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AQA GCSE Physics · Topic 4.4 Atomic structure · Physics only
Edexcel International GCSE Physics · Section 7 Radioactivity and particles
Background Radiation
Physics only This spec point is triple / physics only. If you are
sitting Combined Science (Trilogy), you can skip it.
In this lesson you'll learn to: say what background radiation is, sort its sources into natural and man-made, explain why it is higher in some places, and work out the count rate of a source in Bq by subtracting the background first.
In this lesson you'll learn to: say what background radiation is, sort its sources into natural and man-made, explain why it is higher in some places, and work out the count rate of a source in Bq by subtracting the background first.
In this lesson you'll learn to: explain the sources of background (ionising) radiation
from the Earth and from space, say which sources are natural and which are man-made,
explain why it is higher in some places, and subtract it from a reading to find the count rate
of a source in Bq.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. Tab 1: click the bars and find the biggest source of the radiation you are getting right now. Tab 2: sort ten sources into natural and man-made — a couple of them are traps. Tab 3: work out the count rate of a source, then see what happens if you forget the background.
natural, from the Earth
natural, from space
man-made
Background radiation
Percentages are the usual textbook values for the UK — enough for the exam, not a measurement.
▲ On tab 3, try your own numbers, then tick Forget to subtract the background to see how far out you would be.
It's always there. Now learn to explain it.
The exam asks where background radiation comes from, why it is different in different places, and expects you to subtract it from every reading. The notes below cover all three.
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Revision notes
What background radiation is
⭐ Definition
Background radiation is the low-level ionising radiation that is around us all the time, whether or not there is a radioactive source nearby. You cannot switch it off.
Where it comes from
| From the Earth (natural) | From space (natural) | Man-made |
|---|---|---|
| Radon gas seeping out of rocks such as granite (about half the UK dose) · rocks, soil and buildings · food and drink, which contain carbon-14 and potassium-40 | Cosmic rays from the Sun and from outer space | Medical X-rays, CT scans and tracers (the biggest man-made part) · nuclear power and nuclear accidents · fallout from nuclear weapons testing |
Most of the dose an average person in the UK gets is natural. Nuclear power and fallout together are well under 1%.
💡 The test for natural or man-made
Ask: would it be there if there were no people at all? Radon, rocks, food and cosmic rays would: natural. X-rays, tracers, power stations and bomb tests would not: man-made. Bricks and stone are natural: the radioactivity is in the rock, people only moved it.
Why it is different in different places
- The rock underneath you. Granite, as in Cornwall, contains more uranium, which decays and releases radon gas, so the background there is much higher than on the clay around London.
- Height. Up a mountain or in an aeroplane there is less air above you to absorb cosmic rays, so you get more of them. Aircrew get a bigger dose than most people.
Measuring a count rate: always subtract the background
A Geiger–Müller tube counts the radiation that ionises the gas inside it. Its count rate is measured in becquerels (Bq), like activity. With a source in the room, every reading is really source + background.
⭐ The three steps, every time
1. Measure the background with the source removed, in the same place.
2. Subtract the background count from the count with the source.
3. Divide by the time in seconds. The answer is the count rate of the source, in Bq.
Both counts must be taken over the same length of time. Write the answer in Bq: “counts per second” often loses the mark.
Try it on tab 3 of the animation: put your own numbers in, then tick Forget to subtract the background and see how much more active the source looks.
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How it comes up in the exam
“Explain why the background radiation is higher in [place].”
[Place] is on granite rock, which contains more uranium.
This decays and releases radon gas, so there is
more natural background radiation.
the cause what it does the conclusion
Exam-style question · 3 marks
Background radiation comes from the Earth and from space. Name two sources from the Earth and one from space.
- Earth: radon gas from rocks (1); rocks / soil / buildings or food and drink (1)
- Space: cosmic rays (1)
No mark: “the Sun's heat” or “UV light”. Background radiation means ionising radiation.
Exam-style question · 6 marks
A student measures the background radiation in two parts of the UK. Each reading is taken over 30 minutes with no source present. Cornwall (granite): 5400 counts. London (clay): 1800 counts.
(a) Calculate the background count-rate in Cornwall, in Bq. (2)
30 minutes = 1800 s (1) → 5400 ÷ 1800 = 3 Bq (1, unit needed)
(b) Suggest why the count-rate is higher in Cornwall than in London. (2)
Cornwall sits on granite, which contains more uranium than clay (1); this decays to release radon gas, so there is more natural background radiation there (1).
(c) The student then puts a source next to the detector in Cornwall and records 9000 counts in 30 minutes. Calculate the count-rate of the source alone. (2)
9000 − 5400 = 3600 counts (1) → 3600 ÷ 1800 = 2 Bq (1)
The trap in (c): you must subtract the Cornwall background (5400), not London's. Using 1800 gives 4 Bq — double the right answer.
📘 Now do it in your workbook
Pick out the natural and man-made sources, work out a count rate in Bq the same way as the example, then the Cornwall and London question.
Pick out the natural and man-made sources, work out a count rate in Bq the same way as the example, then the Cornwall and London question.
AQA GCSE Physics Workbook · ATOMIC STRUCTURE · page 12 Physics only Edexcel iGCSE Physics Workbook · RADIOACTIVITY AND PARTICLES · page 8✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 12 of the workbook: natural and man-made sources, a count rate in Bq, and the Cornwall and London question.
Page 8 of the workbook: natural and man-made sources, a count rate in Bq, and the Cornwall and London question.
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