Gamma-ray flux in the Fréjus underground laboratory measured with NaI detector

The γ-ray flux in the Fréjus underground laboratory has been studied using an NaI detector surrounded by different diagonostic shields. Below 4MeV, the spectrum is dominated by radioactivities in the surrounding materials and rocks. Between 4 and 6MeV, the shape of the spectrum is well explained by U, Th and daughters, which are internal contaminations in the NaI crystal. Between 6 and 10MeV, the γ-ray flux is strongly correlated with neutron captures in the surrounding materials. Finally, the γ-ray flux above 10MeV falls off and is related to the very weak cosmic muon flux via muon bremsstrahlung.