AQA A-Level Physics 3.8 Nuclear physics: a complete overview of radioactivity, the nucleus and nuclear energy

What module 3.8 actually demands

Nuclear physics takes you from the discovery of the nucleus to the energy that powers stars and reactors. The examiners test historical reasoning, the properties and equations of radioactive decay, and the mass-energy accounting that explains fission and fusion. This guide walks through the eight topics in specification order, then sets out the exam patterns AQA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Rutherford scattering and the nuclear atom

The alpha scattering experiment fired alpha particles at thin gold foil. Most passed straight through, a few were deflected by large angles, and a very few bounced back. These observations show the atom is mostly empty space with a tiny, dense, positively charged nucleus, establishing the nuclear model and overturning the plum pudding model.

Properties of alpha, beta and gamma radiation

Alpha is a helium nucleus, strongly ionising and least penetrating; beta is a fast electron, moderately ionising; gamma is a high energy photon, weakly ionising and most penetrating. Background radiation must always be subtracted from a count rate. The intensity of gamma radiation from a point source obeys an inverse square law, and the penetration properties determine the uses of each radiation in industry and medicine.

Radioactive decay and half-life

Decay is random and spontaneous. The decay constant $\lambda$ is the probability of decay per unit time, the activity is $A = \lambda N$, and the number of undecayed nuclei falls exponentially as $N = N_0 e^{-\lambda t}$. The half-life is $T_{1/2} = \frac{\ln 2}{\lambda}$. These laws underpin radioactive dating, including carbon-14 dating of once-living material.

Nuclear instability and stability

Stable nuclei lie on a band of stability on a graph of neutron number against proton number. Nuclei above the band decay by beta-minus, those below by beta-plus, and very heavy nuclei by alpha; gamma emission removes excess energy from an excited nucleus without changing its composition.

Nuclear radius and density

The nuclear radius is estimated from the closest approach of alpha particles and, more accurately, from electron diffraction. Measurements give $R = r_0 A^{1/3}$, so volume is proportional to nucleon number and nuclear density is constant for all nuclei.

Mass, energy, fission and reactors

Mass and energy are equivalent through $E = mc^2$. The mass defect corresponds to the binding energy, and the binding energy per nucleon curve peaks at iron-56. Energy is released in fission and fusion because the products are more tightly bound. In induced fission a thermal neutron splits a heavy nucleus, releasing neutrons that can sustain a chain reaction above the critical mass. A reactor uses a moderator, control rods and a coolant to keep the reaction controlled and to generate electricity, with careful handling of radioactive waste.

How module 3.8 is examined

A typical AQA profile for this module:

• Explanations. Interpreting the scattering observations, comparing radiations, and explaining the roles of reactor components.
• Calculations. Half-life and activity, decay equations, nuclear radius, mass defect and binding energy in MeV, and energy released in reactions.
• Graphical questions. Decay curves, log-linear plots, the N against Z graph, and the binding energy per nucleon curve.
• Extended answers. Explaining why both fission and fusion release energy, and how a reactor is controlled safely.

Check your knowledge

A mix of recall and calculation questions covering module 3.8. Attempt them under timed conditions, then check against the solutions.

1. State what the backward scattering of a few alpha particles tells us about the atom. (1 mark)
2. State which radiation is stopped by a few millimetres of aluminium. (1 mark)
3. An isotope has a half-life of $10 \text{ minutes}$. What fraction remains after $30 \text{ minutes}$? (2 marks)
4. A nucleus lies below the band of stability. State its likely decay mode. (1 mark)
5. State the relationship between nuclear radius and nucleon number. (1 mark)
6. A reaction has a mass defect of $0.10 \text{ u}$. Calculate the energy released in MeV. (2 marks)
7. Explain why a chain reaction needs a critical mass. (2 marks)
8. State the function of the moderator in a thermal reactor. (1 mark)