IGCSE Physics Detection of Radioactivity Questions and Worked Solutions (5.2.1) | 0625

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IGCSE Physics 0625 · Topic 5.2.1

IGCSE Physics Detection of Radioactivity Questions and Worked Solutions

50 topic-wise practice questions on Detection of Radioactivity (Unit 5: Nuclear Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions24 Core16 Extended10 Challenge121 marksTextbook §5.2 · pp. 260–274
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: Detection of Radioactivity

Background radiation is ionising radiation that is always around us from natural and artificial sources. Radiation is detected with a detector (e.g. Geiger–Müller tube) connected to a counter.

Key formulas

count rate = counts ÷ timecounts/s or counts/minCore
corrected count rate = measured − backgroundExtendedExtended

Prerequisite knowledge

  • Ionising radiation basics

Exam tips & common mistakes

  • Convert counts/min to counts/s by dividing by 60
  • Always correct for background before finding half-life (Extended)
  • Background is never zero

Key facts from the syllabus (5.2.1)

  • Background radiation is always present
  • Main sources: radon gas in the air, rocks and buildings, food and drink, cosmic rays
  • Ionising radiation is measured with a detector connected to a counter
  • Count rate is measured in counts/s or counts/minute
  • Extended subtract the background count rate to find the count rate due to the source alone

Textbook: §5.2 Radioactivity, pp. 260–274

1
Core[1 mark]

Define background radiation.

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Method: Always present.

Ionising radiation from natural and artificial sources that is always present around us
2
Core[2 marks]

State four sources of background radiation.

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Method: Syllabus list.

Radon gas in the air; rocks and buildings; food and drink; cosmic rays
3
Core[1 mark]

Name a detector used to measure ionising radiation.

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Method: GM tube.

A Geiger–Müller tube connected to a counter (ratemeter)
4
Core[1 mark]

State what is meant by count rate.

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Method: Counts per unit time.

The number of counts detected per second (or per minute)
5
Core[2 marks]

A counter records 1800 counts in 5.0 minutes. Calculate the count rate in counts/minute.

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Method: Divide by time.

Count rate = 1800 ÷ 5.0 = 360 counts/minute
6
Core[2 marks]

A counter records 240 counts in 1.0 minute. Calculate the count rate in counts/s.

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Method: Divide by 60.

Count rate = 240 ÷ 60 = 4.00 counts/s
7
Core[2 marks]

Explain why a count rate is still recorded when no source is present.

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Method: Background radiation.

The detector measures background radiation
8
Core[1 mark]

State the background source that contributes most in many places.

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Method: Radon.

Radon gas
9
Core[2 marks]

Explain why background count rates are measured over a long time.

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Method: Random decay.

Radioactive emission is random, so a long time gives a more reliable average
10
Core[2 marks]

State one man-made source of background radiation.

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Method: Medical.

Medical X-rays (or nuclear weapons testing, nuclear power)
11
Core[2 marks]

Explain why people living in some areas receive higher background doses.

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Method: Rocks e.g. granite.

Some rocks (e.g. granite) contain more radioactive elements and release more radon
12
Core[2 marks]

Explain why airline pilots receive more background radiation than people on the ground.

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Method: Cosmic rays.

At high altitude there is less atmosphere to absorb cosmic rays
13
Core[2 marks]

State the unit used for count rate.

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Method: Counts per time.

counts/s or counts/minute
14
Core[2 marks]

A detector records 450 counts in 3.0 minutes. Calculate the count rate.

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Method: Divide.

150 counts/minute
15
Core[2 marks]

Calculate the count rate in counts/s if 1200 counts are recorded in 5.0 minutes.

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Method: t = 300 s.

4.00 counts/s
16
Core[2 marks]

Explain why background radiation is higher in some granite areas.

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Method: Radon from rocks.

Granite contains uranium which produces radon gas
17
Core[2 marks]

State which source of background radiation comes from outer space.

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Method: Cosmic rays.

Cosmic rays
18
Core[2 marks]

State one natural source of radiation in food.

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Method: e.g. potassium-40.

Radioactive isotopes such as potassium-40 in foods like bananas
19
Core[2 marks]

State why background radiation cannot be avoided.

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Method: Everywhere.

It comes from natural sources all around us (air, rocks, food, space)
20
Core[2 marks]

Describe how a Geiger–Müller tube and counter are used to measure radiation.

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Method: Detector + counter.

Radiation entering the tube ionises gas, producing pulses that the counter records
21
Core[2 marks]

Explain why radon is a significant health risk in some homes.

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Method: Inhaled alpha.

It is a radioactive gas that can be breathed in, emitting alpha particles inside the lungs
22
Core[2 marks]

Suggest one way to reduce radon levels in a house.

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Method: Ventilation.

Improve ventilation (or seal floors)
23
Core[2 marks]

State one medical source of artificial background radiation.

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Method: X-rays.

Medical X-rays (or nuclear medicine)
24
Core[2 marks]

State the most likely detector reading if a GM tube is placed in a room with no radioactive source.

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Method: Background.

A low count rate from background radiation, not zero
25
Extended[2 marks]

A count rate near a source is 85 counts/minute. Background is 15 counts/minute. Calculate the corrected count rate.

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Method: Subtract background.

Corrected = 85 − 15 = 70 counts/minute
26
Extended[2 marks]

A source gives 3.6 counts/s with background 0.40 counts/s. Calculate the corrected count rate.

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Method: Subtract.

Corrected = 3.20 counts/s
27
Extended[3 marks]

A counter records 3000 counts in 10 minutes near a source. Background is 20 counts/minute. Calculate the corrected count rate.

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Method: Count rate = 300/min.

Corrected = 300 − 20 = 280 counts/minute
28
Extended[3 marks]

A background count of 120 is recorded in 5.0 minutes. With a source, 1720 counts are recorded in 5.0 minutes. Calculate the corrected count rate per minute.

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Method: (1720 − 120) ÷ 5.

Corrected = 1600 ÷ 5 = 320 counts/minute
29
Extended[2 marks]

Explain why background radiation must be subtracted before calculating the half-life of a source.

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Method: Otherwise count rate never halves correctly.

Background is not from the source and does not decay, so it would give a wrong half-life
30
Extended[3 marks]

A detector reads 95 counts/min near a source and 25 counts/min without it. The source is shielded with paper and the reading becomes 25 counts/min. Suggest the radiation emitted.

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Method: Paper stops alpha.

Alpha (all of the source's count is stopped by paper)
31
Extended[3 marks]

The corrected count rate of a source falls from 400 to 100 counts/min. Background is 30 counts/min. State the measured (uncorrected) readings.

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Method: Add background.

430 and 130 counts/min
32
Extended[2 marks]

A count rate reading fluctuates between 22 and 31 counts/min with no source. Explain why.

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Method: Randomness.

Radioactive decay and cosmic events are random, so counts vary
33
Extended[2 marks]

A count rate is 52 counts/min with a source and 18 counts/min without. Calculate the corrected count rate.

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Method: Subtract.

34.0 counts/minute
34
Extended[3 marks]

Background is 0.50 counts/s. A source gives 2400 counts in 10 minutes. Calculate the corrected count rate in counts/s.

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Method: Measured = 4.0 counts/s.

Corrected = 4.0 − 0.50 = 3.50 counts/s
35
Extended[3 marks]

A count of 90 is recorded in 3 minutes with no source. With a source, 1290 counts are recorded in 3 minutes. Calculate the corrected count rate per minute.

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Method: (1290 − 90) ÷ 3.

400 counts/minute
36
Extended[2 marks]

Explain why a corrected count rate can never be negative in a correct experiment.

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Method: Source adds.

The source can only add counts to the background
37
Extended[3 marks]

A source is placed at increasing distances from a detector. At large distance the count rate levels off at 22 counts/min. Explain this.

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Method: Background only.

At large distance the source's radiation hardly reaches the detector, so only background (22 counts/min) is detected
38
Extended[2 marks]

State why the background count should be measured both before and after an experiment.

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Method: Check consistency.

To check it has not changed and to obtain a more reliable average
39
Extended[3 marks]

Measured count rates: 0 min: 1030, 10 min: 530, 20 min: 280 counts/min. Background is 30 counts/min. Find the corrected values.

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Method: Subtract.

1000, 500, 250 counts/min
40
Extended[2 marks]

Explain why a long counting time gives a more accurate count rate.

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Method: Randomness.

Random fluctuations average out over a larger number of counts

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Name four sources of background radiation and explain why the background count varies from place to place.

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Method: Sources.

Radon gas, rocks and buildings, food and drink, cosmic rays; levels of radon and radioactive rocks vary with geology, and cosmic rays vary with altitude
42
ChallengeCore[3 marks]

Explain how a Geiger–Müller tube and counter can be used to measure a count rate.

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Method: Counts/time.

Record the number of counts over a measured time and divide (counts/s or counts/min)
43
ChallengeCore[3 marks]

Explain why a single 10-second count is not a reliable measure of background radiation.

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Method: Random.

Decay is random so counts fluctuate; a longer time or repeated counts give a more reliable average
44
ChallengeCore[3 marks]

Explain why people working underground in some mines may receive higher background radiation.

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Method: Radon, rocks.

Radon gas from surrounding rocks can collect in enclosed spaces
45
ChallengeExtended[4 marks]

A source gives 1860 counts in 5.0 minutes. The background is 0.40 counts/s. Calculate the corrected count rate in counts/min.

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Method: Convert units.

Measured = 372 counts/min; background = 24 counts/min; corrected = 348 counts/min
46
ChallengeExtended[4 marks]

Measured count rates: 0 min: 620, 20 min: 320, 40 min: 170 counts/min. Background = 20 counts/min. Find the half-life.

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Method: Correct first.

Corrected: 600, 300, 150 → half-life = 20 min
47
ChallengeExtended[4 marks]

A student forgets to subtract background (30 counts/min). The measured count rate falls from 330 to 180 counts/min in 6.0 h. State what the student would wrongly conclude and find the correct half-life.

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Method: Correct both readings first.

The measured rate has not halved, so the student would think the half-life is longer than 6 h; corrected: 300 → 150 counts/min, so t½ = 6.0 h
48
ChallengeExtended[4 marks]

Explain why the corrected count rate of a long-lived source stays constant over a lesson, but the measured rate fluctuates.

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Method: Randomness.

Its activity hardly changes in an hour, but random emission makes individual counts vary around the average
49
ChallengeExtended[4 marks]

A detector 1.0 m from a source records 25 counts/min, equal to background. Explain what this suggests about the source's radiation.

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Method: Absorbed by air or weak.

The radiation does not reach 1.0 m — likely alpha (absorbed by a few cm of air) or a very weak source
50
ChallengeExtended[4 marks]

A source placed near a GM tube gives 150 counts/min. With 3 mm aluminium between them it gives 150 counts/min, and with 5 cm lead it gives 22 counts/min. Background is 20 counts/min. Identify the radiation.

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Method: Penetration test.

Gamma: not stopped by aluminium but greatly reduced by lead (to near background)
All IGCSE Physics topics← 5.1.2 The Nucleus5.2.2 The Three Types of Nuclear Emission →

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