AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2023: Question 4

✅ Correct answer

Peer review

💡 Key knowledge

• Scientists check each other’s methods, results and conclusions before publication.
• This helps spot mistakes, improve reliability, and reduce bias.

🧠 Exam technique

• Use the exact phrase peer review .
• “Checked by other scientists” is the clue.

❌ Common errors

• “Repeating the experiment” — that is not the named process.
• “Quality control” or “verification” — too vague for the mark scheme.

✅ Correct answer

It is ionising.

💡 Key knowledge

• Ionising radiation removes electrons from atoms.
• This can damage DNA in cells.
• Damaged DNA can lead to uncontrolled cell division, which may cause cancer.

🧠 Exam technique

• The question asks for the property, not the effect.
• Write one clear physics word: ionising .

❌ Common errors

• “Penetrating” is not the mark scheme answer here.
• “Dangerous” or “harmful” is too vague.

✅ Correct answer

becquerel (Bq)

💡 Key knowledge

• Activity = rate of decay.
• 1 becquerel = 1 decay per second.

🧠 Exam technique

• You can write becquerel or Bq .
• If asked for count-rate, remember the unit is counts/s , but that is not the same as activity.

❌ Common errors

• Writing counts/s instead of Bq.
• Giving no unit.

✅ Correct answer

Not all the radiation emitted by the sample is detected.

Acceptable reasons:

• the radiation spreads out in all directions
• some radiation is absorbed before reaching the detector
• some radiation entering the detector is not detected

💡 Key knowledge

• Activity is the total number of decays happening in the source.
• Count-rate is only what the detector records.
• So count-rate is usually smaller than activity.

🧠 Exam technique

• Link the answer to the detector: it only measures part of the radiation.
• A strong answer names one reason clearly and fully.

❌ Common errors

• “The detector is faulty” — not needed and not the mark scheme point.
• “Because the source is weak” — does not explain the difference.
• Leaving out the idea that radiation spreads out in all directions.

✅ Correct answer

Alpha radiation

Because the radiation is not detected beyond 5 cm, or because alpha cannot travel 10 cm in air.

💡 Key knowledge

• Alpha has the shortest range in air.
• Beta travels further than alpha.
• Gamma is the most penetrating and can travel the furthest.

🧠 Exam technique

• Use the results: 300 at 2.0 cm, 24 at 5.0 cm, 0 at 10.0 cm.
• The key clue is 0 counts at 10 cm.
• Top answers compare the data with the known ranges of alpha, beta and gamma.

❌ Common errors

• Choosing beta or gamma — these would still be detected at 10 cm.
• Writing only “it has low count-rate” without naming the radiation.
• Saying “alpha is more ionising” — true, but not relevant to this question.

📐 Calculations: step-by-step

1. At 60 minutes, the count-rate is 98 counts/s.
2. The question says 30 minutes is a whole number of half-lives.
3. Work backwards from 98 to 1568 by doubling:
   98 → 196 → 392 → 784 → 1568
4. This is 4 half-lives in 60 minutes.
5. So 30 minutes is 2 half-lives.
6. Start from 1568 at 0 min:
   1 half-life: 1568 → 784
   2 half-lives: 784 → 392

X = 392 counts/second

💡 Key knowledge

• Half-life is the time taken for the activity or count-rate to halve.
• Count-rate should halve each half-life: 1568, 784, 392, 196, 98.

🧠 Exam technique

• Always keep the same units: counts/second .
• Show the halving pattern clearly for method marks.
• If the final answer is correct but working is unclear, you may lose method marks.

❌ Common errors

• Dividing by 4 instead of halving twice.
• Using subtraction instead of halving.
• Forgetting that 30 minutes is 2 half-lives, not 1.
• Leaving out the unit in the final answer.

✅ Correct answer

Radioactive decay is a random process.

💡 Key knowledge

• Even identical samples do not decay in exactly the same way at every moment.
• Radioactive decay happens unpredictably for individual nuclei.

🧠 Exam technique

• The key word is random.
• For 1 mark, a short statement is enough.

❌ Common errors

• “The count-rate is predictable” — opposite of the mark scheme.
• “Different temperatures” — not relevant here.
• “Different sample sizes” — not the reason given by the mark scheme.

💡 What top answers did

• Used the exact science term: peer review , ionising , becquerel , alpha .
• Linked count-rate to detector limitations.
• Used the data to justify the radiation type.
• Showed a clear halving pattern in the calculation.

❌ Typical misconceptions

• Mixing up activity and count-rate.
• Using “penetrating” instead of “ionising” in part (b).
• Forgetting that alpha has a very short range in air.
• Doing the half-life calculation without showing the sequence of halving.