The shape of the lesson
By the end of the lesson, learners can
- Match the source: Pick the radiation and half-life for each application and see why it fits.
- Right radiation, right half-life: Each use needs a particular penetrating power and a suitable half-life.
- Type: alpha (short range, safe), beta (thickness gauging), gamma (through body or packaging).
- Half-life: short to decay away fast, long to last for years.
- Always justify both choices.
One picture every idea returns to
The use of a radioactive isotope is matched to its radiation type and half-life. Smoke alarms use alpha, sterilising equipment and treating cancer use gamma, medical tracers use short-lived sources, and thickness gauges use beta.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Putting decay to work.. The same radiation that can harm is, used carefully, a precise tool. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Match the source | Pick the radiation and half-life for each application and see why it fits. Project tab 6 of the lab (Applications) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Right radiation, right half-life | Each use needs a particular penetrating power and a suitable half-life. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Right radiation, right half-life, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Each use needs the right radiation and half-life. | A medical tracer needs a short half-life so it does not stay radioactive in the body, while a smoke alarm uses alpha because it ionises the air over a short range. |
Applications of radioactivity: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
A smoke alarm contains a small radioactive source. State the type of radiation used and give two reasons why it is suitable.
A factory checks the thickness of metal foil using a radioactive source and detector. Explain why a beta source is chosen rather than alpha or gamma.
Medical instruments are sterilised using a gamma source while still sealed in their packaging. Explain why gamma is suitable.
A tracer is injected into a patient so that doctors can follow it with a detector outside the body. State the type of radiation and the kind of half-life needed, with reasons.
Explain why a source used to treat a tumour deep inside the body is a gamma emitter rather than an alpha emitter.
Carbon-14 is used to find the age of old wood. Explain why a source with a very long half-life is needed for dating.
Answer key and marking notes
- Alpha radiation. ✓
- It ionises the air over a short gap and is blocked by smoke, and it is safe because the casing stops it. ✓
- It has a long half-life, so it lasts for the life of the alarm. ✓
- Alpha would be completely stopped by the foil, giving no reading. ✓
- Gamma would pass straight through with almost no change, so it would not detect thickness changes. ✓
- Beta is partly absorbed, so the amount getting through depends on the thickness. ✓
- Gamma is very penetrating, so it passes through the packaging. ✓
- It kills the bacteria inside without opening the package. ✓
- Gamma radiation, because it can escape the body to reach the detector. ✓
- A short half-life. ✓
- So the source decays away quickly and the patient receives only a small dose. ✓
- Gamma is penetrating, so it can reach the tumour deep inside the body. ✓
- Alpha would be stopped by the skin and never reach it. ✓
- The material may be thousands of years old. ✓
- A long half-life means enough of the source is still left to measure after that time. ✓
6 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A household smoke alarm contains a radioactive source. Which type of radiation does it use?
- Aalpha
- Bgamma
- Cbeta
- Dit uses no radiation
Why is alpha radiation suitable for a smoke alarm but gamma radiation is not?
- AAlpha is strongly absorbed over a short range, so smoke can block it; gamma would pass straight through.
- BAlpha travels further than gamma.
- CGamma cannot ionise air at all.
- DAlpha is not affected by smoke.
Which radiation is used to sterilise medical equipment by killing bacteria?
- Agamma, because it is penetrating and can pass through sealed packaging
- Balpha, because it is easily stopped
- Cbeta, because it has no charge
- Dnone, sterilising does not use radiation
In a factory, beta radiation is used to control the thickness of aluminium foil as it is rolled. Why is beta the best choice?
- ABeta is partly absorbed by the foil, so a change in thickness changes the amount getting through.
- BBeta passes through without being affected at all.
- CBeta is completely stopped by the foil.
- DBeta makes the foil radioactive.
A radioactive tracer is injected to track blood flow in the body and detected from outside. What properties should it have?
- Aa short half-life and emitting gamma radiation, so it is detectable outside but does not stay active long
- Ba long half-life and emitting alpha, so it stays in the body
- Ca long half-life and emitting gamma, so it lasts for years
- Dalpha radiation, so it is absorbed inside the body
Carbon-14 dating is used to find the age of ancient wood. Which property of carbon-14 makes this possible?
- AIt decays with a known, constant half-life, so the fraction remaining indicates the age.
- BIt never decays, so the amount stays the same.
- CIts half-life changes with temperature.
- DIt turns back into living material over time.
Quiz key
Smoke alarms use an alpha source, because alpha is easily absorbed: smoke entering the alarm blocks the alpha particles and changes the current, triggering the alarm. Gamma would pass straight through smoke without being affected.
Alpha has a very short range and is easily absorbed, so a little smoke is enough to block it and change the reading. Gamma is highly penetrating and would pass through the smoke unaffected, so the alarm could not detect anything.
Gamma radiation is used because it is highly penetrating and can pass through sealed packaging to kill bacteria inside. Alpha would be stopped by the packaging and could not reach the contents.
Beta is partly absorbed by the foil, so the amount reaching the detector depends on the thickness; the reading can then control the rollers. Alpha would be fully stopped (no signal change), and gamma would pass through almost unaffected.
A medical tracer should emit gamma (so it can be detected outside the body) and have a short half-life (so its activity falls quickly and it does not remain a hazard). Alpha would be absorbed inside the body and could not be detected externally.
Carbon-14 decays at a known, constant rate (its half-life), so measuring the fraction left compared with a living sample gives the age. A changing or zero decay rate would make dating impossible.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.