Radioactivity is a topic where a few key facts and one simple method can earn you a lot of marks.
The three types of radiation
- Alpha (α): 2 protons and 2 neutrons (a helium nucleus). Strongly ionising, travels only a few centimetres in air and is stopped by paper.
- Beta (β): a high-speed electron emitted from the nucleus. Moderately ionising and stopped by a few millimetres of aluminium.
- Gamma (γ): an electromagnetic wave. Weakly ionising, very penetrating, and only reduced by thick lead or concrete.
Decay equations
The top numbers (mass numbers) and bottom numbers (atomic numbers) must balance on both sides.
- Alpha decay: mass number goes down by 4, atomic number down by 2. For example, uranium-238 decays to thorium-234.
- Beta decay: mass number stays the same, atomic number goes up by 1. For example, carbon-14 decays to nitrogen-14.
- Gamma emission: no change to either number.
What is half-life?
Half-life is the time it takes for the number of unstable nuclei in a sample to halve, or the time for the activity of a source to fall to half its starting value.
Worked example
A source has an activity of 800 Bq and a half-life of 2 hours. What is its activity after 6 hours?
- Number of half-lives: 6 ÷ 2 = 3
- Halve three times: 800 → 400 → 200 → 100
- Answer: 100 Bq
Contamination vs irradiation
Irradiation is being exposed to radiation from outside; it doesn’t make you radioactive. Contamination is when radioactive atoms get onto or into an object, so the exposure continues. Examiners love this distinction!
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