## 1 Introduction  
Gamma-ray survey methods have been widely used for a long time in geological and radioecological research. It is desirable to increase survey performance in both cases and avoid people on foot moving around the survey site as this can be harmful to specialists in radioecological cases. Work is often carried out on sites with poor pedestrian accessibility during geological surveys because all easily accessible sites already been explored. In this regard, unmanned aircraft systems for radioecological [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] and geological [11, 12, 13] cases have developed rapidly in recent years. Unmanned aerial vehicle (UAV) complexes are increasingly used in known man-made disasters areas [2, 3, 5, 6] and abandoned uranium fields [7] study. UAV gamma surveys have been used in Russian geological practice since 2017 to replace ground-based or aerogeophysical surveys at specific sites, even within the mineral resources study state programs [12]. Unmanned aircraft systems permit gamma-ray studies at lower altitudes, reducing risks for aircraft crew, and remain cost-effective even in small areas, starting from the first kilometres.  
UAV-radiometric surveys [1, 2, 5, 8, 9] are much more widespread than spectrometric ones [4, 7, 10, 11, 13, 14]. This is because light unmanned aerial carriers impose technical limitations that do not allow the effective use of classical methods of terrestrial or aerial gamma-ray spectrometry. For instance, ground-based gamma-ray surveys are performed with stops and long-term measurements. Modern helicopter aerogammaspectrometry systems use scintillation detectors with a volume of tens or hundreds of litres, which is considered necessary for high-quality spectrometric measurements in motion and at a significant altitude. A variant that reduces the payload mass may be radiation semiconductor detectors, which have a small mass and high energy resolution [1, 4, 7, 10]. Recently, such detectors have been used more often, but are still quite expensive and not used widely. Classical scintillation detectors continue to be used [2, 5, 6, 11, 12], in this case, for spectrometric measurements in motion. Unmanned systems carry detection units composed of several crystals, with a mass reaching 4 [11]-7.5 kg [2, 5]. Whether the detectors' size is justified by any parameters other than the maximum UAV payload capacity is unclear, but the mass of more than the first kilograms is quite significant in light of the UAV market's current evolution. Mass significantly affects flight time and range of the unmanned system, factors that are extremely important for geological applications. Geological surveys must be carried out on tens or hundreds of square kilometres. The problem of cost-effective surveys cannot be solved by increasing UAV-carriers' size since heavy UAV carriers are expensive, have high operation cost, and do not fall under Russia's simplified operating procedure.  
Empirical experience, gained carrying out UAV gamma surveys in geological mapping and prospecting [12, 15, 16], shows that it is possible to obtain sufficient high-quality data to solve real geological problems with significantly smaller scintillation detectors than in the examples [5, 8, 11]. The current paper's hypothesis is as follows: small in weight and cheap in production scintillation detectors allow reliable and practical gamma-ray spectrometry and radiometry results to be obtained, even if the area's gamma field is formed by small and not very contrasting anomalous objects. The technical result is achieved using the correct design parameters of the measuring equipment and the appropriate method of data collection and processing. Field experiments were carried in conditions more complicated than in actual geological practice to prove this hypothesis. The experimental work results, as well as the important design features of the unmanned system and the data collection and processing methods necessary to achieve the declared technical result, are the subject of this paper.