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A-Level Physics October/November 2024 Q9(b)(i): Describe how the interaction of a β⁺ particle with an electron in the body enables the…
A-Level Physics · Paper 9702/41 · October/November 2024 · Question 9(b)(i) · [3 marks]
Describe how the interaction of a β⁺ particle with an electron in the body enables the formation of an image.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
When a β⁺ particle (a positron) encounters an electron within the body, they undergo pair annihilation. [1]
As a result of this interaction, the mass of both particles is converted into energy in the form of two identical gamma-ray photons. To conserve momentum, these two photons travel in opposite directions away from the point of annihilation. [2]
A ring of detectors surrounding the patient detects these photons. By measuring the difference in the arrival times of the pair of photons at different points on the detector ring, the location of the annihilation event can be precisely calculated. A computer then processes the locations of many such events to build up a 3D image. [3]
How the marks are awarded
- B1 — Correctly identifying the fundamental interaction as '(pair) annihilation'.
- B1 — Stating that two gamma photons are produced and that they travel in opposite directions.
- B1 — Explaining that the difference in arrival times of the photons at the detector is processed to determine the location of the source and form the image.
Common mistakes
- Stating that only one gamma photon is produced, or that the photons do not travel in opposite directions.
- Using vague language like 'the particles collide and release energy' instead of the specific term 'annihilation'.
- Failing to link the detection of photons to the formation of an image, for example by omitting any mention of timing differences.
- Confusing β⁺ (positron) with β⁻ (electron) and describing an incorrect interaction.
Examiner tip: For questions describing physical processes, structure your answer as a logical sequence of events, using precise scientific terminology for each stage.
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