Given today's revelation of the detection of terrestrial gammay-ray flashes (TGF) produced by thunderstorms and the associated pair production of an electron and a positron, how feasible is it to set up ground-based gamma ray detection station on the cheap using a Geiger-Muller tube?
The electron/positron pair travels along the magnetic field of the earth and bounces back (additional question: why do they bounce back?) This is how Fermi could detect the positrons: they interacted with the satellite which happened to be in the way. The loss of energy due the interaction slow down the positrons which interact with electrons and produce two 511keV photons flying in opposite directions (source.)
My question is: is it feasible at all to capture and detect positron annihilation as the pairs bounce back using a G-M tube? Given that there are 1500 thunderstorms a day I'm assuming each location on earth has a good chance of being a "bounce site" at least a few times a year.