ZIMSEC O Level · Physics · Form 4
The nucleus
Protons and neutrons in the nucleus; electrons outside. Some nuclei are unstable and decay, giving out radiation randomly.
Types of radiation
- Alpha (α): helium nucleus; strongly ionising; stopped by paper or a few cm of air.
- Beta (β): fast electron; stopped by a few mm of aluminium.
- Gamma (γ): electromagnetic wave; weakly ionising; reduced by thick lead or concrete.
Half-life
The time for half the radioactive nuclei in a sample to decay (or the activity to halve).
Worked example: A sample has an activity of 800 counts/min and a half-life of 2 days. Find its activity after 6 days.
6 days = 3 half-lives: 800 → 400 → 200 → 100 counts/min
Background radiation
From rocks, the air (radon), cosmic rays and medical uses. Subtract it from readings.
Uses and safety
Uses: radiotherapy for cancer, sterilising medical equipment, tracers, thickness gauges, carbon dating. Safety: keep a distance, use tongs, limit exposure time and use shielding; store sources in lead-lined boxes.
Check yourself
- Which radiation is stopped by a sheet of paper?
- The half-life of a substance is 5 years. What fraction remains after 15 years?
- Give two safety precautions when handling radioactive sources.
Show answers
- Alpha.
- 1/8
- Any two: use tongs, keep at a distance, minimise time, use shielding, never point at people.
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