Positron Annihilation in Bananas

If positrons are emitted during decay, why don't they interact with electrons and annihilate? And if they do, why don't we see flashes of light coming out from bananas when potassium turns to potassium 40 and emits positrons?
- Andrew (age 28)
Lansdale PA

The positrons do pretty quickly annihilate with electrons. However, the energy released is ~500keV per particle, so the resulting gamma rays are far from the visible range.
The annihilation process and its output is described here:
http://en.wikipedia.org/wiki/Positron_emission_tomography.

Mike W.

(published on 01/10/2012)