Positron electron annihilation reaction
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The positron electron annihilation reaction refers to the physical phenomena of a positron particle (the physical opposite of an electron), generated by a beta decay and slowed to "thermal speeds", and combines/reacts with an electron. The two particles completely disappear (annihilate) each other, leaving only the corresponding energy, released in a specific form; i.e. E=mc².
The energy released from the annihilation is released as two gamma rays photons, each radiating from the point of annihilation in exactly opposite directions, i.e. 180 degrees apart, (conservation of momentum, i.e net momentum = zero). The energy of each gamma ray is 511 KeV (511,000 eV).
This conversion of combined total matter and energy is in accordance with Einstein’s equation: E = mc2 (where E represents energy, m represents mass; and c represents the velocity of light and 2 means squared).
The reaction is the physical phenomena relied on as the basis for PET imaging.