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Edexcel International GCSE Physics · Section 7 Radioactivity and particles
Units for Radioactivity
In this lesson you'll learn to: use the five units of Section 7 — the becquerel (Bq),
the centimetre (cm), the hour (h), the minute (min) and the second (s) —
write Bq correctly (capital B, small q), tell activity from count rate, and convert
hours, minutes and centimetres before you calculate.
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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. Watch the source: every red dot is one nucleus decaying. Turn up the decays per second, then slide the GM tube further away — which number changes and which one doesn't? Then type the unit of activity and see what the examiner would accept. Tabs 2 and 3 are the add the unit and conversion questions from your workbook page.
Activity — nuclei that decayed last second
—Bq
1 Bq = 1 decay per second. Every decay counts, whichever way it went.
Count rate — radiation detected last second
—Bq
Only the radiation that ionises the gas inside the tube is counted. Always lower than the activity.
Exam style: give the unit. Type it in the box.
Activity of the source = 40
Hint: it is named after Henri Becquerel.
right first time 00 / 0
—
Pick the unit that fills the gap.
right 0streak 0
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Choose the correct value.
Going to a smaller unit, the number gets bigger — so you multiply.
The golden rule: convert before you calculate. Equations need time in s and
distance in m. Minutes → seconds × 60. Hours → seconds × 3600. Centimetres → metres ÷ 100.
▲ Moving the tube changes the count rate. It never changes the activity: the source decays just as fast wherever the detector is.
Five units, one golden rule
Bq for activity, cm for how far radiation gets, and h, min or s for time. The notes below show which goes where, the mark students lose by writing “counts per second”, and how to convert first.
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Revision notes
The five units in Section 7
Every answer needs a number and a unit. Radioactivity only uses five:
| Unit | Symbol | Used for |
|---|---|---|
| becquerel | Bq | the activity of a source |
| centimetre | cm | how far radiation travels in air (range), and the thickness of an absorber like lead |
| hour | h | half-lives and times |
| minute | min | half-lives, times, and counting for a fixed time |
| second | s | half-lives, times, and every time you put a time into an equation |
The becquerel
The activity of a source is the number of nuclei that decay each second. It has its own unit, the becquerel, named after Henri Becquerel, who discovered radioactivity.
⭐ Memorise this
Activity is measured in becquerels (Bq).
1 Bq = 1 decay per second. So an activity of 4800 Bq means 4800 nuclei decay every second.
⚠ Where the mark goes — write it Bq
When a question asks you to give the unit of activity, write Bq: a capital B and a small q. Not BQ, not bq.
“Counts per second” is often not accepted, and nor are “cps” or “decays/s”. The becquerel is the answer they want, for activity and for count rate.
Activity and count rate: same unit, different number
Both are measured in becquerels (Bq), because both are “so many per second”:
- Activity = the number of nuclei that decay each second in the source.
- Count rate = the radiation detected each second: the number of times radiation ionises the gas inside the Geiger–Müller (GM) tube.
A source sends radiation off at random in every direction, and only the radiation that ionises the gas in the tube is counted, so the count rate is always lower than the activity. Move the tube further away and the count rate falls, but the activity stays exactly the same.
Convert before you calculate
Half-lives can be given in seconds, minutes, hours, or even days and years. That is fine for stating a half-life: give it in the unit the question uses. But when a time goes into an equation, it must be in seconds, and a distance must be in metres.
| From | To | Do this | Example |
|---|---|---|---|
| hours | seconds | × 3600 | 2 h = 7200 s |
| hours | minutes | × 60 | 1.5 h = 90 min |
| minutes | seconds | × 60 | 5 min = 300 s |
| centimetres | metres | ÷ 100 | 250 cm = 2.5 m |
| metres | centimetres | × 100 | 0.5 m = 50 cm |
| counts in 1 minute | count rate in Bq | ÷ 60 | 1200 counts in 1 min = 20 Bq |
💡 Multiply or divide?
Going to a smaller unit (hours → seconds, metres → centimetres), you need more of them, so the number gets bigger: multiply. Going to a bigger unit, divide. Then check: 2 hours can't be 0.03 s.
How to write it in the exam
“State what an activity of 4800 Bq tells you about the source.”
4800 nuclei decay in the source
every second.
the number from the question what happens how often
This is the exam-style question from page 3 of the workbook, answered part by part.
(a) State the unit that is used to measure the activity of the source. [1]
becquerel (Bq). No mark for “counts per second”.
(b) The activity of the source is 4800 Bq. State what this tells you about the source. [1]
4800 nuclei decay every second (4800 decays per second).
(c) The half-life of the source is 2 hours. Calculate the half-life in seconds. [2]
2 × 3600 (1) = 7200 s (1)
(d) The range of the radiation in air is 4.5 cm. Give this distance in metres. [1]
4.5 ÷ 100 = 0.045 m
(e) A detector records 1200 counts in 1 minute. Calculate the count rate. [2]
1 minute = 60 s, so 1200 ÷ 60 (1) = 20 Bq (1) — write Bq, not “counts per second”.
✅ Check every answer
1 Is there a unit? 2 Is activity written Bq, capital B and small q? 3 Did every time go into the equation in seconds, and every distance in metres?
📘 Now do it in your workbook
Choose the correct unit six times, add the missing units, do the conversions, then the 7-mark exam-style question — the same one worked through above.
Edexcel iGCSE Physics Workbook · RADIOACTIVITY AND PARTICLES · page 3Free sample = the page for this lesson. The full workbook covers the whole of Section 7 Radioactivity and Particles, 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.
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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
Page 3 of the Radioactivity and Particles workbook is free: choose the unit, add the unit, convert, then the 7-mark exam-style question.
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