Here is a high-energy background gamma spectrum I have captured for 10 hours using GAMMA-PRO 5″ system – Fig. 1. I had to lower detector bias to 600V to prevent the highest energy gamma events from clipping.

The spectrum extends all the way to 220 MeV as this enormous NaI(Tl) crystal was able to register gammas at those energies. There is a ‘shelf’ at about 60 MeV (which is consistent with measurements conducted using an HPGe system), and the majority of gammas fall under 3 MeV range. The magnified “low-energy” portion of the high energy background gamma spectrum is shown on Fig. 2 (consistent with ANAIS data, Fig. 3).

Fig. 2. Magnified “low-energy” portion of the high energy background gamma spectrum.

The peaks are at 150 keV, 380 keV, 650 keV, 800 keV, 960 keV, 1.15 MeV, 1.47 MeV, 1.78 MeV, 2.2 MeV, and 2.6 MeV. There is a decaying continuum past 2.8 MeV, which is thought to e caused by cosmic muons.