Results from applying an advanced spatial-unfolding technique to outdoor-trial data acquired with a Silicon photomultiplier-based Compton Telescope for Safety and Security (SCoTSS) gamma imager during perimeter survey of a distributed La-140 source lying within a 500 m x 500 m exclusion zone are presented. A synthetic-data version of the experiment was also modelled using Monte Carlo simulations and reconstructed. For both experimental and synthetic data the method faithfully reproduces the shape of the activity distribution, and for synthetic data the total activity is reproduced as well. However, for the experimental data the method underestimates the total activity by a factor of six. This can be accounted for by environmental snow and soil conditions not included in the detector response functions. This application of the Compton gamma imager survey-data inversion method demonstrates its applicability under austere conditions wherein extreme weather and transportation constraints severely impacted the quality of the data collected. The trial shows that the method has widespread applicability in the radiological and nuclear safety and security field, particularly for scenarios in which a threat material or contaminated area lies within a no-entry or no-fly zone.

An advanced spatial-unfolding technique capable of reconstructing the activity distribution within an exclusion zone from Compton gamma imager measurements taken outside of it is introduced. Although the method is generally applicable to extended sources, we demonstrate it here on a calibrated Cs-137 point source through Monte Carlo simulation studies as well as with measurements made using a Silicon Compton Telescope for Safety and Security (SCoTSS) gamma imager. For synthetic data the method accurately reconstructs the total activity contained within the mapped zone of interest, even when the size of the basis elements used to reconstruct the activity distribution is larger than the source itself. For experimental data, the method reliably located the source but underestimated its activity by up to 17%. This is accurate enough for real-world security applications. The underestimation is likely due to effects not yet included in the simulated response of the detector. The method has widespread applicability in the radiological/nuclear safety and security field, particularly for scenarios in which a threat material or contaminated area lies within a no-entry or no-fly zone.

This work presents a feasibility study of utilizing Health Canada's terrestrial radiation monitoring network, the Fixed Point Surveil-lance (FPS) network, for space weather monitoring through demonstrating detections of Forbush decrease and ground level enhance-ment events. The network is currently comprised of more than eighty sodium iodide spectrometers distributed across Canada. It was designed for terrestrial radiation monitoring but is also capable of registering cosmic radiation in a high-energy channel. Data from four-teen FPS stations for the period from 2003 to 2018 were analyzed and compared with data obtained by other ground-level cosmic radi-ation monitoring systems. The level of atmospheric impacts on measurements can be well explained, and signatures of both long-term solar cycle variations and sporadic solar events have been detected in the FPS network. The Forbush decrease amplitudes in FPS were found to be comparable to those obtained in the global muon detector network but about 2-3 times lower than those recorded by the global neutron monitoring network. This study suggests that the 20 years of cosmic ray data from the FPS network can be used for cli-matological space weather studies. In addition, the network can be readily available for real-time space weather monitoring. Crown Copyright (c) 2023 Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Compton imagers determine the location of a gamma-emitting radioactive source by tracking the interactions of a gamma ray within a position-sensitive spectroscopic gamma detector. The Compton imaging technology is naturally capable of delineating multiple and extended sources as well as localizing point sources. With their graphic output of source location probability contours overlaid on a photograph, Compton imagers are finding application in astronomy, medical imaging, environmental remediation, nuclear nonproliferation and national security. Over the past several years this group has been developing the Silicon photomultiplier-based Compton Telescope for Safety and Security (SCoTSS) imager using a traditional two-plane design with a forward "scatter" plane and a rear "absorber’ plane [1]. At the 2019 IEEE NSS MIC conference we described adapted SCoTSS designs optimized for 4π imaging including cubic, spherical and cruciform geometries and discussed their performance determined from GEANT4 simulations [2]. In this submission we present the results of studies performed on the realized instruments in a laboratory setting, quantifying and comparing their point source response and their imaging uniformity over large fractions the 4π incident angle space. These results, using real data, permit verification of the performance expectations for the various design principles of the detectors.

It is a standard procedure in many countries that response to a nuclear or radiological accident or incident would involve mobile aerial- or ground-based survey with highly sensitive gamma-ray detectors to map the distribution of radioactivity. There may however arise situations in which ground- or air-based detectors are not able to access an area to survey for radioactive materials, therefore technologies and techniques that can estimate the position and activity of radioactive materials from a distance are under development. Tomographic reconstruction methods, well-known in medical physics, permit the reconstruction of an N-dimensional map or image, from a number of N-1-dimensional cross-sectional images, or back-projections. We are investigating a tomographic reconstruction method to reconstruct the radioactivity distribution within a restricted-access zone using measurements from a Compton gamma imager placed at several locations around the perimeter of the zone. In this work an extended source of La-140 with an activity of 35 GBq was deposited within a 500 m by 500 m zone that was surveyed from the perimeter at six locations using a Silicon photomultiplier-based Compton Telescope for Safety and Security (SCoTSS) gamma imager. The reconstructed Compton images from multiple viewpoints were then projected back into the zone to reconstruct the distribution of La-140 within it. This tomographic method reconstructed high intensity along the known location of the La-140 source, suggesting that the method is able to localize the radioactive material. A simple fit to measured counts using a point-source approximation of the source distribution yielded a strength estimate of (7 ± 2) GBq at time of deposition, a reasonable result given the presence of soil and snow attenuation. Our method provides an expedient estimate of the distribution of radioactivity using tomographic techniques. It may be used to inform decisions made on the scene in urgent situations where the distribution of radioactivity must be reconstructed from a distance.

The Silicon photomultiplier-based Compton Telescope for Safety and Security (SCoTSS) has been developed incorporating end-user requirements into the design process. The end-user group includes those responsible for mobile survey in the event of a radiological or nuclear accident, those responsible for radiation survey support to security operations at major events and at Canadian borders, as well as some of those responsible for Canadian defense applications. The SCoTSS development program has reached a technology readiness level of eight, and we are proceeding with field trials of the instrument in high-fidelity operational environments. Prospective end users have been involved in trial set up and execution, assuring applicability in their mission spaces. SCoTSS has been subject to trials involving hidden sources, heavily shielded sources, imager moving with respect to source, and complicated man-made surroundings. Our operators value high sensitivity for anomaly geolocation and mapping. End users also require an instrument which is capable of direction reconstruction in motion, as well as rapid imaging of a field of view. We have developed a "time to image" measure which allows for quantitative comparison of imagers of fundamentally different technology, where one design may have an advantage in terms of energy resolution and compactness and another design may have an advantage in terms of efficiency and cost effectiveness. We present here the performance of the SCoTSS imager in rapid direction finding. As well, we compare the time to image quantity for the SCoTSS imager and the H3D Polaris-H Quad imager where the data were taken under equivalent conditions. This quantitative measure of imaging performance can allow operators to make an informed choice of the design that meets their needs taking into consideration also weight and size as well as budgetary constraints.

Natural Resources Canada (NRCan) is responsible for the provision of aerial radiometric surveys in the event of a radiological or nuclear emergency in Canada. Manned aerial surveys are an essential element of the planned consequence management operation, as demonstrated by the recovery work following the 2011 Tohoku earthquake and tsunami, and their effects in Fukushima, Japan. Flying lower and slower than manned aircraft, an unmanned aerial vehicle (UAV) can provide improved spatial resolution. In particular, hot spot activity can be underestimated in manned survey results as the higher flight altitude and wider line spacing effectively average the hot spot over a larger area. Moreover, a UAV can enter an area which is hazardous for humans. NRCan has been investigating the inclusion of UAV-borne radiation survey spectrometers into its aerial survey response procedures. The Advanced Radiation Detector for UAV Operations (ARDUO) was developed to exploit the flight and lift capabilities available in the under 25kg class of UAVs. The detector features eight 2.8cm x 2.8cm x 5.6cm CsI(Tl) crystals in a self-shielding configuration, read out with silicon photomultipliers and digitized using miniaturized custom electronics. The ARDUO is flown on a main- and tail-rotor UAV called Responder which has a 6kg lift capacity and up to 40min. endurance. The performance of the ARDUO-Responder UAV system was characterized in both lab and outdoor trials. Outdoor trials consisted of aerial surveys over sealed point sources and over a distributed source. Results show how the directional response of the ARDUO can provide an indication in real time of source location for in-flight guidance. As well, the results show how use of the directional information in post-acquisition processing can result in improved spatial resolution of radiation features for both point and distributed sources.

Under the comprehensive nuclear-test-ban treaty a State Party may request an on-site inspection to establish whether a nuclear explosion has taken place. The on-site inspection may include high- and low-resolution gamma spectroscopy surveys as well as environmental sampling and analysis. Aerial radiometric survey in particular has been demonstrated to provide efficient coverage of large areas. We have developed the Advanced Radiation Detector for Unmanned Aerial Vehicle (UAV) Operations (ARDUO) and are investigating its possible contribution to aerial radiometric survey in on-site inspection. This gamma spectrometer is direction-capable; it can point out the direction toward a source of radioactivity while in flight. We have collected data with the ARDUO flown over one and two point radiation sources in a grid survey covering 5,000 m(2). With this data we demonstrate how directional techniques can be implemented in mobile aerial surveys to improve the spatial precision of the resulting radioactivity map. In particular, in an on-site inspection, an Inspected State Party may declare up to 50 km(2) of restricted-access sites, each of area up to 4 km(2). We show how perimeter survey outside of a restricted-access site of area 920 m(2) with the direction-capable ARDUO permits a reconstruction of the distribution of radioactivity within the no-fly zone.

In 2018, Defence Research and Development Canada, in partnership with Natural Resources Canada, led a field trial of survey and mapping of a large dispersion of radioactivity using Unmanned Aerial Vehicles (UAVs). The intent was to disperse La material in a 3,200 m L-polygon with an approximate activity level of 10 MBq m and to measure the radioactive material using sensors carried by UAVs. Due to the potential radiological hazard to personnel, the activity was approved only if Unmanned Ground Vehicles (UGVs) were able to completely handle and disperse the material remotely. One UGV was equipped with a traditional agricultural sprayer to disperse the material, and one UGV was equipped with a force feedback manipulator arm. Due to the freezing temperatures during dispersal, the 35 GBq of La was dispersed non-uniformly as one sprayer boom failed to perform as tested. However, rough analysis of the electronic dosimetry on the UGV concluded that 99% of the material was dispersed on the ground. The dosimeter placed closest to the robot manipulator arm, used for dispersal of material, indicated a contact dose of 33.5 mSv. The electronic dosimeter placed where the driver would have sat on the sprayer vehicle if it were not unmanned indicated a dose of 22.3 mSv. Thus, the use of UGVs for material dispersion substantially reduced the external exposure to personnel. The use of UGVs eliminated the potential of internal exposure as well. The Radiation Safety Officer received the highest dose at approximately 3 μSv, with the majority of the exposure coming from the handling of the Type A container.

The spectrum of cosmogenic neutrons at Earth's surface covers a wide energy range, from thermal to several GeV. The flux of secondary neutrons varies with latitude, elevation, solar activity, and nearby material, including ground moisture. We report the results from a campaign to measure count rates in neutron detectors responding to three different energy ranges conducted near the geomagnetic North Pole at CFS Alert, Nunavut, Canada (82.5°N, 62.5°W; vertical geomagnetic cutoff rigidity, RC = 0 GV) in June of 2016. In November 2016, we performed a follow-on measurement campaign in southern Canada at similar RC (1.5 GV) and elevations. We conducted these measurements, at varying elevation and ground moisture content, with unmoderated and moderated 3He detectors for thermal and epithermal-to-MeV sensitivity, and with EJ-299-33 pulse shape discrimination plastic scintillator detectors for fast neutrons. Background gamma rays were monitored with NaI(Tl) detectors. Using these data sets, we compared the measured count rates to a predictive model. This is the first ever data set taken from this location on Earth. We find that for the thermal and epithermal-to-MeV neutron measurements the predictive model and data are in good agreement, except at one location on rock-covered ground near 1 km elevation. The discrepancy at that location may be attributable to ground moisture variability. Other measurements, during this campaign and prior, support the assertion that ground moisture plays a critical role in determining neutron flux.

Mapping radioactive contamination using aerial survey measurements is an area under active investigation today. The radiometric aerial survey technique has been extensively applied following reactor accidents and also would provide a key tool for response to a malicious radiological or nuclear incident. Methods exist to calibrate the aerial survey system for quantification of the concentration of natural radionuclides, which can provide guidance. However, these methods have anticipated a spatial distribution of the source which is large in comparison to the survey altitude. In rapid emergency-response aerial surveys of areas of safety concern, deposits of relatively small spatial extent may be expected. The activity of such spatially restricted hot spots is underestimated using the traditional methods. We present here a spatial deconvolution method which can recover some of the variation smoothed out by the averaging due to survey at altitude. We show that the method can recover the true spatial distribution of concentration of a synthetic source. We then apply the method to real aerial survey data collected following detonation of a radiological dispersal device. The findings and implications of the deconvolution are then discussed by reference to a groundbased truckborne survey over the same contamination.

Compton gamma imagers are instruments which use the physics of Compton scattering to locate gamma emitters. This makes Compton gamma imagers useful tools for determining the distribution of radionuclides within an area which has restricted access. Our group has developed a Compton gamma imager which we call the silicon photomultiplier-based Compton telescope for safety and security (SCoTSS). Using a SCoTSS instrument, we made measurements of an extended source of La-140 that lay within an inaccessible area and implemented a tomographic back-projection algorithm to reconstruct the distribution. We demonstrate our method on simulated data of a point source of La-140 and two extended sources of La-140 for initial validation, and then apply our method to experimental measurements. Our method demonstrates good ability to localize the distribution of La-140 in all cases. Future improvements to our method will include the implementation of iterative methods and quantitative methods to estimate the reconstructed activity.

With funding from the Department of National Defence's (DND) Centre for Security Science, over the years from 2007 to 2012 a research team composed of scientists from Natural Resources Canada (NRCan), the National Research Council (NRC), and McGill University developed imagers to find radioactive sources and show their location overlaid on a photograph. These imagers were developed primarily for use in security/surveillance, and in consequence management. A follow-on DND-funded project called "Compton Imaging for Standoff Radiation Detection", governed by memoranda of understanding between DND and NRCan [1] and between DND and NRC [2], has been established in order to provide information useful in determining whether the Canadian Forces should procure Compton imagers. This is Report A specified in those agreements. We provide an introduction to Compton imaging, discuss the current technology readiness level of Compton imagers in Canada, and provide a status report of work under the project to date.

The SiPM-based Compton Telescope for Safety and Security (SCoTSS) has been developed with inorganic crystalline scintillator material for gamma detection. The instrument is sensitive enough to be used in a mobile survey mode, accumulating energy deposited in any crystal second-by-second and tagging these spectra with GPS position. The SCoTSS imager of course has the additional advantage of being able to produce an image of the radioactive objects in its field of view using events that satisfy a coincidence trigger between the scatter and absorber layers. The Advanced Radiation Detector for UAV Operations (ARDUO) on the other hand, is a non-imaging directional detector intended for use aboard a small unmanned aerial vehicle (UAV). The ARDUO detector features exactly the same volume of CsI(Tl) as is used in the absorber layer of a single SCoTSS module, giving it similar detection and alarming sensitivity, and mapmaking capability. However, in the ARDUO detector, the crystals are arranged closely together to optimize direction determination from self-shielding effects. Flown in a grid pattern with a UAV over an area of extended contamination, the ARDUO detector is also capable of making a map or image of that area. With its close-packed crystal arrangement, the ARDUO detector makes a poor Compton imager but does have some ability to produce a peripheral image in a fly-by. In this presentation we investigate the relative merits of Compton imaging versus mobile directional detection.

A series of experiments was conducted in 2012 at the Defence Research and Development Canada's Suffield Research Centre in Alberta, Canada, during which three radiological dispersal devices were detonated. The detonations released radioactive La-140 into the air, which was then carried by winds and detectable over distances of up to 2 km. The Nuclear Emergency Response group of Natural Resources Canada conducted airborne radiometric surveys shortly following the explosions to map the pattern of radioactivity deposited on the ground. The survey instrument suite was based on large volume NaI(Tl) scintillation gamma radiation detectors, which were situated in a basket mounted exterior to the helicopter and oriented end-to-end to maximize the sensitivity. A standard geophysical data treatment was used to subtract backgrounds and to correct the data to produce counts due to La-140 at the nominal altitude. Sensitivity conversion factors obtained from Monte Carlo simulations were then applied to express the measurements in terms of surface activity concentration in kBq m(-2). Integrated over the survey area, the results indicate that only 20 to 25% of the bomb's original inventory of radioactive material is deposited within a 1.5-km radius of ground zero. These results can be accommodated with a simple model for the RDD behavior and atmospheric dispersion.

We present the development of a mobile survey spectrometer and fieldable Compton gamma-ray imager. The detector employs CsI(Tl) scintillator coupled to SensL silicon photomultipliers (SiPMs) and incorporates a unique modular design, where individual units can be employed separately in applications requiring a compact detector, e.g. military operations, or combined together for those requiring a more sensitive detector, e.g. aerial surveying. Each module is a fully functional imager, providing both mapping and imaging capabilities along with isotope detection and identification. We describe the design of an imager module, its custom electronics readout, its integration into the Radiation Solutions Inc (RSI) RadAssist software, and its performance in the lab and field. The energy reconstruction is shown to perform well across the full spectrum of interest up to 3 MeV, with resolutions and noise levels suitable for low-energy measurement in both the scatter and absorber parts of the detector. Successful truck-borne field tests of the imager demonstrate that it is capable of localizing a shielded 10 mCi Cs-137 source at distances up to 40 m to within a few degrees in tens of seconds.

With uranium exploration shifting to greater depths and more indirect targets, new tools and better use of existing data are needed to maximize exploration efficiency. Recent and historical research has demonstrated that although deeply-buried beneath Athabasca Group sandstones, unconformity-related uranium deposits still may be targeted by understanding subtle surficial geochemical anomalies. Airborne gamma-ray spectrometric surveys can be used for effective surficial geochemical mapping of K, U and Th over large areas and have been conducted by the Geological Survey of Canada across much of the Canadian Shield over the past 50 years, notably in partnership with Saskatchewan over the Athabasca Basin. To apply the results of these surveys to uranium exploration, the effects of deposit-related geochemical anomalies on airborne gamma-ray measurements must be predicted. And, just as importantly, the background in terms of K, U and Th above which these anomalies have to be detected, has to be quantified and its genetic linkages understood. The "Eastern Athabasca Basin" airborne gamma-ray survey was conducted in partnership with the Saskatchewan Geological Survey in 2009. This regional survey was ground-truthed along the corridor between Key Lake and the McArthur River mine site in 2013. High-resolution helicopter-borne gamma-ray acquisition, ground gamma-ray spectrometry, surficial material mapping, sampling and laboratory analyses were performed. Results indicate that the relationships between subsurface processes, glacial dispersal and airborne gamma-ray measurements are very intricate and responsive to detailed local surficial geological processes that have modified elemental dispersion from bedrock sources. In many cases, surficial sediments and landforms can be discriminated based on their K, U and Th geochemistry, relating in turn, to their provenance. Quantitative analysis of the airborne data, integrated with surficial geological knowledge, can differentiate between the complex patchwork of background domain levels and deposit-related surficial geochemical anomalies.