6.3.1Radioactivity
Dangers of Radiation — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 19 of 20
Part 1 of 7Ionising radiation and living cells
Alpha, beta, gamma and X-rays are ionising radiations: when they pass through matter they knock electrons off atoms, leaving charged ions. In a living cell, ionisation can damage important molecules, in particular DNA. The consequences depend on the dose and on which cells are affected.
- Rapidly dividing cells such as bone marrow and the lining of the gut are especially vulnerable.
- The result may be cancer.
- Mutation in reproductive cells could pass on genetic changes to children.
Low doses over a long time carry a small, random increase in the risk of cancer rather than a sure effect.
Part 2 of 7Comparing the three radiations
The danger depends on whether the source is outside or inside the body.
| Radiation | Outside the body | Inside the body |
|---|---|---|
| Alpha | Low risk: absorbed by skin and air | Very high risk: strong ionisation of nearby cells |
| Beta | Moderate risk: penetrates skin and can burn it | Moderate to high risk |
| Gamma | High risk: penetrates deep into the body | Lower ionisation per unit path, but much escapes the body |
Gamma is hard to stop and ionises weakly, whereas alpha ionises strongly but cannot penetrate. This is the reason why the two examples contamination (alpha) and irradiation (gamma) matter so much.
Part 3 of 7Factors that affect the harm done
- A source of long half-life stays active for years, while a short half-life source decays away fast but is more active at the start.
Part 4 of 7Precautions
People who work with radiation follow the principle of keeping doses as low as reasonably practicable.
Medical staff who carry out X-rays stand behind screens, while patients receive the smallest dose that gives a usable image: the benefit to the patient is judged to be greater than the small risk.
Part 5 of 7Radioactive waste
Nuclear power and medicine produce radioactive waste. Low-level waste (gloves, equipment, clothing) is sealed in containers and buried in controlled sites. High-level waste from nuclear fuel stays dangerous for thousands of years because of long half-lives.
- then sealed in glass (vitrified) and in steel containers, and stored deep underground in geologically stable rock.
Part 6 of 7Assessing risk sensibly
Risk depends on how likely harm is and how serious it would be.
- so the risk of a dental X-ray is tiny and the benefit of finding a problem is large.
- which is why measurements and proper figures matter.
Part 7 of 7Worked examples
Explain why alpha radiation is the most dangerous radiation if the source is swallowed, but not the most dangerous when held outside the body.
Alpha particles are strongly ionising, and they release all of their energy within a very short distance. Inside the body, that damage is concentrated on nearby living cells, so it causes severe damage and may cause cancer.
Outside the body, alpha has a range of only a few centimetres in air and is absorbed by dead surface layers of skin, so it cannot reach living cells. Gamma and beta, which penetrate further, are a bigger hazard from outside.
A technician works at a distance of 1.0 m from a gamma source, receiving a dose in 1 hour. Without changing the dose rate, she works for only 15 minutes instead. By what fraction is her dose reduced?
Dose ∝ time at a constant dose rate.
15 minutes is ¼ of an hour, so the dose is ¼ of the original.
The dose is reduced by 3/4 (75%).
Give two reasons why high-level nuclear waste needs to be stored for thousands of years.
1. It contains isotopes with long half-lives, so its activity falls very slowly and it remains radioactive for a long time.
2. It emits ionising radiation, which can damage cells and cause cancer; it also may contaminate groundwater, so it must be kept isolated until its activity is safe.
Key terms
| Term | Meaning |
|---|---|
| Ionising radiation | Radiation that can remove electrons from atoms: alpha, beta, gamma and X-rays. |
| Mutation | A change in the DNA of a cell, which may lead to cancer. |
| Radiation dose | Measure of the energy absorbed by tissue and the harm that it does. |
| Film badge | Badge that darkens when exposed to radiation, worn to monitor a worker’s dose. |
| Radioactive waste | Unwanted material that emits ionising radiation. |
| Vitrification | Sealing high-level waste in glass for long-term storage. |
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