## 2 Materials and Methods
### Unmanned Aerial Vehicle (UAV) and Scintillation Detectors  
The experimental surveys conducted include a comparative analysis of the results of UAV gamma surveys with small scintillation detectors on the ground and at several altitudes. Studies were carried out with serial and self-developed prototype equipment in radiometric and spectrometric modes. The SibGIS UAS unmanned system with a multi-rotor carrier (Figure 1) was used for the UAV gamma survey. This option provides geophysical surveys with an accurate terrain trapping. The UAV is a hexacopter with a 1-metre frame diagonal, and is equipped with a real time kinematics (RTK) system and a laser altimeter that supports a series of reflections. With a 28 Ah battery and an optimal 2-2.5 kg payload, this UAV can carry out a geophysical survey in medium-mountain terrain conditions, during not less than 40 min. This carrier is designed specifically for geophysical surveys in severe conditions of geological expeditions; its maximum payload is significantly higher than that of conventional serial UAVs such as DJI S1000 or Matrice 600; it is more durable and optimized to reduce electromagnetic interference to geophysical sensors. Maximum load capacity reaches 7 kg (the weight of the sensor for time-domain airborne electromagnetic survey (TDEM) figure in Section 4 of this article). The radiometric equipment used in this work included:  
1. For ground surveys, the most common Russian and the post-Soviet space scintillation radiometer SRP-68 were used. It was equipped with a 30 \(\times\) 25 mm (15 cm\({}^{3}\)) NaI(Tl) detector and a vacuum photomultiplier (PMT). The following equipment was used as payloads for the unmanned system:
2. A radiometer with a CsI(Tl) 8 \(\times\) 8 \(\times\) 100 mm (volume 6.5 cm\({}^{3}\)) crystal with a silicon photoelectron multiplier (SiPM). The measurement frequency was 0.5 Hz. The device's weight was less than 70 g (hereunder this sensor will be referred to as a 'lightweight' or'small radiometer'). 3. A spectrometer with a CsI(Na) 30 \(\times\) 150 mm crystal and a Hamamatsu 6095 photomultiplier, the detector's volume was about 100 cm\({}^{3}\). The measurement frequency in the integral channel was 0.5 Hz. The gamma spectrum was recorded over 1024 channels with a configurable frequency range of 0.1-1 Hz. The weight of the device is 1 kg. 4. A spectrometer with a CsI(Tl) crystal of 40 \(\times\) 80 mm and SiPM, with a detector volume also of about 100 cm\({}^{3}\). The measurement frequency in the integral channel was 0.5 Hz. The gamma spectrum was recorded over 1024 channels with a configurable frequency range of 0.1-1 Hz. The device's weight was 0.75 kg. Thus, the detectors used for spectrometric measurements have a volume of about 100 cm\({}^{3}\) (compared to those used in [2; 5; 7; 12] that are 400 and 1000 cm\({}^{3}\) in volume). The detector used for radiometric measurements in the air has a volume 2 times smaller than a serial radiometer for pedestrian survey. However, it important to note that evaluating the measuring system's efficiency and other characteristics only on the basis of the detector's volume is not correct. The material from which the detector is made also needs to be considered. Our detectors' scintillation material is CsI (thallium/sodium), while traditional devices for ground and aerogeophysical research, are usually equipped with detectors from NaI(Tl). Different scintillation materials have significantly different properties (Table 1): compared to NaI, CsI scintillators are less brittle, denser (which allows for greater efficiency), less hygroscopic, and have more temperature stability. CsI(Tl)'s main disadvantage for geophysical field measurements equipment is the different wavelength at maximum emission from NaI, which makes it difficult to select suitable PMTs. Figure 2 shows the resolution of the detectors used. They are noticeably higher.