Positron emission tomography (PET) relies on the decay of
PHYSICS · Multi-part · A Level · Physics · Particles and radiation · Particles and radiation
Problem
Positron emission tomography (PET) relies on the decay of 18F, which emits a positron. After slowing to thermal energies in a patient’s tissue, the positron annihilates with an electron, producing two γ photons of energy 511 keV that travel in nearly opposite directions. A PET facility encloses its detector electronics inside a 5 mm thick lead casing and uses no additional local shielding for the patient area.
(1) Describe the sequence of particle events from the instant the nucleus of 18F decays to the emergence of the two γ photons, identifying the conservation laws satisfied at each stage.
(2) Compare qualitatively the penetration abilities of the emitted positron and the annihilation γ photons in biological tissue, citing the relevant particle properties responsible for the difference.
(3) Evaluate whether the 5 mm lead casing is adequate to protect personnel from (i) direct exposure to positrons and (ii) exposure to the annihilation γ photons. Support your reasoning without performing numerical calculations.
(4) The patient’s body itself provides some self-shielding. Discuss how the density and composition of human tissue influence the detection efficiency of the 511 keV γ photons by the PET scanner.