IGCSE Physics 5.2 Radioactivity MCQs – 30 Practice Questions with Answers (Cambridge 0625)
Free practice for Cambridge IGCSE Physics (0625) Section 5.2 Radioactivity, covering Core and Supplement material on alpha, beta and gamma radiation, half-life calculations, decay equations, uses and safety. Every question has a worked explanation, so you can check your reasoning as you go.
Quick recap: after each half-life the activity (or number of undecayed nuclei) halves. Alpha decay: mass number −4, proton number −2. Beta-minus decay: mass number unchanged, proton number +1. Always subtract the background count before doing a half-life calculation. See all the key equations on our IGCSE Physics Formula Sheet.
Alpha, Beta & Gamma Radiation
Q1. An alpha particle consists of
- (A) 1 proton and 1 neutron
- (B) 2 protons and 2 neutrons
- (C) 2 protons and 2 electrons
- (D) 4 electrons
Show Answer
Answer: B – An alpha particle is a helium nucleus, 42He, made of 2 protons and 2 neutrons. It has a charge of +2e.
Q2. Which radiation is completely stopped by a single sheet of paper?
- (A) Alpha radiation
- (B) Beta radiation
- (C) Gamma radiation
- (D) None of them
Show Answer
Answer: A – Alpha particles are large and strongly ionising, so they lose their energy quickly and are stopped by paper or a few centimetres of air. Beta needs a few millimetres of aluminium, and gamma is only reduced by thick lead or concrete.
Q3. Which type of radiation has the strongest ionising effect?
- (A) Beta radiation
- (B) Gamma radiation
- (C) Alpha radiation
- (D) All three are equally ionising
Show Answer
Answer: C – Alpha particles carry a double positive charge and are relatively massive, so they knock electrons off many atoms along a short path. This strong ionisation is why alpha has the shortest range.
Q4. A beta particle is best described as
- (A) a helium nucleus
- (B) an electromagnetic wave
- (C) an electron in orbit around the nucleus
- (D) a fast-moving electron emitted from the nucleus
Show Answer
Answer: D – A beta-minus particle is an electron created inside the nucleus and ejected at high speed. It is not one of the orbiting electrons.
Q5. Which radiation passes through an electric field without being deflected?
- (A) Gamma radiation
- (B) Alpha radiation
- (C) Beta radiation
- (D) All three are deflected
Show Answer
Answer: A – Gamma radiation is an electromagnetic wave with no charge, so electric and magnetic fields do not deflect it. Alpha (+ charge) and beta (− charge) are deflected in opposite directions.
Q6. A few millimetres of aluminium sheet will stop which radiation?
- (A) Gamma radiation only
- (B) Beta radiation
- (C) Gamma and beta radiation
- (D) No radiation at all
Show Answer
Answer: B – Beta particles penetrate paper but are stopped by about 2-3 mm of aluminium. Gamma rays pass through aluminium and are only absorbed gradually by dense materials such as lead.
Q7. Ionising radiation from a radioactive source is commonly detected using
- (A) a voltmeter
- (B) a thermocouple
- (C) a Geiger-Muller tube
- (D) a barometer
Show Answer
Answer: C – In a Geiger-Muller tube, incoming radiation ionises the gas inside, producing a pulse of current for each particle or photon detected. The pulses are counted by a ratemeter or counter.
Q8. Which of these is a natural source of background radiation?
- (A) Radio waves from a TV transmitter
- (B) Ultraviolet light from the Sun
- (C) Sound waves in the air
- (D) Radon gas released from rocks
Show Answer
Answer: D – Background radiation comes from sources such as radon gas seeping from rocks and soil, cosmic rays from space, and naturally radioactive substances in food and building materials. It is present even when no radioactive source is nearby.
Q9. Separate narrow beams of alpha, beta and gamma radiation enter a magnetic field at the same speed. Which beam is deflected the most?
- (A) Alpha
- (B) Gamma
- (C) Beta
- (D) All three are deflected equally
Show Answer
Answer: C – Beta particles have a very small mass and a single negative charge, so the magnetic force bends their path the most. Alpha particles are much heavier and deflect only slightly; gamma rays are uncharged and are not deflected at all.
Q10. Gamma radiation is
- (A) a particle with mass number 4
- (B) an electromagnetic wave of very short wavelength
- (C) a stream of fast-moving electrons
- (D) a stream of neutrons
Show Answer
Answer: B – Gamma rays are high-frequency, short-wavelength electromagnetic waves emitted from the nucleus. They travel at the speed of light and carry no charge or mass.
Half-Life & Decay Calculations
Q11. The half-life of a radioactive isotope is the time taken for
- (A) the count rate of a sample to fall to zero
- (B) every nucleus in a sample to decay
- (C) half of the sample’s mass to disappear as energy
- (D) half of the nuclei in a sample to decay
Show Answer
Answer: D – After one half-life, half the original nuclei have decayed, so the activity and the (background-corrected) count rate also fall to half their starting values.
Q12. The corrected count rate of a sample falls from 800 counts/minute to 100 counts/minute. How many half-lives have passed?
- (A) 2
- (B) 3
- (C) 4
- (D) 8
Show Answer
Answer: B – Work halving step by step: 800 → 400 → 200 → 100. That is three halvings, so 3 half-lives have passed.
Q13. An isotope has a half-life of 6 hours. A sample has an initial activity of 480 Bq. What is its activity after 18 hours?
- (A) 60 Bq
- (B) 120 Bq
- (C) 160 Bq
- (D) 27 Bq
Show Answer
Answer: A – 18 hours ÷ 6 hours = 3 half-lives. Halve three times: 480 → 240 → 120 → 60 Bq.
Q14. A detector near a radioactive source measures 340 counts/minute. With the source removed, the background count rate is 40 counts/minute. What is the corrected count rate due to the source?
- (A) 380 counts/minute
- (B) 40 counts/minute
- (C) 300 counts/minute
- (D) 340 counts/minute
Show Answer
Answer: C – Corrected count rate = measured count rate − background count rate = 340 − 40 = 300 counts/minute. The background must always be subtracted before half-life calculations.
Q15. After the background has been subtracted, the count rate of a sample falls from 160 counts/second to 40 counts/second in 20 minutes. What is the half-life of the sample?
- (A) 5 minutes
- (B) 10 minutes
- (C) 20 minutes
- (D) 40 minutes
Show Answer
Answer: B – 160 → 80 → 40 is two halvings, i.e. 2 half-lives in 20 minutes. Half-life = 20 ÷ 2 = 10 minutes.
Q16. What fraction of a radioactive sample remains undecayed after 4 half-lives?
- (A) 1/4
- (B) 1/8
- (C) 1/2
- (D) 1/16
Show Answer
Answer: D – Each half-life multiplies the amount remaining by 1/2. After 4 half-lives the fraction left is (1/2)4 = 1/16.
Q17. Radioactive decay is described as random and spontaneous. This means that
- (A) it is impossible to predict when a particular nucleus will decay, and decay is unaffected by temperature or pressure
- (B) all the nuclei in a sample decay at exactly the same moment
- (C) decay can be speeded up by heating the sample
- (D) nuclei only decay when the sample is shaken or disturbed
Show Answer
Answer: A – Decay happens without any external trigger (spontaneous) and by chance (random). We can predict the fraction that decays in a given time, but never which individual nucleus will go next.
Q18. An isotope has a half-life of 5 days. A sample initially contains 64 g of this isotope. What mass of the isotope remains after 15 days?
- (A) 4 g
- (B) 16 g
- (C) 8 g
- (D) 32 g
Show Answer
Answer: C – 15 days ÷ 5 days = 3 half-lives. Halve three times: 64 → 32 → 16 → 8 g of the original isotope remains (the rest has decayed into other nuclides).
Q19. The corrected count rate of a source is 320 counts/second. After 30 minutes it is 80 counts/second. What is the half-life?
- (A) 5 minutes
- (B) 7.5 minutes
- (C) 10 minutes
- (D) 15 minutes
Show Answer
Answer: D – 320 → 160 → 80 is two halvings in 30 minutes, so one half-life = 30 ÷ 2 = 15 minutes.
Q20. A radioactive sample is heated strongly and then compressed to a very high pressure. What happens to its half-life?
- (A) It decreases
- (B) It stays the same
- (C) It increases
- (D) It becomes zero
Show Answer
Answer: B – Decay is a nuclear process. Physical conditions such as temperature, pressure and chemical state do not change the half-life.
Decay Equations, Uses & Safety
Q21. Uranium-238, 23892U, decays by emitting an alpha particle. The nuclide formed has [Supplement]
- (A) mass number 242 and proton number 94
- (B) mass number 234 and proton number 92
- (C) mass number 234 and proton number 90
- (D) mass number 238 and proton number 90
Show Answer
Answer: C – An alpha particle is 42He, so the mass number falls by 4 (238 − 4 = 234) and the proton number falls by 2 (92 − 2 = 90). The new nuclide is thorium-234, 23490Th.
Q22. Carbon-14, 146C, decays by beta-minus emission. The product nuclide has [Supplement]
- (A) mass number 14 and proton number 7
- (B) mass number 10 and proton number 4
- (C) mass number 14 and proton number 5
- (D) mass number 18 and proton number 8
Show Answer
Answer: A – In beta-minus decay the mass number is unchanged (14) but the proton number increases by 1 (6 + 1 = 7), forming nitrogen-14, 147N.
Q23. During beta-minus decay, the change that takes place inside the nucleus is [Supplement]
- (A) a proton changes into a neutron and an electron
- (B) an orbiting electron falls into the nucleus
- (C) two protons join together to form a helium nucleus
- (D) a neutron changes into a proton, and the electron created is emitted as the beta particle
Show Answer
Answer: D – A neutron in the nucleus turns into a proton plus an electron. The proton stays in the nucleus (so the proton number rises by 1) and the electron is ejected at high speed as the beta particle.
Q24. A domestic smoke detector contains a weak source of americium-241. Which radiation does this source emit?
- (A) Beta radiation
- (B) Alpha radiation
- (C) Gamma radiation
- (D) X-rays
Show Answer
Answer: B – The alpha particles ionise the air inside the detector, allowing a small current to flow. Smoke absorbs the alpha particles, the current falls and the alarm sounds. The alpha radiation is fully absorbed inside the detector, so it is safe in normal use.
Q25. The thickness of a moving aluminium sheet is monitored by placing a radioactive source on one side and a detector on the other. The most suitable radiation to use is
- (A) beta radiation
- (B) alpha radiation
- (C) gamma radiation
- (D) ultraviolet radiation
Show Answer
Answer: A – Beta radiation is partly absorbed by the sheet, so a small change in thickness causes a clear change in the count rate. Alpha would be absorbed completely, and gamma would pass through almost unaffected.
Q26. A radioactive tracer is injected into a patient, and detectors outside the body follow its movement through an organ. The radiation emitted must be
- (A) alpha, because it is the most strongly ionising
- (B) beta, because it is absorbed by the body
- (C) gamma, because it can pass through the body to reach the detector
- (D) alpha, beta or gamma – all are detected equally well
Show Answer
Answer: C – Only gamma radiation is penetrating enough to escape from inside the body and be detected externally. Alpha and beta would be absorbed by the patient’s tissues (and would cause unnecessary damage).
Q27. In radiotherapy, a tumour deep inside the body is treated using
- (A) an alpha source painted onto the skin
- (B) a beam of low-energy radio waves
- (C) ultraviolet light shone on the skin
- (D) a focused beam of gamma radiation
Show Answer
Answer: D – Gamma rays penetrate to the tumour and their ionising effect kills the cancer cells. The beam is often rotated around the patient so the tumour receives the full dose while healthy tissue receives less.
Q28. A doctor chooses a radioactive isotope to use as a tracer in the body. Which half-life is the most suitable?
- (A) 100 years
- (B) About 6 hours
- (C) 10 seconds
- (D) 50 days
Show Answer
Answer: B – A half-life of a few hours is long enough for the investigation to be completed, but short enough that the isotope decays away soon afterwards, keeping the patient’s radiation dose low. Very short half-lives decay before the scan finishes; very long ones keep irradiating the patient.
Q29. Which action does NOT reduce the radiation dose received by a worker handling a gamma source?
- (A) Wearing thin rubber gloves
- (B) Holding the source with long tongs
- (C) Standing behind a lead screen
- (D) Spending as little time near the source as possible
Show Answer
Answer: A – Thin gloves give no shielding against penetrating gamma rays (they only protect against contamination). Real precautions are: increase the distance (tongs), use shielding (lead screen), and limit the exposure time.
Q30. Nuclear waste with a very long half-life causes a long-term storage problem because
- (A) it decays so quickly that it overheats
- (B) it stops being radioactive after a few weeks
- (C) it remains radioactive for thousands of years and must be stored safely for all that time
- (D) it cannot be detected with a Geiger-Muller tube
Show Answer
Answer: C – A long half-life means the activity falls only very slowly, so the waste stays hazardous for thousands of years and needs secure storage that will last far into the future.
Keep practising: explore all our physics boards and topics on the Physics MCQs hub, and keep the key equations beside you with the IGCSE Physics Formula Sheet.
All questions are original practice questions written for QuaidianPoint – not Cambridge past-paper questions.