Radiological and Nuclear Threat Detection Using Small Unmanned Aerial Systems

The aim of this research is to demonstrate the feasibility of remotely sensing nuclear and radiological threat materials by leveraging recent advances in radiation detectors, unmanned systems, and contextual sensors. The broad intent is to get detectors out of the hands of humans and onto semi-autonomous systems for a wide range of use cases. The search for special nuclear material is one specific mission area where radiation detectors employed on small unmanned aerial systems could provide significant operational value by exploiting the advantages that remote access enables: improved collection time, decreased source-to-detector distance, and reduced unintentional shielding. The goals of this study are fivefold: (1) assess current capabilities for directed search and substantiate the improvement that an unmanned approach would provide, (2) expand the understanding of the background radiation environment to include building rooftops, (3) establish system requirements and map out the parameter space of trade-offs (i.e., trade space) based on an analysis of current sensor and platform capabilities, (4) investigate and optimize search methods, and (5) identify and characterize additional mission areas for further investigation.To achieve these five goals, we started by identifying boundary conditions for signal collection time, source-to-detector distance (i.e., standoff), and intervening material attenuation for three different search modes: vehicle-mounted standoff detection, rotary-wing aerial detection, and small unmanned aerial system-based remote detection. The objective of this analysis was to calculate the theoretical reduction in detector area required to achieve the same minimum detectable activity of a Cs-137 source for a given detector material. We found that measuring from the rooftop with just 50 cm2 of detector area should detect smaller activity sources than 10,000 cm2 in a vehicle-borne approach or 5,000 cm2 in an aerial helicopter-borne approach.Our next objective was to characterize the background radiation environment sensed from the rooftops of light industrial buildings. We conducted a measurement campaign across fifteen buildings varying in geographic location, size, shape, height, wall construction, and roofing material. We discovered the variation in the background radiation ranged up to ±50% when analyzing contributions from seven prominent background peaks. Across a single building, this variation ranged 25–40% for contributions from potassium, uranium, and thorium. We also examined the attenuation of radiation by roofing materials both in simulation and experiment. We found that typical roof construction attenuates 1461 keV gamma-rays by approximately 50% when passing normal to the roof and continues to increase as the incident angle between the source and the detector increases. This observation directly influenced our approach to developing an optimal search scheme.With knowledge of the background and consideration of threat signatures, we then initiated an effort to develop a system architecture and design a sensor suite capable of detecting relevant threats in the anticipated environment. We employed established requirements analysis techniques to frame the development of a system that will provide tangible operational value to the user. We examined the trade space for platforms and sensors in terms of size, weight, power, cost, and visibility profile. Although our survey of capabilities is a snapshot in time, it lays the foundation for future analysis of alternatives. We recommend a platform that can move both through the air and on the ground and suggest further exploration of tube-launched systems for several military mission areas employing radiation sensors. For detectors, we recommend room temperature semi-conductors: cadmium zinc telluride for gamma-ray spectroscopy and lithium-backfilled etched-silicon diodes for neutron detection. Technologies such as real-time kinematic positioning, solid-state light depth and ranging, and thermal infrared cameras warrant further study as auxiliary contextual sensors in the system.Assuming an overmatched system is attainable, we then constructed a method to select advantageous measurement locations and developed techniques to optimize a search pattern. We devised a nonlinear programming routine and applied threshold cuts to reduce the time to converge to a near-optimal solution. We also explored several parameters that might be used as the objective quantity depending on the mission requirements and intelligence assessment.Finally, with the intent of removing humans from the task of operating detectors in elevated radiation areas, we sought to expand our inquiry to seven additional military mission areas. We briefly examined a historical vignette where unmanned radiation detection assets would have provided considerable value, summarized the general operational conditions, assessed the impact that remote detection might have on the speed, accuracy, fidelity, safety, or feasibility of a given mission, and identified unique challenges that might arise in developing a materiel solution. These additional areas are ripe for exploration and contribution from the broader community of researchers