[en] The intrinsic radioactivity of gamma scintillation units SU has been investigated and screening conditions provided by different shielding chambers are compared. The SU consists mainly of a NaI(Tl) gamma detector with the diameter of 76 mm and a photomultiplier enclosed in a copper case. The energy resolution of the SU equals (7.0+-0.1)% for the ¹³⁷Cs photopeak. The investigations were carried out in a low-back-ground surface and underground cells with one-or multi-layered shields. Major specifications of the NaI(Tl) detector are presented as well as measured SU background gamma spectra in the range from 0.3 to 7.0 MeV and the background parameters of the SU major components. The background has been established to be mainly due to α particles being the indicators of the NaI(Tl) crystal contamination with U-Ra and Th nuclides. The contributions of the SU structural materials can be neglected when compared to the contributions of the photomultiplier and the NaI(Tl) crystal. The measurements have reaffirmed that there is no sense in strengthening the bulky shields of the underground cells when no special measures are taken to purify the air from emanations (sup(220,222)Rn)