The shape of the lesson
By the end of the lesson, learners can
- Measure the background: See the steady count from background radiation and how a nearby source adds to it.
- Where it comes from: Most background radiation is natural, with a smaller artificial contribution.
- Sources: radon, rocks and soil, cosmic rays, food, medical.
- Detection: Geiger-Muller tube and counter, giving a count rate.
- Correct: corrected = measured - background.
One picture every idea returns to
Background radiation is the low level of ionising radiation always around us, from natural sources such as radon gas, rocks, cosmic rays and food, and from artificial sources such as medical uses. It is detected with a Geiger-Muller tube, and its count must be subtracted when measuring a source.
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, The radiation that is always there.. A radiation detector never reads zero, even with no source in sight. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Measure the background | See the steady count from background radiation and how a nearby source adds to it. Project tab 1 of the lab (Background radiation) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Where it comes from | Most background radiation is natural, with a smaller artificial contribution. | Cooperative Concept Mapping |
| 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, 12 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 Where it comes from, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| Background radiation is present everywhere, so when measuring a source you must subtract the background count rate to get the true reading. | For most people the largest natural contributor is radon gas. |
Background radiation: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by background radiation.
State three natural sources of background radiation.
Name the instrument used to detect radioactivity, and state what quantity it measures.
The background count rate is 18 counts per minute. With a source present, the counter reads 96 counts per minute. Calculate the count rate due to the source.
A student measures the count rate from a source but forgets to subtract the background. Explain what is wrong with their value for the source.
A student takes several one-minute background readings and gets slightly different values each time. Explain why, and suggest how to get a more reliable background value.
Answer key and marking notes
The low level of radiation that is present around us all the time, from natural and man-made sources. ✓
- Radon gas from rocks and soil. ✓
- Cosmic rays from space. ✓
- Radioactive substances in food and drink (also rocks and building materials). ✓
- A Geiger-Muller tube connected to a counter. ✓
- It measures the count rate (counts per second or per minute). ✓
- corrected = measured - background = 96 - 18
corrected count rate = 78 counts per minute
- Their value is too high. ✓
- The counter also detects background radiation, which has not been taken away. ✓
- Radioactive decay is random, so the count varies from minute to minute. ✓
- Take several readings and find the average (or count over a longer time). ✓
2 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.
Which of these is a natural source of background radiation?
- Aradon gas released from rocks and soil
- Bthe light from a desk lamp
- Csound from passing traffic
- Dheat from a radiator
Before measuring the count rate from a radioactive source, a student records the count with no source present. Why?
- Ato measure the background radiation and subtract it, giving the true count from the source
- Bto warm up the detector
- Cto charge the source
- Dbackground radiation does not need to be considered
Quiz key
Radon gas from rocks and soil is a major natural source of background radiation, along with cosmic rays and radioactive materials in the ground and food. Light, sound, and heat from a radiator are not ionising radiation.
Background radiation is always present and adds to the reading, so it must be measured and subtracted to find the true count rate due to the source alone. Ignoring it would make the source seem more active than it is.
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.