Radioactive decay test with answers for A Level physics (free PDF download)

Quick answer: Radioactive decay is the random change of an unstable nucleus, and a sample decays exponentially with a fixed half-life. This Physics 11-12 test on radioactive decay has 8 questions worth 20 marks, to do in 45 minutes: a plutonium decay, reading a decay graph and Polonium-210 source. The mark scheme is on a separate page; free printable PDF.

Answer all the questions. Show your working, give units, and use c = 3.00 × 10⁸ m/s, 1 u = 931.5 MeV/c² and ln 2 = 0.693. Total 20 marks. A calculator is allowed.

Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB E.3-E.5

Time: 45 minutes · 8 questions · Total: 20 marks · Calculator allowed

Contents

Question 1 A plutonium decay (3 marks)

Plutonium-239 (Z = 94) decays by emitting an alpha particle.

Determine the nucleon number and proton number of the daughter nuclide. (2 marks)
Name the daughter element (uranium has Z = 92). (1 mark)

Question 2 Reading a decay graph (3 marks)

The graph shows the activity of a sample of an isotope X.

Activity of isotope X.

Use the graph to determine the half-life. (1 mark)
Calculate the activity after 10 days. (2 marks)

Question 3 Polonium-210 source (3 marks)

A source contains 4.0 × 10¹⁶ nuclei of polonium-210, which has a half-life of 138 days.

Calculate the decay constant in s⁻¹. (2 marks)
Calculate the activity of the source. (1 mark)

Question 4 Binding energy of lithium-7 (3 marks)

Use atomic masses: hydrogen atom 1.007825 u, neutron 1.008665 u, lithium-7 atom 7.016004 u (lithium has Z = 3).

Calculate the mass defect in u. (1 mark)
Calculate the binding energy in MeV, using 1 u = 931.5 MeV/c². (1 mark)
Calculate the binding energy per nucleon. (1 mark)

Question 5 Why fusion and fission release energy (2 marks)

Use the shape of the graph of binding energy per nucleon against nucleon number to explain why both the fusion of light nuclei and the fission of heavy nuclei release energy.

Question 6 A chain reaction (3 marks)

Explain how a chain reaction occurs in a sample of uranium-235, and how control rods keep a reactor at a steady rate.

Question 7 Fusion conditions (2 marks)

Explain why nuclear fusion requires temperatures of millions of kelvin.

Question 8 Activity (1 mark)

Define the activity of a radioactive source and give its SI unit.

Alpha decay: change in A and Z? — A falls by 4, Z falls by 2.

Beta-minus decay: what changes in the nucleus? — A neutron becomes a proton; Z rises by 1.

Activity equals what? — A = λN, in becquerel (decays per second).

Exponential decay law? — N = N₀ e^(−λt)

Link between half-life and decay constant? — T½ = ln 2 / λ ≈ 0.693 / λ

Fraction left after n half-lives? — (½) to the power n

What is the mass defect? — Mass of separate nucleons minus the mass of the nucleus.

Convert mass defect in u to MeV? — Multiply by 931.5.

Which nucleus is near the peak of the curve? — Iron-56, about 8.8 MeV per nucleon.

Why do fission and fusion release energy? — The products have higher binding energy per nucleon.