Development and Flight Trial of a UAV-based Gamma Ray and Neutron Detection System for Large-Area Radioactivity Mapping and Source Activity Estimation

TL;DR

This work develops and validates a UAV-mounted SiPM-based NaIL scintillation payload capable of dual gamma and neutron detection for large-area radioactivity mapping. It introduces an analytic radionuclide detection efficiency framework that accounts for branching ratios, detector solid angle, air attenuation, and intrinsic efficiency, and demonstrates ground-level activity estimation via a non-negative least squares approach using Dirac-delta bases. Indoor flight trials with Cs-137 and Co-60 show localisation within 0.5 m and activity estimates with errors around 10% or less, illustrating a practical pathway to autonomous aerial radiological surveying. The combination of a compact dual-mode detector, a physics-informed estimation method, and raster-based localisation supports rapid, low-exposure monitoring with potential applications in security, decontamination, and NORM detection.

Abstract

Advances in scintillation crystal and Silicon PhotoMultiplier (SiPM) technologies have enabled the development of compact, lightweight, and low-power radiation detectors that are suitable for integration with Unmanned Aerial Vehicles (UAVs). This integration enables efficient and cost-effective large-area radiation monitoring while minimising occupational exposure. In this work, a SiPM-based NaIL scintillation detection payload was developed, characterised, and mounted on a multirotor UAV for gamma ray and neutron source localisation and activity estimation applications. To support these capabilities, an analytic radionuclide detection efficiency model was developed and used to estimate radioactivity on the ground from aerial energy spectrum measurements. The analytic expression for the detection efficiency incorporated physical phenomena, including the branching ratio, detector solid angle, air attenuation, and intrinsic peak efficiency, leading to agreement within 10% of experimental radionuclide detection efficiencies. The UAV-based radiation detection system was physically validated through a controlled indoor live radioactive source demonstration at 1.5 m, 3 m, and 4.5 m flight heights. Using the developed ground-level radioactivity estimation method, Cs-137 and Co-60 sources were successfully localised within 0.5 m, and their activities were estimated with errors on the order of 10% or less.

Figures (9)

• Figure 1: SiPM-based NaIL scintillation detection payload integrated with the MR4 quadrotor UAV from Bask Aerospace.
• Figure 2: NaIL scintillation detector measurement process. A new thread is created in step (1) and an initialisation message is sent to the MultiChannel Analyzer (MCA) in preparation for a new measurement. After a 1 s delay, allowing for the SiPM operating voltage to ramp up and stabilise, radiation is measured from step (2) to step (3) over the detector run time $\tau_r$. The detector data is retrieved by the thread after time delay $\Delta t_p$ in step (4), and saved by the main thread after $\Delta t_m$ in step (5). Step (1) is repeated after 1 s for the platform operating in the circular ArduPilot flight mode or until the system reaches the target waypoint in guided mode.
• Figure 3: Experimental $^{133}$Ba (a), $^{152}$Eu (b), $^{22}$Na (c), $^{137}$Cs (d), $^{60}$Co (e), and $^{252}$Cf (f) energy spectra measured with the NaIL scintillation detector.
• Figure 4: Photopeak Full Width Half Maximum (FWHM) derived from the energy spectra in Figure \ref{['fig:nail_spectra']}. The points correspond to the FWHM for gamma ray energies between 81 keV ($^{133}$Ba) and 1408 keV ($^{152}$Eu), whereas the x-mark at 3200 keV is the full energy equivalent photopeak FWHM from $^6$Li neutron capture in the NaIL scintillator. The power function (dotted-line) fitted to the gamma ray photopeak FWHMs takes the form of FWHM = $(95 \pm 8) \cdot \gamma$-energy$^{(-0.40 \pm 0.01)}$.
• Figure 5: Total count (step plot) and lateral distance from the radioactive source (dashed line) along the circular flight path at 2 m flight height. The detector measured radiation over 1 s (a)-(b), 5 s (c)-(d), and 10 s (e)-(f) windows for flight speeds of 10 deg/s (left) and 20 deg/s (right) around a 3 m diameter circle.