AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), June 2025: Question 4

✅ Model Answer

• Alpha particle: Consists of two protons and two neutrons (or a helium nucleus). [1 mark]
• Beta particle: A fast-moving / high-speed electron (emitted from the nucleus). [1 mark]
• Gamma ray: A high-energy / high-frequency electromagnetic wave (or electromagnetic radiation). [1 mark]

💡 Key Scientific Facts

• Alpha (α) is identical to a Helium-4 nucleus (⁴₂He).
• Beta (β) is formed when a neutron in an unstable nucleus turns into a proton and an electron; the electron is ejected at high speed.
• Gamma (γ) has no mass and no charge; it is an electromagnetic wave from the top end of the EM spectrum.

❌ Common Traps

• Beta: Saying "an orbital electron" or "an electron from an outer shell". Beta particles come strictly from inside the nucleus.
• Alpha: Stating "a helium atom" instead of "a helium nucleus" (atoms include electrons; alpha particles have no electrons).

✅ Correct Values

• Top row (mass numbers): 198 for Hg and 0 for β [1 mark]
• Bottom row (atomic numbers): 80 for Hg and -1 for β [1 mark]

🧠 Conservation Law Method

Total Mass Number and Total Atomic Number must balance across the arrow:

• Top: 198 = 198 + 0 ✓
• Bottom: 79 = 80 + (-1) ✓

❌ Common Error

Writing the atomic number of Hg as 78 instead of 80 . Remember, a neutron becomes a proton, so the atomic number increases by 1!

🔧 Step-by-Step Graphical Working

1. Find initial activity: At time = 0 days, Activity = 22 kBq .
2. Halve the initial activity: 22 ÷ 2 = 11 kBq .
3. Read from the graph: Draw a horizontal line from 11 kBq to the decay curve, then drop straight down to the time axis.
4. Value: Reads exactly at 2.75 days (acceptable range: 2.7 to 2.8 days).

✅ Mark Allocation

• 1 mark: Showing evidence of halving the activity (e.g. a horizontal construction line drawn from 11 kBq across to the curve).
• 1 mark: Final answer in range 2.75 days (allow 2.7 – 2.8).

🧠 Examiner Tip

Always draw your construction lines clearly on the exam graph with a ruler. If your final reading has a slight misread, you can still secure the method mark if examiners see your line drawn from 11 kBq!

🔧 Step-by-Step Calculation

1. Step 1: Read activity at 5 days from graph:
   At t = 5 days, Activity = 6.0 kBq (allow 7.0 kBq). [1 mark]
2. Step 2: Find decrease in activity:
   Initial = 22 kBq, New = 6.0 kBq
   Decrease = 22 − 6.0 = 16.0 kBq .
3. Step 3: Calculate percentage decrease:
   Formula: (Decrease ÷ Original) × 100
   (22 − 6.0) ÷ 22 × 100 [1 mark]
   = 16 ÷ 22 × 100 = 72.7272...% [1 mark]
4. Step 4: Round to 2 significant figures:
   72.72...% rounds to 73%. [1 mark]
   (Note: If 7.0 kBq was read, (15/22)×100 = 68.18...% → 68%)

❌ Pitfalls to Avoid

• Calculating remaining percentage instead of decrease: Doing (6.0 ÷ 22) × 100 = 27% gives the percentage remaining, not the percentage decrease.
• Ignoring Significant Figures: Leaving the answer as 72.7% loses the final mark. The question explicitly states: "Give your answer to 2 significant figures".
• Wrong denominator: Always divide by the original value (22), never the new value (6.0).

✅ Model Answer (Any TWO)

• Beta radiation has a short range in human tissue (a few millimetres to centimetres). [1 mark]
• It increases the chance of destroying/killing the cancer cells. [1 mark]
• It minimises/decreases radiation damage to surrounding healthy cells. [1 mark]

🧠 Exam Technique: The "Dual Benefit"

In radiotherapy questions, top students always link the properties of the radiation to two contrasting impacts:

• Maximising harm to the target (cancer cells).
• Minimising harm to the surroundings (healthy tissue).

✅ Accepted Marking Points (Any ONE)

• The risk (or trauma) of a second surgical procedure to remove the sample is greater than the risk of leaving it inside. [1 mark]
• The activity will quickly decrease to a safe/negligible level due to its short half-life (~2.8 days). [1 mark]
• Also allowed: The risk of leaving the sample in the patient is very small.

💡 Connecting the Half-Life

With a half-life of roughly 2.75 days, after about a month (around 10 half-lives), the activity of Gold-198 drops to less than 0.1% of its starting level, rendering it virtually harmless without requiring invasive surgery.