That technique doesn't work on Earth; the atmosphere's too thick to pass gamma rays (in either direction). An atmospheric column has the mass of a ~10 meter col

That technique doesn't work on Earth; the atmosphere's too thick to pass gamma rays (in either direction). An atmospheric column has the mass of a ~10 meter column of water. It's pretty effective on other planetoids, though: https://en.wikipedia.org/wiki/Lunar_Prospector?useskin=vector#Gamma_Ray_Spectrometer_(GRS) https://en.wikipedia.org/wiki/Lunar_Prospector?useskin=vecto...