AQA GCSE Physics Physics Paper 1 (Higher), June 2024: Question 9

✅ Correct Answer

• Radiation from source A travels approximately 3 cm in air.
• After this distance, the count rate drops off to background radiation levels.
• This shows that alpha radiation has a very short range in air due to its low penetrating ability.

💡 Key Knowledge

• Alpha particles (a) are helium nuclei. Because they are relatively large and doubly charged, they strongly ionise air molecules, losing energy quickly and stopping within 2–5 cm in air.
• Background radiation must be accounted for: when count rates flatten out at zero or a steady low baseline, it indicates the radiation is fully absorbed.

🧠 Exam Technique

To score all 3 marks, structure your answer chronologically using the graph: state the distance it travels (read from the x-axis where source A drops to baseline), state what happens to the count rate at that point, and link it directly to the property of alpha radiation.

❌ Common Errors

Students often lose marks by just naming "alpha" without explaining *how* the graph proves it (failing to reference the distance value or the drop in count rate).

✅ Correct Answer

• Use an aluminium sheet (or another suitable absorber like thin lead or a few mm of metal).
• Beta radiation will be absorbed/stopped by the aluminium, whereas gamma radiation will penetrate through it (or: only gamma radiation will pass through).

💡 Key Knowledge

• Beta particles (b) are fast-moving electrons blocked by a few millimeters of aluminium.
• Gamma rays (y) are high-frequency electromagnetic waves that can penetrate several centimeters of lead or meters of concrete. Paper stops alpha, aluminium stops beta, and thick lead stops gamma.

🧠 Exam Technique

Note that the second mark is strictly dependent on scoring the first mark. You must name the correct absorber material *and* state the contrasting outcomes for beta and gamma radiation.

❌ Common Errors

Suggesting paper as the absorber. Paper only stops alpha; it will not help distinguish between beta and gamma because both pass right through paper.

✅ Correct Answer

Any one of the following:

• Increase the distance between the source and the teacher.
• Limit exposure time.
• Use tongs or forceps to handle sources.
• Wear a lead apron or stand behind a safety screen.
• Keep the source in a lead-lined box when not in use.

💡 Key Knowledge

Radiation dose received decreases with distance (inverse square law) and time of exposure. Shielding absorbs radiation before it reaches the body.

✅ Correct Answer

Any one of the following:

• Wear gloves / wear a lab coat.
• Handle the source with tongs or forceps.
• Do not eat or drink in the laboratory.
• Do not touch the source with bare hands.

🧠 Exam Technique

Understand the difference between *irradiation* (exposure to radiation externally) and *contamination* (transferring radioactive material onto or into an object/body). Precautions for contamination focus on barrier protection (gloves, coats) and preventing ingestion.

📐 Step-by-Step Calculation

1. Locate the point: Find 300 s on the x-axis of Figure 16 and move up to the curve.
2. Draw a tangent: Use a ruler to draw a straight line that touches the curve precisely at t = 300 s , matching the slope of the curve without crossing through it.
3. Calculate the gradient: Form a large triangle on your tangent line. Find the change in y ( Delta y ) and change in x ( Delta x ).
4. Account for scale multipliers: Remember the y-axis is scaled by × 10²³ .
5. Apply values: Gradient = Change in y / Change in x. An acceptable answer falls in the range 6.5 × 10²⁰ to 7.6 × 10²⁰ .

✅ Final Answer & Unit

Activity = 7.1 × 10²⁰ (Acceptable range: 6.5 × 10²⁰ to 7.6 × 10²⁰ )

Unit = becquerel / Bq

🧠 Exam Technique

Activity is defined as the rate of decay (number of decays per second), which equals the gradient of a atoms-vs-time graph. Make your gradient triangle as large as possible to minimize reading errors!

❌ Common Calculation Traps

• Forgetting to multiply your final answer by 10²³ from the axis label.
• Writing "decays/second" for the unit instead of the required SI unit Bq (or becquerel).