AQA GCSE Physics Physics Paper 1 (Higher), June 2023: Question 5

✅ Mark Scheme Model Answer

• Similarity 1: Both have the same number of protons (6 protons) [1 mark]
• Similarity 2: Both have the same number of electrons (6 electrons) [1 mark]
• Difference: Carbon-14 has 8 neutrons whereas carbon-12 has 6 neutrons (or different numbers of neutrons) [1 mark]

🧠 Exam Technique: "Compare" Questions

When asked to "compare", you must provide both similarities and differences:

• State the three subatomic particles: protons, neutrons, and electrons.
• Give specific numbers if known (mass number 14 − 6 = 8 neutrons; mass number 12 − 6 = 6 neutrons).

✅ Mark Scheme Model Answer

Any one of the following:

• The time it takes for the number of nuclei (in a sample) to halve is 5700 years. [1 mark]
• The time it takes for the count rate (or activity) to halve is 5700 years. [1 mark]

❌ Common Errors

• Saying "the time it takes for radioactivity to halve" — the mark scheme explicitly ignores "radioactivity". You must use activity, count rate, or number of nuclei.
• Saying "half the time for an atom to decay" — individual decay events are completely random.

✅ Mark Scheme Model Answer

• Isotope chosen: Nitrogen-18 [1 mark]
• Reason: It has the highest activity / emits the most radiation per second / decays fastest [1 mark]
• Consequence: Therefore delivers the greatest dose of radiation absorbed (per second) [1 mark]

🧠 Examiner Insight (AO3 Logic Chain)

Marks 2 and 3 depend on choosing Nitrogen-18. Avoid circular reasoning:

• Merely stating "it has the shortest half-life" does not gain Mark 2 because that is already given in the table!
• You must explain the physical consequence: a shorter half-life means rapid decay, which creates a higher activity/count rate and thus delivers a concentrated dose in a short time.

💡 Key Knowledge

• Irradiation: An object is exposed to radiation from an external source. The object itself does not become radioactive.
• Contamination: Radioactive particles/atoms get onto skin, clothing, or inside the body. The object continues to emit radiation.

✅ Mark Scheme Breakdown

• Irradiation: Exposure of an object/person to radiation. [1 mark]
• Contamination: The unwanted presence of radioactive material/atoms on or inside an object/person. [1 mark]

✅ Acceptable Answers (Any One)

• Cancer / tumour formation [1 mark]
• DNA mutation / damage to genes [1 mark]
• Cell death / tissue damage [1 mark]
• Radiation sickness / radiation burns [1 mark]

❌ Rejected Answers

• "Mutates cells" (incorrect phrasing — radiation mutates DNA, not cells).
• "Death" is too vague and gets 0 marks.

✅ Mark Scheme Model Answer

1. Some radioactive materials emit alpha radiation [1 mark]
2. Alpha radiation has a very short range in air (only a few cm) / is weakly penetrating [1 mark]

🧠 Why 1 cm matters

Alpha particles (helium nuclei) collide heavily with air molecules, stopping after approximately 3 to 5 cm. If the worker stood 10 cm away, any alpha particles would be absorbed by the air before reaching the detector window.

⚠️ Calculation Traps

• Mixing time units: The pilot's rate is given hourly, while the nuclear worker's dose is given daily. Always convert both to matching time periods before dividing!
• Significant figures: Both inputs are 2 significant figures (0.0030 and 0.00050). The answer 144 is an exact division: 72 ÷ 0.5 = 144.

💡 Alternative Method

Compare the rates directly:

• Hourly dose ÷ Daily dose = 0.0030 ÷ 0.00050 = 6
• Number of days = 6 × 24 = 144 days