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.

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.

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.

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.

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.

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.