Half-life
Time for half the nuclei (or the activity) to decay.
Worked example: A source has an activity of 1 600 Bq and a half-life of 6 hours. Find its activity after 24 hours.
4 half-lives: 1 600 → 800 → 400 → 200 → 100 Bq
Uses and safety
Medical tracers and radiotherapy, sterilisation, smoke detectors (alpha), thickness gauges, carbon dating. Safety: minimise time, maximise distance, use shielding.
Fission and fusion
Fission of large nuclei (uranium-235) in nuclear reactors; fusion of light nuclei in stars.
Check yourself
- Which radiation is used in smoke detectors and why?
- Complete: ²³⁸U → ²³⁴Th + ?
- What fraction remains after 3 half-lives?
Show answers
- Alpha – it is strongly ionising but has a short range and is stopped easily.
- ⁴He (an alpha particle)
- ⅛
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