S6 Physics Practice Paper: Radioactivity, Half-life And Nuclear Energy (12 Questions, 100 Marks, 75 Minutes, With Answers)

S6 Physics Practice Paper: Radioactivity, Half-life and Nuclear Energy (12 Questions, 100 Marks, 75 Minutes, with Answers)

2026-11-09

HKDSE Physics: Radioactivity, Half-life and Nuclear Energy

12 questions, 100 marks, 75 minutes: alpha, beta and gamma radiation, decay equations, half-life, carbon-14 dating, fission, fusion and mass-energy conversion, with marking points.

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Instructions: This paper has three sections, A, B and C. Answer all questions. Total marks: 100. Time allowed: 75 minutes.

Show all working. Answers without working earn no method marks. Write down a formula before substituting numbers.

Give numerical answers to three significant figures unless stated otherwise, and state the correct units.

Spend about 35 minutes on Section A, then use the remaining time for the half-life calculations in Section B and the nuclear energy calculations in Section C.

Section A: Properties of Radiation and Nuclear Decay Equations

Alpha, beta and gamma rays, and decay equations

#	Question	Answer and marking notes
1	Describe the nature of alpha, beta and gamma radiation, and compare their charge, penetrating power and ionising power.	(a) Alpha radiation is a helium nucleus of two protons and two neutrons and is positively charged (3 marks). (b) Beta radiation is a fast electron and is negatively charged (2 marks). (c) Gamma radiation is a high-frequency electromagnetic wave with no charge (1 mark). (d) Penetrating power increases from alpha to beta to gamma, while ionising power decreases in the same order (2 marks).
2	Radium-226 (mass number 226, atomic number 88) emits one alpha particle. Write the nuclear equation and state the mass number and atomic number of the daughter nucleus.	(a) The equation is Ra-226 gives Rn-222 + He-4 (3 marks). (b) The mass number falls from 226 to 222 (2 marks). (c) The atomic number falls from 88 to 86, so the daughter is radon (2 marks). (d) Mass number and charge are conserved on both sides (1 mark).
3	Carbon-14 decays by beta emission to form nitrogen-14. Write the nuclear equation and explain why the mass number does not change in beta decay.	(a) The equation is C-14 gives N-14 + electron (3 marks). (b) In beta decay a neutron in the nucleus turns into a proton and an electron is emitted (2 marks). (c) The proton number rises from 6 to 7, so the atomic number increases by one (2 marks). (d) The total number of nucleons is unchanged, so the mass number stays 14 (1 mark).
4	The net count rate of a radioactive source is initially 8000 Bq and its half-life is 5 years. Find the count rate after (a) 10 years and (b) 20 years.	(a) 10 years is two half-lives (2 marks), so the count rate is 8000 divided by 4, which is 2000 Bq (2 marks). (b) 20 years is four half-lives (2 marks), so the count rate is 8000 divided by 16, which is 500 Bq (2 marks).

Section B: Half-life, Dating and Radiation Protection

Half-life calculations, carbon-14 dating and background radiation

#	Question	Answer and marking notes
5	The count rate of a radioactive source falls to one-sixteenth of its original value after 40 days. Find the half-life of the nuclide.	(a) One-sixteenth equals one half to the power four, so four half-lives have passed (3 marks). (b) Four half-lives take 40 days (2 marks). (c) The half-life is 40 divided by 4, which is 10 days (3 marks).
6	The carbon-14 activity of an ancient wooden sample is one-eighth that of living wood. Given that the half-life of carbon-14 is 5730 years, find the age of the sample and state one assumption of this dating method.	(a) One-eighth equals one half to the power three, so three half-lives have passed (3 marks). (b) Age = 3 times 5730 = 17190 years (4 marks). (c) Assume the proportion of carbon-14 in the atmosphere stayed constant while the sample was alive and that no carbon was exchanged afterwards (3 marks).
7	An experiment records 400 counts per minute from a sample, while the background count is 40 counts per minute. Find the net count rate and explain why counting is repeated many times.	(a) Net count rate = 400 – 40 = 360 counts per minute (3 marks). (b) Radioactive decay is a random process, so a single count fluctuates widely (3 marks). (c) Repeating the count and averaging reduces the random error (2 marks).
8	Uranium-235 absorbs a neutron and undergoes fission: U-235 + n gives Ba-141 + Kr-92 + 3n. (a) Verify that mass number and atomic number are conserved. (b) The mass defect per fission is 0.215 u and 1 u is equivalent to 931 MeV. Find the energy released in MeV and in joules.	(a) Mass number: 235 + 1 = 236 on the left and 141 + 92 + 3 = 236 on the right (2 marks); atomic number: 92 = 56 + 36, so both are conserved (2 marks). (b) Energy = 0.215 × 931 = 200 MeV (3 marks). (c) 200 MeV = 2.0 × 10⁸ × 1.6 × 10⁻¹⁹ J = 3.2 × 10⁻¹¹ J (3 marks).

Section C: Fission, Fusion and Nuclear Energy Applications

Mass defect, reactors and radioactive waste

#	Question	Answer and marking notes
9	Four hydrogen nuclei fuse into one helium nucleus inside the Sun, with a mass defect of 0.0287 u. Find the energy released per reaction in MeV and name this type of reaction.	(a) Energy = 0.0287 × 931 = 26.7 MeV to three significant figures (4 marks). (b) The reaction is nuclear fusion (2 marks). (c) Fusion needs very high temperature and pressure to overcome the electrostatic repulsion between protons (2 marks).
10	Explain the function of (a) the chain reaction, (b) the control rods and (c) the moderator in a nuclear reactor.	(a) Neutrons released in each fission trigger further fissions, so the reaction sustains itself as a chain reaction (3 marks). (b) Control rods absorb neutrons and adjust their number, thereby controlling the reaction rate (3 marks). (c) The moderator slows fast neutrons into thermal neutrons, raising the chance of fission in uranium-235 (2 marks).
11	Give two uses of radioactive isotopes and state three principles of protection when handling radioactive sources.	(a) Cobalt-60 is used in radiotherapy, americium-241 in smoke detectors and iridium-192 to inspect welds in industry (any two) (2 marks). (b) Keep the exposure time short (2 marks). (c) Increase the distance from the source (2 marks). (d) Use shielding such as lead or concrete (2 marks).
12	Compare nuclear fission power with coal-fired power by giving two advantages and one disadvantage, and briefly describe how high-level radioactive waste is handled.	(a) Nuclear power emits no carbon dioxide or sulphur dioxide, so it causes far less air pollution (2 marks). (b) The energy released per unit mass of fuel is much greater than that of coal, so less fuel is needed (2 marks). (c) It produces high-level radioactive waste, and a reactor accident has severe consequences (2 marks). (d) Waste is first cooled in storage ponds, then vitrified and sealed in a deep geological repository (2 marks).

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