CAIE A-Level Physics 24.3.4 Annihilation Photons in PET

Practise explaining how positron-electron annihilation in a PET tracer produces two opposing gamma photons, converting particle mass into detectable gamma-ray energy.

Exam points

explain how positron-electron annihilation in PET converts particle mass into gamma-ray energy
explain why two gamma photons travel in opposite directions and are detected outside the body

[Maximum number: 2]

Fluorine-18 $\left({ }_{9}^{18} \mathrm{~F}\right)$ is a radioactive nuclide that is used as a tracer in positron emission tomography (PET scanning). Fluorine-18 decays to a nuclide of oxygen ( O ) according to

{ }_{9}^{18} \mathrm{~F} \longrightarrow{ }_{P}^{Q} \mathrm{X}+{ }_{8}^{R} \mathrm{O} \text {. }

Explain how the radioactive decay of fluorine-18 results in the emission from the body of the gamma-ray photons that are detected during a PET scan.

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24.3.4—That, in PET scanning, positrons emitted by the decay of the tracer question 1