7.2.3Radioactivity and Particles
Uses of Radioactivity — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 10 of 17
Part 1 of 7Choosing a source: the key properties
When deciding which radioactive source to use for a job, three questions are asked: which type of radiation does it emit (how far does it penetrate and how strongly does it ionise), what is its half-life (how long will it stay active), and is the source a solid, liquid or gas (can it be contained and handled safely)? All of the uses in this lesson follow from matching those answers to the job.
Part 2 of 7Smoke detectors
A smoke detector contains a small source of alpha radiation (typically americium-241). The alpha particles ionise the air between two metal plates, so a small steady current flows between them. When smoke enters the gap, smoke particles absorb or attach to the ions, the current falls, and the circuit sounds the alarm.
Why alpha? It is a strongly ionising source, so a tiny amount makes enough ions. Because alpha is stopped by a few centimetres of air, or by the case, it is harmless to people in the room. The source needs a long half-life (hundreds of years) so that the detector works for decades.
Part 3 of 7Thickness monitoring
In a paper, plastic or metal-foil mill, a beta source is placed on one side of the sheet and a detector on the other.
Why beta?
- while gamma would pass through almost unchanged,
- so changes in thickness would hardly alter the reading.
Part 4 of 7Tracers in medicine and industry
A tracer is a small amount of radioactive material added to a system so its movement can be followed with a detector outside.
- A gamma camera forms an image showing how well the organ works, for example the kidneys or the heart.
- Gamma is used so that it escapes the body to the detector, and the short half-life means the patient is not radioactive for long.
- A detector moved along the surface finds the leak, where the count rate rises over the soil.
- A tracer with a short half-life leaves the pipe safe.
Part 5 of 7Sterilisation, radiotherapy and dating
Sterilising equipment: medical instruments and some foods are sealed in packaging and exposed to a strong gamma source (cobalt-60). The radiation kills bacteria and other microbes without heating the item. The packaging means the goods are not made radioactive and the process works even inside sealed packages because gamma penetrates well.
Radiotherapy beams of gamma rays are aimed at a tumour from several directions so that the tumour gets a high dose while the surrounding healthy tissue gets only a small dose from each beam.
Dating (extension): living things take in carbon-14, which decays with a half-life of about 5700 years. After death no more is taken in, so the proportion of carbon-14 falls and measuring it gives the age of an organic object. Rocks are dated with isotopes that have much longer half-lives, such as uranium.
Part 6 of 7Summary of reasoning
Always give three linked points in an exam: the type of radiation and why it suits the job, the half-life and why that length is suitable, and a safety consideration. For example: a beta source is used for monitoring paper because it is partly absorbed by thin sheets, and it has a long half-life so it needs replacing only rarely.
Part 7 of 7Worked examples
A paper mill uses a source to monitor the thickness of paper. State which type of radiation is suitable, and explain your choice.
Beta radiation is suitable.
Alpha would be completely absorbed by paper, so no count rate would reach the detector. Gamma would pass through nearly unchanged, so changes in thickness would hardly change the count rate. Beta is partly absorbed, so the count rate changes noticeably with the thickness.
A hospital buys a gamma source for a tracer test. One has a half-life of 6 hours and the other has a half-life of 6 years. Which should be used and why?
The 6-hour source.
It stays radioactive long enough to complete the scan, but most of it has decayed within a day or two, so the patient receives a smaller total dose. A 6-year half-life source would remain active in the body for a very long time and would harm the patient.
A gamma source used for sterilising instruments has activity 8.0 × 1014 Bq. Its half-life is 5.3 years. What is its activity after 10.6 years, and why might the factory replace it earlier?
10.6 years is 2 half-lives, so the activity is 8.0 × 1014 × ¼ = 2.0 × 1014 Bq.
With a quarter of the original activity, items would need four times the exposure time to receive the same dose, so the factory may replace the source sooner to keep production efficient.
Key terms
| Term | Meaning |
|---|---|
| Tracer | Small amount of radioactive substance used to follow the movement of a material. |
| Radiotherapy | Using ionising radiation to destroy cancer cells. |
| Sterilisation | Killing microorganisms, here using gamma radiation. |
| Ionisation | The removal of electrons from atoms to make ions. |
| Gamma camera | Detector that forms an image from gamma rays emitted by a tracer in the body. |
| Carbon dating | Estimating the age of organic material from the remaining proportion of carbon-14. |
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