CCEA A Level Physics 4.5 The Nucleus and 4.6 Nuclear Decay Exam Questions

About CCEA A Level Physics 4.5 The Nucleus and 4.6 Nuclear Decay

These sections in Unit A2 1: Deformation of Solids, Thermal Physics, Circular Motion, Oscillations and Atomic and Nuclear Physics cover the structure of the nucleus and how unstable nuclei decay. You need to describe the alpha-particle scattering experiment and what it showed, use the relationship between nuclear radius and nucleon number, and show that nuclear density is roughly constant. Nuclear decay covers the penetrating power of alpha, beta and gamma radiation, changes in proton and nucleon numbers, the random and exponential nature of decay, activity, decay constant and half-life, and a method for measuring half-life. Expect decay equation, exponential decay and nuclear density calculations.

What you need to know

• Alpha-particle scattering Most alpha particles passed straight through thin gold foil, a few were deflected through large angles and a very small number bounced back. This showed the atom is mostly empty space with a tiny, dense, positively charged nucleus.
• Nuclear radius and density r = r0 A^(1/3), where A is the nucleon number and r0 is about 1.2 x 10^-15 m. Since volume is proportional to A, nuclear density is roughly the same for all nuclei, about 10^17 kg m-3.
• Radiations Alpha (helium nucleus) is stopped by paper or a few cm of air. Beta (fast electron) is stopped by a few mm of aluminium. Gamma (electromagnetic) is reduced by several cm of lead.
• Decay equations Alpha decay: nucleon number falls by 4, proton number by 2. Beta-minus decay: proton number rises by 1, nucleon number unchanged. Gamma emission changes neither.
• Exponential decay Decay is random and spontaneous. Activity A = λN, and N = N0 e^(-λt), A = A0 e^(-λt). Half-life t1/2 = ln 2 / λ = 0.693/λ.
• Measuring half-life Use a GM tube and counter to record count rate over time, subtract background count, and plot ln(corrected count rate) against time; the gradient is -λ.

Common mistakes in 4.5 The Nucleus and 4.6 Nuclear Decay

• Saying the scattering experiment showed electrons orbit the nucleus; it showed the nucleus is small, dense and positive.
• Forgetting to subtract background radiation before analysing count rates.
• Using half-life in days with λ in s-1; keep the time units consistent.
• Saying a nucleus decays after one half-life; half-life is the time for half the nuclei in a sample to decay, and individual decays are random.
• Writing a beta-minus decay that lowers the proton number; it raises it by one.

Try three 4.5 The Nucleus and 4.6 Nuclear Decay exam-style questions