Annihilation in PET Scanning (Cambridge (CIE) A Level Physics): Revision Note

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

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