## 2 Materials and Methods
### Unmanned Aerial Vehicle (UAV) and Scintillation Detectors
\begin{table}
\begin{tabular}{c c c c c} \hline
**Scintillator** & **Density** & **Energy Resolution 1** & **Lmax of Emission [nm]** & **Hygroscopicity** \\ \hline NaI(Tl) & 3.67 & 6.5–7.5\% & 415 & High \\ CsI(Tl) & 4.51 & \(\sim\)8\% & 540 & Low \\ CsI(Na) & 4.51 & \(\sim\)8\% & 420 & Medium \\ Bismuth Germanate (BGO) & 7.13 & \(\sim\)10\% & 480 & Low \\ LaBr3 & 5.08 & 2.5–4\% & 380 & High \\ \hline \end{tabular}  
* Typical values are given for crystals with a size of no more than 8 \(\times\) 8 cm. With a size increase, the resolution usually decreases.  
\end{table} 
Table 1: Some properties of various scintillation materials. Figure 1: Unmanned geophysical system SibGIS unmanned aerial system (UAS): (**a**) with a spectrometer 40 \(\times\) 80 mm (100 cm\({}^{3}\)); (**b**) with a radiometer 8 \(\times\) 8 \(\times\) 100 mm (6.5 cm\({}^{3}\)); (**c**) with a spectrometer 30 \(\times\) 150 mm (100 cm\({}^{3}\)). The ideal scintillation material has not yet been found, so researchers use different detectors, depending on the tasks carried out and the survey and data processing methods. In the previously mentioned UAV systems, BGO detectors were used [8], which had a high density but low energy resolution. Therefore this material is convenient for making compact and efficient detectors, but the spectra analysis possibilities in the low-energy range are quite limited; NaI [2; 5; 9; 13]; CsI [6; 12; 13]. In the case [6; 13], only a 33 cm\({}^{3}\) detector was used, but the spectrometric survey was performed only at individual points due to long-term measurements. Thus, the radiometric system can have significantly different characteristics, but it should allow implementing a specific survey and data processing method. When choosing materials for the production of detection units for UAV systems, the following factors are priorities: availability (price and physical), durability, an energy resolution of at least 8%, temperature stability, non-hygroscopicity, density higher than NaI(Tl). Even though, in theory, the energy resolution of CsI is several percent worse than that of NaI, resolution of the scintillation crystals used in these experiments was relatively high for low-cost equipment: 6.7% for Cs-137 for CsI(Na) 30 \(\times\) 150 mm detector and 7.5% for CsI(Tl) 40 \(\times\) 80 mm detector (Figure 2). These values are not unique--CsI crystals are easy to produce, so we can usually choose a good crystal with high resolution. It is important to note that the detectors used in this work have a mass inferior to 1 kg and are, thus, excessively light. Commercially available medium-sized production UAVs, such as the DJI S1000, can carry significantly larger detection units (thus more efficient). In fact, for real radiometric surveys, not one but two CsI(Tl) blocks of 8 \(\times\) 8 \(\times\) 100 mm [15] are used, and for spectrometric measurements--spectrometers with CsI(Tl) crystals with a 63 \(\times\) 63 mm [16] and 80 \(\times\) 80 mm [12] crystal volume. In such cases, the payload weight will be 2.2 kg and 3.5 kg, twice and four times the mass, respectively. This is acceptable for the SibGIS UAS unmanned system. However, in order to write this article, the experimental conditions were specifically complicated compared to real practice--in doing so, the results will, arguably, be even more interesting. Also, as shown below, even with small detectors, a geologically significant result can be achieved. Figure 2: Evaluation of the spectrometers energy resolution: (**a**) CsI(Na) 30 \(\times\) 150 mm; (**b**) CsI(Tl) 40 \(\times\) 80 mm.