What you’ll learn6 learning objectivesChoose one objective for a focused lesson, or study the complete topic.24.3.1A tracer is a substance containing radioactive nuclei that can be introduced• understand that a tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studiedSyllabus objective24.3.2That a tracer that decays by β+ decay is used in positron emission• recall that a tracer that decays by β+ decay is used in positron emission tomography (PET scanning)Syllabus objective24.3.3Annihilation occurs when a particle interacts with its antiparticle and that• understand that annihilation occurs when a particle interacts with its antiparticle and that mass–energy and momentum are conserved in the processSyllabus objective24.3.4That, in PET scanning, positrons emitted by the decay of the tracer• explain that, in PET scanning, positrons emitted by the decay of the tracer annihilate when they interact with electrons in the tissue, producing a pair of gamma-ray photons travelling in opposite directionsSyllabus objective24.3.5The energy of the gamma-ray photons emitted during the annihilation of an• calculate the energy of the gamma-ray photons emitted during the annihilation of an electron-positron pairSyllabus objective24.3.6The gamma-ray photons from an annihilation event travel outside the body and• understand that the gamma-ray photons from an annihilation event travel outside the body and can be detected, and an image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photonsSyllabus objective