The positron will have some initial energy after emission from the parent nucleus. It travels a short distance from the nucleus, scatters and collides with loosely bound electrons nearby before fusing with one of them to form positro-nium (which has a very short half-life, «10-7 s) and then annihilates. Their
Positron scattering from multiple electrons in tissue
Figure 2.1: Positron emission and annihilation. A positron is emitted from a proton-rich nucleus, losing energy by scattering from atomic electrons in tissue before annihilating with an electron to produce two 511 keV photons (or gamma rays) which are moving 180° (±0.25° FWHM) apart.
mass converts into energy in the form of two 511 keV photons, which are indistinguishable from gamma rays. To simultaneously conserve both momentum and energy, the photons are emitted 180° to each other. Figure 2.1 shows the positron annihilation and the emission of two 511 keV photons. The detection of these two 511 keV photons forms the basis of PET imaging.
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