Retrieval Practice
Nuclear Structure & Radiation — AQA A-Level Physics
Q1. What were the three key observations from Rutherford's alpha scattering experiment?
- Most alpha particles passed straight through (atom is mostly empty space).
- Some were deflected through small angles (positive charge concentrated at the centre).
- Very few bounced back at >90° (nucleus is extremely small, dense, and contains most of the mass).
Q2. State the properties of an alpha particle.
- Helium nucleus: 2 protons + 2 neutrons.
- Mass = 4 u, charge = +2e.
- Highly ionising (~10,000 ion pairs per cm), weakly penetrating (3-7 cm range in air, stopped by paper).
Q3. State the properties of a beta-minus particle.
- High-energy electron emitted from the nucleus.
- Charge = -e.
- Moderately ionising (~100 ion pairs per cm), moderately penetrating (20 cm - 3 m in air, stopped by ~3 mm aluminium).
Q4. What is the difference between beta-plus emission and electron capture?
- Both convert a proton to a neutron.
- Beta-plus: a positron and neutrino are emitted.
- Electron capture: an orbital electron is absorbed by the nucleus, emitting a neutrino and often a gamma ray.
- Both decrease Z by 1 and increase N by 1.
Q5. Why does the inverse square law apply to gamma radiation but not alpha or beta?
- Gamma is not easily absorbed, so it spreads out uniformly as a sphere.
- Alpha and beta are absorbed quickly by matter before they can spread out.
Q6. State the largest source of background radiation in the UK.
- Radon gas from rocks and buildings.
- It is a naturally occurring alpha emitter produced by the decay of uranium in rocks and soil.
Q7. How do you calculate the corrected count rate?
Corrected count rate = measured count rate with source present − background count rate (measured with no source).
Q8. On the N-Z graph, where are beta-minus emitters found and why?
- To the left of the stability line (neutron-rich region).
- These nuclei have too many neutrons, so a neutron converts to a proton via beta-minus decay to move closer to stability.
Q9. Write the general equation for alpha decay.
- A/Z X → (A-4)/(Z-2) Y + 4/2 α.
- The nucleon number decreases by 4 and the proton number decreases by 2.
Q10. State the equation linking nuclear radius R to mass number A.
- R = R₀\(A^{1/3}\), where R₀ ≈ 1.05 fm.
- A graph of R against \(A^{1/3}\) is a straight line through the origin.
Q11. Explain why nuclear density is constant for all nuclei.
- V = (4/3)πR₀³A, so volume is proportional to A.
- Mass m = Au.
- Therefore ρ = m/V = 3u/(4πR₀³), which contains only constants.
- The A cancels, so density is the same for all nuclei.
Q12. State the approximate value of nuclear density.
ρ ≈ 3.4 × 10¹⁷ kg m⁻³.
Q13. Give one advantage and one disadvantage of using electron diffraction to measure nuclear radius.
- Advantage: gives a direct, accurate measurement (not just an upper limit).
- Electrons are leptons, unaffected by the strong nuclear force.
- Disadvantage: electrons must be accelerated to very high speeds to achieve a de Broglie wavelength comparable to nuclear size.
Q14. In the inverse square law practical, what do you plot and what does a straight line confirm?
- Plot 1/√C against distance x.
- A straight line through (or near) the origin confirms that C ∝ 1/x², verifying the inverse square law.
Q15. State the equation for estimating nuclear radius by closest approach.
- Eₖ = Qq/(4πε₀r), so r = Qq/(4πε₀Eₖ).
- For an alpha particle, Q = 2e and q = Ze.
- This gives an upper limit for the nuclear radius.