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Annihilation in PET Scanning (CIE A Level Physics)

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The process of annihilation

  • When a positron is emitted from a tracer in the body, it travels less than a millimetre before it collides with an electron
  • The positron and the electron will annihilate, which means
    • their mass is converted into energy
    • two gamma rays are produced and move apart in opposite directions
  • Annihilation doesn’t just happen with electrons and positrons, it can occur between any particle and its antimatter counterpart
  • Annihilation is defined as:

When a particle meets its equivalent antiparticle they are both destroyed and their mass is converted into energy

  • As with any collision, mass, energy and momentum are always conserved

Annihilation between an electron and positron

The Process of Annihilation, downloadable AS & A Level Physics revision notes

When a positron and electron annihilate, their mass is converted into two gamma-ray photons

Positron emission tomography (PET) scanning

  • Once the tracer is introduced to the body it has a short half-life, so, it begins emitting positrons (β+) immediately
    • This allows for a short exposure time to the radiation
    • A short half-life does mean the patient needs to be scanned quickly and not all hospitals have access to expensive PET scanners
  • In PET scanning:
    • Positrons are emitted by the decay of the tracer
    • They travel a small distance and annihilate when they interact with electrons in the tissue
    • This annihilation produces a pair of gamma-ray photons which travel in opposite directions

PET scan

PET Scanning Machine (1), downloadable AS & A Level Physics revision notesPET Scanning Machine (2), downloadable AS & A Level Physics revision notes

Annihilation of a positron and an electron is the basis of PET Scanning

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.