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AQA GCSE Physics · Topic 4.4 Atomic structure
Radioactive Decay and Nuclear Radiation
In this lesson you'll learn to: explain why an unstable nucleus gives out radiation,
describe what alpha, beta, gamma and neutron radiation are made of, compare how far each one
penetrates and how strongly it ionises, define activity and count rate,
and use count rates behind paper, aluminium and lead to identify the radiation a source gives out.
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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 — identify the mystery source
Don't read anything yet. Put the GM tube at 2 cm with no absorber and note the count rate. Then try paper, then 3 mm aluminium, then 5 cm lead, and move the tube back to 2 m. Start with Sources A, B and C, which are labelled alpha only, beta only and gamma only, and watch which radiation stops where. Then choose mystery source D or E, tick alpha, beta or gamma and press Check. For a real challenge, untick Show the radiation and do it from the numbers alone, like a real experiment.
Identifying radiation — alpha, beta or gamma?
GEIGER–MÜLLER COUNTER
0 Bq
average: — Bq
| Distance | Absorber | Count rate |
|---|
Take a reading, then press Record reading.
▲ Each absorber rules one radiation in or out. Paper tests for alpha, aluminium for beta, lead for gamma.
Found the source? Now learn the facts behind it.
The exam won't show you the animation. It will ask what each radiation is made of, what stops it, and how to prove from count rates which ones a source gives out. The notes below give you all three.
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Revision notes
1. Why a nucleus decays
Some nuclei are unstable: they have the wrong balance of protons and neutrons, or too much energy. An unstable nucleus becomes more stable by giving out radiation. This is radioactive decay, and it is random: you cannot predict which nucleus will decay next, or when.
⭐ Memorise this
Radioactive decay — an unstable nucleus gives out radiation to become more stable.
Random — you cannot predict which nucleus will decay next.
2. The four types of nuclear radiation
Learn each card: what it consists of, what stops it, how strongly it ionises, and its charge. These are exactly the four boxes on page 6 of the workbook.
α Alpha
Consists of2 protons + 2 neutrons
a helium nucleus
a helium nucleus
Penetratesstopped by paper
or a few cm of air
or a few cm of air
Ionisesstrongly
Charge+2
β Beta
Consists ofa fast electron
from the nucleus
from the nucleus
Penetratesstopped by ~3 mm aluminium
or about 1 m of air
or about 1 m of air
Ionisesmoderately
Charge−1
γ Gamma
Consists ofan EM wave
from the nucleus
from the nucleus
Penetratesstopped by thick lead
or about 1 m of concrete
or about 1 m of concrete
Ionisesweakly
Charge0
n Neutron
Consists ofa neutron
from the nucleus
from the nucleus
Penetratesvery penetrating
needs thick concrete or water
needs thick concrete or water
Ionisesnot directly
Charge0
💡 The pattern that saves you learning it twice
The more ionising a radiation is, the less penetrating it is. Every time it ionises an atom it loses some energy, so alpha, which ionises most strongly, runs out of energy soonest. Gamma ionises weakly, so it keeps going.
3. Penetration
Two different things stop radiation, and the exam tests both: air, if the detector is far enough away, and the material you put in the way. Keep them apart.
1. Range in air. With nothing in the way, each radiation only travels so far before it has ionised enough air to lose its energy.
2. Absorbers. Now put paper, aluminium and lead right up close, within about 2 cm of the source. All three radiations can easily travel that far in air, so if one stops, it is the material that has stopped it.
4. Activity and count rate
You can't see radiation, so you detect it. A Geiger–Müller (GM) tube connected to a counter clicks every time radiation passes into the tube and ionises the gas inside it.
⭐ Memorise this
Activity — the number of decays per second. It is measured in becquerel (Bq): 1 Bq = 1 decay per second.
Count rate — the number of measured decays per second (the number of ionisations per second in the detector).
The count rate is always lower than the activity. The radiation goes off in every direction, and the tube only counts the radiation that happens to travel into it.
5. Identifying radiation from count rates
This is the practical on page 7. Put a GM tube a few centimetres from the source, then put each absorber between them in turn. Each absorber tests for one radiation:
| What you see | What it tells you |
|---|---|
| The count rate drops when you put paper in the way | alpha is present |
| The count rate drops again when you swap the paper for 3 mm aluminium | beta is present |
| The count rate drops again when thick lead is put in the way | gamma is present |
💡 Exam tip — subtract the background first
There is always a little background radiation, from rocks, cosmic rays and the air. Measure the count rate with no source present first, then subtract it from every reading. A corrected count of zero means nothing is getting through that absorber: everything you're reading is background.
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How to write each answer
These questions come up again and again. These are the sentences that get the marks — learn the shape, then swap in the details from your question.
“Describe the structure of an alpha particle.”
An alpha particle is made of 2 protons and 2 neutrons, the same as a
helium nucleus.
“Compare the penetration of alpha, beta and gamma.”
Alpha is stopped by paper,
beta is stopped by aluminium, and
gamma is stopped by thick lead or a metre of concrete, so gamma is the
most penetrating.
“Define activity.”
Activity is the number of decays per second, measured in
becquerel (Bq).
“Define count rate.”
Count rate is the number of measured decays per second
(the number of ionisations per second in the detector).
“Explain how the count rates show which radiation is present.”
The count rate falls when paper is added, so
alpha is present. It falls again with
aluminium, so beta is present. It falls again when
thick lead is added, so
gamma is present.
the radiation or absorber what happens the conclusion
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Worked examples
Cover the answer, try it, then check. Subtract the background from every reading before you decide anything.
Example 1 — which radiation is it?
The background count rate is 20 counts/s. A student measures the count rate from source B with different absorbers in the way: none 500, paper 140, aluminium 140, lead 30 counts/s. Which types of radiation does source B give out? Explain how you know.3 marks
Subtract 20 from each: none 480, paper 120, aluminium 120, lead 10
Paper: 480 → 120, a big drop → alpha is present
Aluminium: 120 → 120, no further drop → no beta
Lead: 120 → 10, drops again → gamma is present
Answer: source B gives out alpha and gamma
Paper: 480 → 120, a big drop → alpha is present
Aluminium: 120 → 120, no further drop → no beta
Lead: 120 → 10, drops again → gamma is present
Answer: source B gives out alpha and gamma
Tip: look at the change each absorber makes, not the size of the reading.
Example 2 — when the count hits background
Source C, with the same 20 counts/s background: none 320, paper 320, aluminium 20, lead 20 counts/s. Which radiation does it give out?3 marks
Subtract 20 from each: none 300, paper 300, aluminium 0, lead 0
Paper: 300 → 300, no change → no alpha
Aluminium: 300 → 0, all stopped → beta is present
Lead: 0, nothing left to stop → no gamma
Answer: source C gives out beta only
Paper: 300 → 300, no change → no alpha
Aluminium: 300 → 0, all stopped → beta is present
Lead: 0, nothing left to stop → no gamma
Answer: source C gives out beta only
Common slip: seeing "20 behind lead" and deciding gamma is present. 20 is the background, so the corrected count is zero.
Example 3 — the exam-style question from page 6
A radioactive source emits alpha, beta and gamma radiation. A student places a detector a short distance from the source, then puts sheets of paper, aluminium and lead between the source and the detector in turn. Explain how the student can use the count rate readings to show that the source emits all three types of radiation.5 marks
1. Measure the background count rate first, with the source removed, and subtract it from every reading.
2. Measure the count rate with no absorber in place, as a starting value.
3. Put the paper in: the count rate falls, so alpha is present (paper stops alpha).
4. Put the aluminium in: the count rate falls again, so beta is present.
5. Put the lead in: the count rate falls again, so gamma is present (thick lead stops gamma).
2. Measure the count rate with no absorber in place, as a starting value.
3. Put the paper in: the count rate falls, so alpha is present (paper stops alpha).
4. Put the aluminium in: the count rate falls again, so beta is present.
5. Put the lead in: the count rate falls again, so gamma is present (thick lead stops gamma).
Mark scheme: one mark for each numbered point.
📘 Now do it in your workbook
Fill in the four radiation cards, draw the penetration diagram, then work out which radiation sources B and C give out from their count rates — with worked answers at the back so you can mark your own.
AQA GCSE Physics Workbook · ATOMIC STRUCTURE · pages 6 & 7Free sample = the pages for this lesson. The 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 notes and the free workbook pages above cover every mark.
Now put it into practice
Watching is the easy bit — identifying radiation only becomes automatic once you have worked through a few sets of count rates yourself. Pages 6 and 7 of the workbook are free, and the full book covers every spec point in Topic 4 Atomic Structure with worked answers.
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