Emerging Technologies and Methods in Wide-Area Search for Nuclear Materials

This paper outlines Canada's Nuclear Emergency Response team's integration of emerging technologies, including uncrewed systems, advanced detectors, and high-performance computing, to enhance wide-area nuclear material search capabilities and improve the accuracy of source localization and error propagation.

The Gist

Imagine you are a firefighter trying to find a hidden fire in a massive forest. In the past, your team had a very sensitive smoke detector, but it was "direction-blind." It could tell you, "There is smoke somewhere nearby," but it couldn't tell you where the smoke was coming from. If you flew a helicopter over the forest, the detector would pick up smoke from a fire far away, making it look like the fire was right under the helicopter. This is the problem Canada's nuclear emergency team faces with traditional radiation detectors.

This paper explains how the team at Natural Resources Canada (NRCan) is upgrading their "smoke detectors" with new technology to find radioactive sources faster, more accurately, and from safer distances.

Here is a breakdown of their new tools and methods:

1. The Old Way: The "Fuzzy Photo"

Traditionally, the team flies helicopters with large, heavy detectors (like giant ears listening for radiation).

• The Problem: Because the helicopter is high up, the detector hears radiation from a huge area on the ground. It's like taking a photo of a crowd from a plane; you see a blur of people, but you can't tell exactly who is standing where. If there is a "hot spot" of radiation, the traditional method blurs it out, making it look weaker and wider than it really is.
• The Fix: They used powerful supercomputers to run simulations. Think of this as using a computer program to "un-blur" the photo. By mathematically reversing the smearing effect, they can sharpen the image and see that a wide, weak signal is actually a small, very intense fire.

2. The New Eye: The "Directional Camera" (SCoTSS)

The team developed a new device called SCoTSS. Instead of just listening for radiation, this device acts like a camera that can see the direction the radiation is coming from.

• How it works: It uses a special type of sensor (Silicon Photomultipliers) to track how radiation bounces inside the machine. It's like a billiard table where you can trace the path of a ball backward to see exactly where it hit the table.
• The Result: They tested this by driving a truck around a restricted area (like a fence line) while a radioactive source was hidden inside. Even though the truck couldn't go inside the fence, the "camera" could look over the fence and create a map of where the source was. It's like standing outside a dark room and being able to point exactly at a glowing lightbulb inside without opening the door.

3. The Drone Pilot: The "Smart Drone" (ARDUO)

Sometimes, sending a human in a helicopter is too dangerous or impossible (like in a "no-fly zone"). The team built a special detector for drones called ARDUO.

• The Challenge: Drones have small batteries and can't fly for long. They need to get the most information possible in a single, short trip.
• The Innovation: This drone detector is "direction-capable." As the drone flies back and forth, it doesn't just count radiation; it constantly calculates a vector (an arrow) pointing toward the source.
• The Magic Trick: The paper describes a new math method to solve a puzzle. If the drone flies a straight line, the arrows might point in confusing directions because there are two different sources. The new method uses a computer to look at all the arrows at once and figure out the best possible location for the sources that explains every single arrow.
  • The Analogy: Imagine you are walking down a street and a compass needle spins wildly. If you only look at the needle for one second, you might think the magnet is in front of you. But if you record the needle's direction for the whole walk, a computer can figure out that there are actually two magnets: one right under your feet and one hidden in a house across the street.

4. Knowing What You Don't See

A crucial part of this new system is knowing where it is safe to go.

• The Uncertainty Map: When the computer guesses where a source is, it also calculates how sure it is. It creates a "confidence map."
• Why it matters: If the computer says, "There is a 95% chance the radiation is here, but there is a small chance it could be 10 meters away," the ground crew knows to be careful in that 10-meter zone. This prevents them from walking into a "false clear" area where they might think it's safe but actually isn't.

Summary

The paper argues that by combining direction-sensing hardware (like the SCoTSS camera and ARDUO drone) with super-fast computer math, Canada can:

1. See through the "blur" of high-altitude surveys.
2. Map radioactive sources from the perimeter of a dangerous zone without entering it.
3. Pinpoint hidden sources using a single, short drone flight.
4. Give ground crews a clear map of where it is truly safe to walk.

The goal is to keep nuclear security tight and ensure that when an emergency happens, responders have the sharpest possible "eyes" to find the danger quickly and safely.

Technical Summary

Technical Summary: Emerging Technologies and Methods in Wide-Area Search for Nuclear Materials

Problem Statement
Traditional wide-area nuclear search operations, primarily conducted by crewed aircraft using large-volume NaI(Tl) detectors, face significant operational limitations. While these systems are robust and sensitive, they are "direction-blind," meaning they cannot distinguish the specific origin of radiation within their field of view. This limitation prevents the extrapolation of measurements from a survey line into unsurveyed areas, potentially leading to false assessments of source strength or location. Furthermore, operational constraints such as no-fly zones, restricted access sites, and limited flight endurance (particularly for uncrewed systems) often force operators to rely on single sortie data or road-constrained ground surveys. In these scenarios, the inability to determine direction can result in misinterpreting a distant strong source as a local weak one, or failing to locate sources entirely when flight paths cannot cover the entire area of interest.

Methodology
The paper outlines a progression from traditional survey methods to advanced, direction-capable systems supported by high-performance computing (HPC).

1. Traditional Aerial Survey and Spatial Deconvolution: The authors first review standard NaI(Tl) aerial surveys, citing a 2012 Radiological Dispersal Device (RDD) trial. They describe a method to correct for the "smearing" effect inherent in altitude-based measurements. Using HPC, researchers modeled the transport of radiation from ground pixels to the aircraft to perform a spatial inversion (deconvolution). This minimization process determines the necessary and sufficient ground contamination distribution to account for the measured altitude data, recovering spatial details lost due to the survey height.

2. Gamma Imaging with SCoTSS: To address the inability to extrapolate off survey lines, the authors introduce the Silicon Photomultiplier-based Compton Telescope for Safety and Security (SCoTSS). This instrument utilizes CsI(Tl) crystals and Silicon Photomultipliers (SiPMs) to track Compton scattering events, reconstructing the angle of incoming gamma rays. The methodology involves collecting data from multiple vantage points around a restricted zone (perimeter survey) and summing the individual Compton images to perform tomographic reconstruction, thereby localizing the source within the zone without entering it.

3. Uncrewed Aerial Systems (ARDUO) and Extrapolation: The paper details the development of the Advanced Radiation Detector for Uncrewed Aerial Vehicle Operations (ARDUO), a compact, direction-capable spectrometer mounted on a remotely piloted aerial system (RPAS). ARDUO uses eight CsI(Tl) crystals read out by SiPMs in a self-shielding configuration to calculate source direction (azimuth and elevation) once per second.

   • Extrapolation Algorithm: The core methodological contribution is a regression technique applied to single-sortie data. The system simulates the response of the detector array to a grid of 4m × 4m ground pixels. A minimization algorithm with hundreds of free parameters simultaneously fits the measured gross counts and directional vectors to determine the optimal placement of radioactive sources. This allows for the extrapolation of source locations to areas not directly flown over.

Key Contributions and Results

• Spatial Deconvolution: Applied to RDD trial data, the deconvolution method successfully narrowed the estimated contaminated area from ~100 m (as observed at 15 m altitude) to 30–40 m on the ground. It also corrected the underestimation of peak concentration, revealing a maximum of ~400 kBq/m² compared to the 180 kBq/m² measured directly.
• Tomographic Reconstruction: Using the SCoTSS imager, the team successfully localized a distributed La-140 source within a restricted access zone by summing images from six perimeter viewpoints. The reconstructed image aligned closely with the actual dispersion pattern, demonstrating the utility of perimeter surveys for evidentiary information in restricted zones.
• ARDUO Directional Extrapolation: In a single sortie test with two Cs-137 sources, the ARDUO system's raw directional arrows were ambiguous regarding a source located off the flight path. However, the spatial inversion method successfully resolved the locations of both sources: one directly under the flight line and a second located approximately 30 meters away from the trajectory.
• Uncertainty Propagation: The methodology includes the propagation of measurement error to the extrapolated region. Results showed that while the source under the flight line was precisely located, the off-line source had higher positional uncertainty. Crucially, the negative uncertainty analysis confirmed that the signal was statistically distinct from zero background, while positive uncertainty maps highlighted regions where ground crews should exercise caution.

Significance and Claims
The paper claims that these emerging technologies, driven by advances in semiconductor physics (SiPMs) and high-performance computing, significantly enhance nuclear security and emergency response capabilities.

• Operational Flexibility: Direction-capable detectors allow for effective source localization even when flight paths are constrained or when only a single sortie is feasible.
• Enhanced Situational Awareness: The ability to extrapolate source locations and propagate error margins provides operators with critical information to distinguish between clear and contaminated regions, reducing the risk of false negatives or unnecessary exposure.
• Global Security Application: The authors assert that these methods are particularly promising for on-site inspections under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) and other nuclear security operations where access is restricted. The work aims to strengthen global nuclear security by sharing these technical advancements in wide-area search.

The paper maintains a modest tone regarding absolute quantification; it notes that current models do not yet account for all dead material or perfect point-source geometries, meaning results are currently presented in relative units. However, the primary claim is the successful demonstration of location determination and uncertainty propagation, which are essential for guiding ground crews and making informed operational decisions.