Optimizing gamma-ray spectrometers for UAV-borne surveys with geophysical applications

Area of Science:

• Geoscience
• Environmental Science
• Nuclear Geophysics

Background:

• Heavy-duty unmanned aerial vehicles (UAVs) enable deployment of gamma-ray spectrometers for mapping natural radionuclides (⁴⁰K, ²³⁸U, ²³²Th).
• UAV platforms offer high spatial resolution over difficult terrain, but mapping low-activity natural radionuclides requires optimized detection systems.
• Determining the minimum practical detector size is crucial for effective UAV-borne radionuclide surveys.

Purpose of the Study:

• To investigate the minimum practical detector size for UAV-borne gamma-ray spectrometry of natural radionuclides.
• To compare different scintillator detector volumes (2000 ml, 1000 ml, 350 ml) for mapping ⁴⁰K, ²³⁸U, and ²³²Th concentrations.
• To evaluate uncertainty estimation methods (Full Spectrum Analysis vs. variogram) in UAV surveys.

Main Methods:

• An agricultural field was mapped using a UAV equipped with three different sized gamma-ray spectrometers (2000 ml, 1000 ml, 350 ml) at 20 m altitude and 5.6 m/s speed.
• Radionuclide concentrations were extracted using Full Spectrum Analysis (FSA).
• Uncertainties were calculated directly from FSA output and via variogram analysis, comparing results to ground-based measurements.

Main Results:

• All three detectors successfully characterized the spatial distribution of ²³²Th, which correlated with topsoil sand and clay fractions.
• Variogram analysis provided lower uncertainty estimates compared to direct FSA output, especially for smaller detectors, by incorporating spatial variation.
• UAV measurements at 20 m altitude showed a tendency to shift towards the mean concentration, being less sensitive to extreme values than ground-based measurements.

Conclusions:

• Smaller gamma-ray detectors can be effectively used in UAV surveys for mapping natural radionuclides, provided appropriate analysis methods are employed.
• Variogram analysis is recommended for estimating uncertainties in UAV-borne gamma-ray surveys to better capture spatial variability.
• Survey parameters like detector volume, height, and speed can be optimized for acceptable accuracy in UAV-based radioactive pollution studies.