Event detection

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The primary annihilation photon excites the BGO crystal which decays by releasing secondary, lower energy photons.
These go through the optical coupling and strike the photocathode(s) of the PMT(s).
This causes electrons to be ejected from the photocathodes via the photoelectric effect.
These are accelerated to secondary photocathodes by the large potential difference where they cause more electrons to be ejected (amplification). By the end of the chain, there are enough electrons present to measurable current pulse.