AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2021: Question 2

Key focus

• Size comparison of a helium atom and an alpha particle
• What ionisation means in terms of electrons
• Why beta cannot trigger a spark detector
• How to test for alpha, beta and gamma using paper and aluminium

How marks are gained

• Ticking the exact correct option
• Choosing the right statements for ionisation
• Giving a reason, not just a yes/no answer
• Describing a step-by-step method with results and conclusions

✅ Correct answer

More than 10 000 times bigger

This is the only option that scores the mark.

💡 Key knowledge

• An alpha particle is the same as a helium nucleus.
• A helium atom is much larger because it includes electrons and has an overall atomic size, not just a nucleus.
• The radius of an atom is vastly bigger than the radius of its nucleus.

🧠 Exam technique

For multiple-choice questions, do not overthink the wording. AQA often tests your memory of scale. If you know atoms are much larger than nuclei, choose the largest option available.

❌ Common errors

• Choosing exactly 5000 times bigger because it sounds precise.
• Mixing up the size of the nucleus with the size of the whole atom.
• Thinking “alpha particle = helium atom” instead of “helium nucleus”.

✅ Correct answers

• The atom becomes a positive ion.
• The atom loses an electron.

1 mark for each correct tick.

💡 Key knowledge

• Ionisation means removing electrons from an atom.
• If an atom loses electrons, it has more protons than electrons, so it becomes positively charged.
• Alpha radiation is strongly ionising, so it removes electrons easily.

🧠 Exam technique

If a question says “ionised”, think electrons are lost. Then link that to the charge:

loses electron → positive ion

❌ Common errors

• Ticking “the atom gains a proton” or “gains a neutron”.
• Thinking the nucleus changes during ionisation.
• Forgetting that ions form because of electron transfer, not proton transfer.

✅ Correct answer

Beta radiation is only weakly ionising.

This matches the mark scheme exactly.

💡 Key knowledge

• A spark detector works because radiation ionises air near the wire mesh.
• The ionised air lets a spark form across the large potential difference.
• Beta particles do not ionise the air strongly enough to trigger the spark reliably.

🧠 Exam technique

To score the mark, give the direct reason: weakly ionising. You do not need a long explanation unless the question asks for one.

❌ Common errors

• Saying beta “is not ionising at all” — it is ionising, just weakly.
• Writing about penetration instead of ionisation.
• Giving vague answers like “beta is too small” without linking to ionisation.

✅ What a full-mark answer must include

• Use the paper sheet.
• Use the aluminium sheet.
• Also compare with a reading using no absorber.
• Explain how the results show alpha, beta or gamma radiation.
• Repeat readings and calculate a mean because radioactive decay is random.

This is what separates Level 3 responses from lower levels: clear sequence + correct interpretation of results.

💡 Key knowledge

• Alpha is stopped by paper.
• Beta passes through paper but is stopped by aluminium.
• Gamma is very penetrating and passes through paper and aluminium much more easily.
• Count rate is the number of decays detected in a given time.

🧠 Exam technique

• Write the method in the order it happens.
• Use phrases like “record the count rate” and “compare with the previous reading”.
• To reach the top level, explain what each result means.
• Use scientific words: absorbed, penetrate, count rate, mean, random.

❌ Common errors

• Only describing one absorber instead of using paper, aluminium and no sheet.
• Saying “alpha is not detected” without explaining that paper stops it.
• Forgetting to repeat readings or calculate a mean.
• Mixing up the order of shielding: paper first, then aluminium.
• Writing “beta goes through aluminium” — it does not.

Alpha

• Same as a helium nucleus
• Strongly ionising
• Stopped by paper

Beta

• Weakly ionising compared with alpha
• Passed by paper, stopped by aluminium
• Can be used in ionisation questions

Gamma

• Very penetrating
• Not stopped by paper
• Not easily stopped by aluminium