Autonomous localization of multiple ionizing radiation sources using miniature single-layer Compton cameras onboard a group of micro aerial vehicles

TL;DR

The paper tackles rapid, autonomous localization of multiple ionizing radiation sources in open environments using a swarm of lightweight MAVs equipped with miniature single-layer Compton detectors. It introduces an online MLEM-like radiation-mapping framework that uses a simplified system matrix and a memory-efficient sensitivity model to reconstruct source intensities over a discretized map, while leveraging an active, centralized search strategy to guide MAV trajectories for information gain. The approach is validated through both realistic simulations and real-world Cs-137 experiments, demonstrating the ability to localize multiple sources online and to improve localization quality as measurements accumulate, albeit with limitations for low-activity sources. This work enables scalable, real-time hazardous-radiation mapping with agile aerial robots by combining direction-based Compton measurements, onboard processing, and cooperative planning, with potential for integration with intensity-based methods for enhanced performance.

Abstract

A novel method for autonomous localization of multiple sources of gamma radiation using a group of Micro Aerial Vehicles (MAVs) is presented in this paper. The method utilizes an extremely lightweight (44 g) Compton camera MiniPIX TPX3. The compact size of the detector allows for deployment onboard safe and agile small-scale Unmanned Aerial Vehicles (UAVs). The proposed radiation mapping approach fuses measurements from multiple distributed Compton camera sensors to accurately estimate the positions of multiple radioactive sources in real time. Unlike commonly used intensity-based detectors, the Compton camera reconstructs the set of possible directions towards a radiation source from just a single ionizing particle. Therefore, the proposed approach can localize radiation sources without having to estimate the gradient of a radiation field or contour lines, which require longer measurements. The instant estimation is able to fully exploit the potential of highly mobile MAVs. The radiation mapping method is combined with an active search strategy, which coordinates the future actions of the MAVs in order to improve the quality of the estimate of the sources' positions, as well as to explore the area of interest faster. The proposed solution is evaluated in simulation and real world experiments with multiple Cesium-137 radiation sources.

Figures (10)

• Figure 1: Estimate of the radioactive hotspots (yellow circles) generated from the Compton measurements (green cones) by the proposed method in a real-world experiment (top). The estimation method builds on a small and lightweight Compton camera (bottom left) that can be deployed onboard sub-1 kg MAV with onboard processing (bottom right).
• Figure 2: Differences between the classical two-layer Compton camera architecture and the single-layer Minipix Timepix3.
• Figure 3: Compton camera produces Compton cones $\tilde{\mathbf{c}}_{i} = (\mathbf{\hat{s}}_{i}, \mathbf{a}_{i}, \beta_{i})$ parameterized by the cones' origin, axis, and the Compton angle, respectively. As the MAV flies through the environment, the viewpoints $\mathbf{\hat{v}}$ are sampled. The area of possible source positions $\mathcal{M}$ is discretized into $J$ cells, where each cell is represented by its center position $\mathbf{m}_{j}$.
• Figure 4: Monte Carlo simulation showing the probability of Compton detections at various sensor orientations (i.e., relative positions of the source) with respect to the detector's geometry.
• Figure 5: The search strategy pipeline. The future waypoints for the MAV are generated based on $\mathbf{s}$ (exploration --- green) and $\bm{\lambda}$ (exploitation --- orange). The waypoints are assigned to individual MAV by dividing them into clusters based on their spatial position (purple circles). The ordering of waypoints is determined using the sequencing method (TSP). Finally, the pipeline outputs non-colliding paths for every UAV.