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In a PET scan, what happens when an emitted positron encounters an electron in the body?
AThey annihilate into two 0.511 MeV gamma rays
BThey combine to form a stable, heavier particle
CThey pass through each other with no interaction
DThey both convert into visible light of random colors
Answer & Solution
Correct answer: A. They annihilate into two 0.511 MeV gamma rays
1. A positron and an electron are matter and antimatter counterparts of each other.
2. When they meet, mutual annihilation occurs, producing two gamma rays, each with an identical energy of 0.511 MeV.
3. These two gamma rays move directly away from each other, letting detectors on opposite sides pinpoint where the annihilation happened.
4. They do not combine into a heavier particle, pass through unaffected, or turn into visible light.
5. So mutual annihilation into two 0.511 MeV gamma rays is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.5 Medical Applications of Radioactivity: Diagnostic Imaging and Radiation_
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