AQA A-Level Physics Paper 2, June 2023: Question 29

✅ Correct Answer

B — β⁻ (Beta-minus particles)

Beta particles are partially absorbed by thin sheets of aluminium (~0.5 mm to a few mm). A small variation in sheet thickness produces a measurable change in detected count rate.

💡 Key Knowledge

• Alpha (α): Extremely low penetration. Completely absorbed by a single sheet of paper or a few centimetres of air. It would be 100% blocked by 0.5 mm of aluminium, showing zero count rate regardless of variations.
• Beta-minus (β⁻): Penetrates several millimetres of aluminium. At 0.5 mm, a significant fraction passes through, making count rate directly sensitive to thickness fluctuations.
• Beta-plus (β⁺): Positrons rapidly encounter electrons in the metal, undergoing matter-antimatter annihilation to produce 511 keV gamma photons, destroying the predictable attenuation profile.
• Gamma (γ): Highly penetrating. Almost entirely passes through 0.5 mm of aluminium with negligible absorption; variations in thickness would cause negligible change in count rate.

🧠 Exam Technique & Elimination

Use the principle of attenuation sensitivity:

1. Step 1: Check extremes. If radiation is completely absorbed (α) or barely absorbed at all (γ), count rate won't correlate sensitively with thickness. Eliminate A and D immediately.
2. Step 2: Compare β⁻ vs β⁺. Beta-plus (β⁺) emits antiparticles (positrons) which annihilate with atomic electrons almost instantly:
   e⁺ + e⁻ → 2γ
   This converts the beta stream into gamma rays, which penetrate straight through. Standard industrial sources therefore use stable β⁻ emitters (such as Strontium-90). Eliminate C.

❌ Common Misconceptions

• "Beta is completely blocked by aluminium": A common GCSE oversimplification is that "aluminium stops beta". In reality, standard fast beta particles are stopped by several millimetres of aluminium (e.g., ~3–5 mm). A sheet of 0.5 mm absorbs only a fraction, which is precisely why it works as a detector.
• Picking β⁺ instead of β⁻: Forgetting that positrons are antimatter and annihilate upon contact with ordinary electrons inside the aluminium foil.
• Choosing Gamma: Gamma rays require dense materials (several centimetres of lead or thick concrete) to see substantial attenuation. A 0.5 mm foil will not alter the gamma count rate detectably.