Yeah, that’s going to be too hard. I only have two SiPMs (besides the current detector) and they are expensive. I figured I could maybe rely on the gamma from the annihilation energy being a quite different energy than the gammas from the more common electron-capture.
However you raise a good point that that would not be a very good demonstration of positron annihilation at all – just evidence that it’s not the other 2 decay modes (and it would take ages to collect that evidence besides). Ah well. Got plenty of other science I can do instead.
Probably I’ll tackle something easier like checking for radon decay products in petrol.