Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Read the notes, then try the questions on paper.
Notes
Radioactive decay
Some atomic nuclei are unstable. They give out radiation to become more stable – this is radioactive decay. It is random: you can’t predict which nucleus will decay next. Activity is the rate at which a source decays, measured in becquerels (Bq). Count-rate is the number of decays recorded each second by a detector such as a Geiger-Muller tube.
| Radiation | What it is | Ionising | Range in air | Stopped by |
|---|---|---|---|---|
| Alpha (α) | 2 protons + 2 neutrons (a helium nucleus) | Strongly | A few cm | Paper or skin |
| Beta (β) | High-speed electron from the nucleus (a neutron turns into a proton) | Moderately | About 1 m | A few mm of aluminium |
| Gamma (γ) | Electromagnetic wave from the nucleus | Weakly | Very far | Reduced by thick lead or concrete |
Nuclei can also emit a neutron.
Nuclear equations
- Alpha decay: mass number goes down by 4, atomic number goes down by 2. E.g. uranium-238 (atomic number 92) → thorium-234 (atomic number 90) + alpha (mass 4, atomic number 2).
- Beta decay: mass number stays the same, atomic number goes up by 1. E.g. carbon-14 (6) → nitrogen-14 (7) + beta (mass 0, charge −1).
- Gamma emission: no change to mass number or atomic number.
The mass numbers and the atomic numbers must add up to the same total on both sides of the equation.
Half-life
The half-life of a radioactive isotope is the time it takes for the number of unstable nuclei in a sample to halve, or for the count-rate (or activity) to fall to half its starting level. After 1 half-life, ½ is left; after 2, ¼; after 3, ⅛.
Contamination and irradiation
Contamination: unwanted radioactive atoms get onto or into an object – the object becomes radioactive. Irradiation: an object is exposed to radiation but does not become radioactive. Precautions: use tongs or gloves, keep your distance, use shielding, and limit the time of exposure. Results about radiation effects should be published and peer reviewed.
Questions
Q1 (F) Which type of radiation is stopped by a sheet of paper?
Show answer
Alpha.
Q2 (F) Which type of radiation is the most strongly ionising?
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Alpha.
Q3 (F) What is meant by the half-life of a radioactive isotope?
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The time it takes for the number of unstable nuclei in a sample to halve – or for the count-rate to fall to half its starting value.
Q4 (F) A source has a count-rate of 800 counts per minute. What will it be after 2 half-lives?
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800 → 400 (1 half-life) → 200 counts per minute (2 half-lives)
Q5 (F/H) An isotope has a half-life of 6 hours. How long does it take for the count-rate to fall from 1600 to 200 counts per minute?
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1600 → 800 → 400 → 200 is 3 half-lives.
3 × 6 = 18 hours
Q6 (F/H) Uranium-238 (atomic number 92) decays by alpha emission. Give the mass number and atomic number of the new nucleus.
Show answer
Mass number: 238 − 4 = 234. Atomic number: 92 − 2 = 90 (thorium).
Q7 (F/H) Carbon-14 (atomic number 6) decays by beta emission. Give the mass number and atomic number of the new nucleus, and explain the change.
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Mass number stays 14; atomic number goes up to 7 (nitrogen). A neutron in the nucleus turns into a proton and a high-speed electron (the beta particle) is emitted.
Q8 (F/H) What is the difference between contamination and irradiation?
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Contamination is when radioactive atoms get onto or into an object, so it becomes radioactive. Irradiation is when an object is exposed to radiation but does not become radioactive.
Q9 (H) What fraction of a radioactive sample is left undecayed after 4 half-lives? Express the decline as a ratio.
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½ × ½ × ½ × ½ = 1/16 remains. The ratio of final to initial is 1 : 16.
Q10 (H) Explain why alpha sources are most dangerous inside the body, but least dangerous outside it.
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Outside the body, alpha is stopped by the outer layer of skin (and has a range of only a few cm in air), so it can’t reach living cells. Inside the body it is next to living cells, and because it is strongly ionising it causes a lot of damage in a small area.
