Annihilation in PET Scanning (CIE A Level Physics)

• 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, and their mass becomes pure energy in the form of two gamma rays which move apart in opposite directions
• Annihilation doesn’t just happen with electrons and positrons, annihilation is defined as:

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

• As with all collisions, the mass, energy and momentum are conserved

Annihilation of a positron and electron to form two gamma-ray photons

• 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

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