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On the omnipresent background gamma radiation of the continuous spectrum

The background spectrum of a germanium detector, shielded from the radiations arriving from the lower and open for the radiations arriving from the upper hemisphere is studied, both in a ground level and in an underground laboratory. It is established that the continuous portion of this background spectrum is mostly due to the radiations that arrive from the upper hemisphere of the continuous spectrum similar to the instrumental one. The intensity of this radiation of an average energy of about 120 keV is estimated to about 8000 photons/m2s 2πsrad in a ground level laboratory, and to about 5000 photons/m2s 2πsrad at the depth of 25 m.w.e. Rough estimates of the dose that it contributes to the skin are about 1.5 nSv/h and 1 nSv/h respectively. Simulations by GEANT4 and CORSIKA demonstrate that this radiation is both of cosmic and terrestrial origin, mixed in proportion that still has to be determined.