I'm not sure what they're trying to show with their gamma ray spectra. To me, they all look like a potassium-40 peak (at 1460 keV), with a bunch of Compton scat

I'm not sure what they're trying to show with their gamma ray spectra. To me, they all look like a potassium-40 peak (at 1460 keV), with a bunch of Compton scattering at lower energies.