Anderson discovered the positron in a bubble chamber, where the positron causes gas to condense during its flyby. However, shouldn't this interaction between the positron and the (non anti-matter) atoms in the bubble chamber cause the positron to annihilate and emit a gamma ray burst before it could ever leave a (nicely curbed) track?
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1 Answer
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To annihilate, the positron needs to combine with an electron to make positronium. But since it's a charged particle, it is perfectly capable of ionizing the medium in the bubble chamber (and thus cause nucleation of bubbles) without annihilating with every electron it meets.
To form positronium, the positron and electron cannot have very high relative velocity. Think of the positron as "running into a room full of electrons", bumping into them. Until it slows down enough (thermalizes) that it can get to know an electron, it can't ask it for a (annihilation) date.
When you look at annihilation, there is a mean free path between the position of emission of the positron and the point of annihilation. This is a factor that limits the spatial resolution of PET scanners for certain isotopes that have high mean energy of emission (e.g. O-15, with 1.7 MeV end point energy and an RMS range of 2.75 mm).
