Double background coincidences at two multidetector spectrometers, which have 6 and 32 NaI(Tl) detectors and registration geometry close to 4π (PRIPYAT-2M and ARGUS), are considered as a sum of true and random ones. They have been analyzed together with the energy resolution and efficiency of ¹³⁷Cs, ⁶⁵Zn and ⁴⁰K (photons with energy of 662 keV, 1116 keV and 1461 keV, respectively) detection in the full absorption peak (individual detectors and the whole spectrometers). The number of detector-duplet combinations registering double coincidences was 15 and 496, respectively (an angle from the spectrometer centers to the detecor centers ranged from 37.38° to ~180°). Double background coincidences in the whole energy range in dependence on the detector arrangement, as well as double coincidences caused by monoenergetic sources in the whole energy range, showed that main contributors to the background double coincidences at the spectrometers PRIPYAT and ARGUS are coinciding photons, which were scattered from detector to detector. In the 32-detector system, the minimum, maximum, arithmetic mean and standard deviation of the background double coincidences counting rates in the whole energy range were found to be 0.034 (detector pairs at 79.19°), 0.142 (37.38°), 0.066, 0.033 cps, respectively. The same values for the background double coincidences counting rates coming from monoenergetic sources were 0.974 (63.43°), 4.646 (41.81°), 3.0724, 1.167 cps, respectively (¹³⁷Cs), and 0.389 (63.43°), 18.706 (~180°), 2.794, 5.294 cps, respectively (⁶⁵Zn), while for the background double coincidences counting rates in the photo-peak regions – 0.003 (63.43°), 0.0114 (41.81°), 0.0074, 0.0029 cps, respectively (region 662 keV), and 0.0056 (63.43°), 0.0241 (37.38°), 0.0148, 0.006 cps, respectively (region 1116 keV). In the 6-detector system, average counting rates of the background double coincidences in the whole energy range were 0.539 cps (90°) and 0.544 cps (180°), those of the double coincidences from monoenergetic sources – 0.867 cps (90°) and 0.862 cps (180°) – ¹³⁷Cs, 1.993 cps (90°) and 1.986 cps (180°) – ⁴⁰K, and those of the background double coincidences in the photo-peak regions – 0.0825 cps (90°) and 0.0749 cps (180°) – region 662 keV, 0.0426 cps (90°) and 0.0428 cps (180°) – region 1461 keV.

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