The lines in this picture are trails of bubbles formed as charged particles force their way through an unstable mixture of neon and hydrogen. They curve in an applied magnetic field, and spiral because they lose energy quickly.
The highlighted tracks are antielectrons, each of which is part of an electron-positron () pair that has 'materialized’ from a high energy photon in the field of a nucleus. This is often referred to as 'pair-production’.
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For more examples of positrons, click on one of the pictures below.