AQA A Level Physics 3.8 Nuclear Physics Exam Questions

About AQA A Level Physics 3.8 Nuclear Physics

Nuclear Physics is AQA section 3.8 and is examined on Paper 2. Section 3.8.1 covers Rutherford scattering and the nuclear model, properties of alpha, beta and gamma radiation, the inverse square law for gamma rays, the exponential law of decay, half-life and decay constant, and nuclear instability, including the N against Z graph and excited nuclear states. It continues with nuclear radius from electron diffraction, mass and energy, binding energy per nucleon, induced fission, thermal reactors and safety. It builds directly on the particles work in section 3.2. Examiners favour decay calculations with logarithms, binding energy curve explanations and the gamma inverse square law required practical.

What you need to know

• 3.8.1.1 Rutherford scattering Most alpha particles passed straight through gold foil and a few were deflected through large angles, showing a small, dense, positively charged nucleus. Know why this replaced the plum pudding model.
• 3.8.1.2 Alpha, beta and gamma Alpha is strongly ionising with a range of a few cm in air; beta-minus is less ionising and stopped by a few mm of aluminium; gamma is weakly ionising and obeys I = k/x^2. Know uses, background radiation and why sources are handled with tongs.
• 3.8.1.3 Radioactive decay Decay is random. Activity A = lambda N, N = N0 e^(-lambda t), and half-life T = ln 2 / lambda. Use log graphs to find lambda and know applications of half-life such as carbon dating.
• 3.8.1.4 Nuclear instability On a graph of N against Z, stable nuclei lie near the line of stability. Neutron-rich nuclei undergo beta-minus decay, proton-rich nuclei beta-plus decay or electron capture, and very heavy nuclei alpha decay. Excited nuclei emit gamma rays; technetium-99m is a medical example.
• 3.8.1.5 Nuclear radius R = R0 A^(1/3), so nuclear density is roughly constant. Radius can be found from the first minimum of an electron diffraction pattern.
• 3.8.1.6 to 3.8.1.8 Mass, energy and fission E = mc^2; mass difference and binding energy, with 1 u equivalent to 931.5 MeV. Binding energy per nucleon peaks near iron, so fission of heavy nuclei and fusion of light nuclei release energy. In a thermal reactor know the moderator, control rods, coolant, critical mass and waste handling.

Common mistakes in 3.8 Nuclear Physics

• Saying radioactive decay is affected by temperature or chemical state. Decay is spontaneous and random and is not affected by external conditions.
• Mixing units of lambda and t. If lambda is in s-1 then t must be in seconds; convert hours or years first.
• Stating that binding energy is energy stored in the nucleus. It is the energy needed to separate a nucleus into its nucleons, equivalent to the mass defect.
• Writing that control rods slow neutrons down. The moderator slows neutrons; control rods absorb neutrons to control the rate of fission.
• Forgetting the background count when analysing count rate data. Subtract background before using the inverse square law or decay equations.

Try three 3.8 Nuclear Physics exam-style questions