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AQA GCSE Physics · Topic 4.4 Atomic structure
Hazards, Uses and 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 and where it comes from, work out a count-rate in becquerels by subtracting the background first, and choose the right radiation and half-life for a job — long for a monitor that must keep working, short for a tracer that has to decay away.
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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
In this lesson you'll learn to: say what background radiation is and where it comes from, work out a count-rate in becquerels by subtracting the background first, and choose the right radiation and half-life for a job — long for a monitor that must keep working, short for a tracer that has to decay away.
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Start here — have a play
Don't read anything yet. On tab 1 you get five real jobs — a smoke alarm, a paper mill, a foil line, a patient and a leaking water pipe. For each one, choose the radiation and the half-life, then press Check. Get them wrong first; the explanation is the exam answer. Tab 2 shows why a smoke alarm and a medical tracer need opposite half-lives, and tab 3 is background radiation, including the sum that catches people out.
Job
Radiation
Half-life
The job, and your answer
Scored: 0 of 0 choices
Half-lives and dose percentages are the usual textbook values — enough for the exam, not a lab measurement.
▲ Every answer comes from two questions: what is the radiation supposed to get through, and does the source need to last or disappear?
Got the pattern? Now learn to write it.
These questions are nearly always "explain why this source is suitable". The marks come from naming the radiation with a reason and the half-life with a reason — never just one.
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Revision notes
Background radiation
Background radiation is the low-level radiation that is around us all the time, whether or not there is a radioactive source nearby. It has always been there, and you cannot switch it off.
| Natural sources — there with or without people | Man-made sources — only because we built something |
|---|---|
| Radon gas from rocks (about half of the UK dose) · rocks and buildings · food and drink · cosmic rays from space | Medical X-rays, CT scans and tracers (much the biggest man-made share) · the nuclear industry · fallout from weapons testing |
Most of the dose an average person in the UK gets is natural. The nuclear industry and fallout together are well under 1%.
⭐ Why background radiation is different from place to place
It depends on the rock underneath you. Granite — as in Cornwall — contains more uranium, which decays to release radon gas, so the background count-rate there is much higher than on the clay around London. It also rises with altitude, because there is less atmosphere to absorb cosmic rays, which is why aircrew get a bigger dose.
Measuring a count-rate — always subtract the background
Activity is the rate at which a source decays, measured in becquerels (Bq), where 1 Bq = 1 decay per second. A detector such as a Geiger-Müller tube measures the count-rate, the number of decays it detects each second.
Every reading taken with a source in the room is really source + background, so:
⭐ The three steps, every time
1. Change each time into seconds, and work out each count-rate: counts ÷ time.
2. Subtract the background count-rate from the count-rate with the source — make sure both
are counts per second (Bq).
3. The answer is the count-rate of the source alone, in Bq.
You can also subtract the counts first and then divide, as long as both readings were taken over the same length of time — same answer either way.
The background must be measured in the same place, on the same day — it is local.
Have a go at this: on tab 3 of the animation at the top of this page there is a count-rate calculator. Put your own numbers in, then tick "forget to subtract the background" and see how much bigger the source looks than it really is.
Monitors need a LONG half-life
A monitor is a device that has to keep working: a smoke alarm, a thickness gauge. It works by comparing the count-rate now with the count-rate when nothing is in the way.
- A long half-life means the activity barely changes over months or years, so the source gives a constant count-rate to compare against.
- So if the count-rate decreases, you know it is because more radiation is being absorbed by the material — and not because the source has decayed.
- A short half-life would make the reading drift, and the monitor would give false readings.
Tracers need a SHORT half-life
A tracer is something you follow — through a body, a pipe or a river — and then want gone.
- A short half-life means the count-rate reduces to almost zero soon after the measurement, which limits harmful exposure to humans. The patient is not left radioactive.
- But it must still be long enough to travel to where it is going and be measured.
Choosing the radiation
⭐ The rule that decides every answer
Pick the radiation that is partly affected by the thing you are measuring — and, if the source is inside a body, the one that can get out.
| Use | Monitor or tracer? | Radiation | Half-life | Why |
|---|---|---|---|---|
| Smoke alarm | Monitor | Alpha | Long | Alpha ionises the air so a small current flows. Smoke absorbs the alpha, the current falls and the alarm sounds. Beta and gamma would pass straight through the smoke. Long half-life so the alarm still works in ten years. |
| Paper or foil thickness gauge | Monitor | Beta | Long | Beta is partly absorbed, so the count-rate changes with thickness: too thick and it falls, too thin and it rises, and the rollers adjust. Alpha would be stopped completely and gamma would pass straight through. |
| Medical tracer (technetium-99m) | Tracer | Gamma | Short (6 hours) | Only gamma is penetrating enough to escape the body and reach the detector. Short half-life so the patient's exposure is small. |
| Leak in a water pipe near the surface | Tracer | Beta | Short | Where the pipe is sound the beta is absorbed; where there is a crack the water carries the tracer out and the count-rate above ground jumps. A pipe buried deep needs gamma instead. |
Risk and benefit
Radiation damages cells, so every use carries a risk. A medical tracer does give the patient a dose — but the scan finds a problem that could not be found otherwise, so the benefit outweighs the risk. The short half-life and the small quantity used keep that risk as low as possible.
The people who work with sources are protected the same way as in any lab: distance and time to reduce irradiation, and gloves and tongs to prevent contamination.
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How it comes up in the exam
Two question types dominate: a count-rate calculation where you must subtract the background, and an "explain why this source is suitable" question where you must give the radiation and the half-life, each with a reason.
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.
Exam-style question · 4 marks
A hospital uses technetium-99m as a tracer. It emits gamma radiation and has a half-life of 6 hours. Explain why these two properties make it suitable.
- Gamma is very penetrating (1), so it can pass out of the patient's body and be detected outside (1)
- A half-life of 6 hours is short, so the count-rate falls to almost nothing soon after the scan (1)
- which limits the patient's exposure — but it is still long enough to travel round the body and be imaged (1)
Marking note: "because gamma is best" scores nothing. Every mark needs the property and what it lets you do.
📘 Now do it in your workbook
Page 12 is background radiation and the count-rate sum; page 13 is the three real uses, where you write the radiation, the half-life and how each one works.
AQA GCSE Physics Workbook · ATOMIC STRUCTURE · pages 12 & 13The full workbook covers the whole of Topic 4 Atomic Structure, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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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.
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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
Pages 12 and 13 of the workbook: the Cornwall and London count-rate question, and the three real uses to annotate in your own words.
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