| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Physics |
| Lesson Topic: explain that, in PET scanning, positrons emitted by the decay of the tracer annihilate when they interact with electrons in the tissue, producing a pair of gamma-ray photons travelling in opposite directions |
Learning Objective/s:
- Describe the annihilation process of a positron with an electron and the resulting gamma photons.
- Explain why the photons travel in opposite directions and how this is used in PET imaging.
- Identify the sequence of events from tracer injection to image reconstruction in a PET scan.
- Interpret the significance of the 511 keV photon energy and the 180° angle for coincidence detection.
- Apply basic calculations of photon energy to relate it to the electron rest mass.
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Materials Needed:
- Projector or interactive whiteboard
- PowerPoint slides with diagrams of annihilation and detector ring
- Printed handout summarizing PET sequence and key quantities
- Clicker or polling tool for quick checks
- Sample tracer decay diagram cards
- Optional: simulation software demonstration (e.g., PhET)
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Introduction:
Begin with a striking image of a PET scanner and ask students how we can see inside the body without opening it. Recall their prior study of radioactive decay and photon interactions, linking that knowledge to positron emission. Explain that today they will understand how annihilation produces paired gamma photons that enable precise imaging, and they will be able to describe each step of the PET process.
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Lesson Structure:
- Do‑now (5'): quick quiz on β⁺ decay and electron‑positron annihilation.
- Mini‑lecture (10'): present the PET sequence with slides, emphasizing the annihilation equation and 511 keV photons.
- Guided activity (12'): students work in pairs using handouts to label the six‑step PET process and calculate photon energy from mₑc².
- Demonstration (8'): run a simulation of coincidence detection and discuss why opposite directions matter.
- Check for understanding (5'): clicker questions on key concepts.
- Consolidation (5'): class creates a flow‑chart on the board summarizing the entire scan.
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Conclusion:
Summarise that PET imaging relies on positron‑electron annihilation producing two 511 keV photons travelling in opposite directions, which the detector ring records simultaneously. Ask each student to write one key takeaway on an exit ticket and assign a short homework to research another medical imaging technique that uses different radiation.
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