Online program Overview Session: NSS-02

Abstract

In high-grade uranium mines such as the Orano Canada Inc. (OCI) operated McClean Lake Mine, gamma logging probes deal with a non-linear behavior due to both gamma self-absorption in uranium and electronics dead-time, which may lead to a significant underestimation of the resources. Up to now, quantitative uranium grade measurements were conducted through coring and subsequent laboratory analysis. However, the SABRE mining method involves larger diameter drilling, making systematic coring impractical due to the additional costs. Therefore, Orano Mining and CEA have developed and qualified a combined correction for electronics dead-time and gamma self- absorption. These phenomena are significant above a uranium grade of about 1% w/w U, leading to huge resource underestimations at high grade, for instance nearly a factor 3 at 50% w/w U, just for self-absorption. In this work, we detail the tests conducted at Orano La Hague irradiator to estimate the dead-time of different NaI(Tl) and Geiger Muller probes, the MCNP Monte Carlo calculations to assess uranium self- absorption on a large uranium grade range, a new method developed to jointly correct for these two phenomena, and its experimental qualification through the comparison of gamma logging with core sample analyses from high-grade boreholes.

Abstract

The 1–30 MeV gamma ray observation holds key astrophysical signatures—nucleosynthesis, cosmic-ray interactions, and possible dark-matter signatures—yet remains the least explored window in high-energy astronomy. The electron-tracking Compton camera (ETCC) tracks the recoil electron to constrain the photon direction to a single point on the sky, enabling bijective imaging. We are developing the ETCC for SMILE-3, planned for 2028, extending the observation band to 5 MeV via a double-hit analysis. Beyond this, reconstructing pair-production events inside the time projection chamber (TPC) offers a further extension above ∼ 10 MeV and enables cross-calibration between Compton and pair-production channels in the same detector—an important advantage for MeV observations, where background control is critical. Additionally, identifying pair events removes a source of misreconstruction in the double-hit analysis, where atmospheric gamma rays above 10 MeV undergoing pair production are misidentified as Compton events and leak through the zenith-angle cut. We developed a geometry-based algorithm to identify pair-production events from the 3D point-cloud of TPC tracks. Voxelization and DBSCAN separate track clusters from noise; local PCA estimates direction vectors, and intersecting pairs define vertex candidates evaluated by a log-likelihood ratio (LLR) test. Accepted events are reconstructed from the vector sum of the two track directions. We validated the algorithm using inverse-Compton-scattered gamma rays at 3.3, 4.8, and 6.6 MeV from the UVSOR BL1U beamline. At 6.6 MeV, the 68% containment angle was 16.2◦ for beam-synchronous events and 12.0◦ for asynchronous events, consistent with the nuclear-recoil kinematic limit. Monte Carlo simulations with the SMILE-3 geometry are underway to quantify identification efficiency and leakage improvement.

Abstract

Dual gamma and neutron imaging is promising owing to their complementary characteristics, and has been widely applied in nuclear material monitoring, defect analysis of aerospace materials, and cultural heritage authentication. A gamma and neutron imager with an active area of about 20 cm × 20 cm is currently under development. The imager employs a home-developed Ce³⁺-doped lithium glass scintillator coupled to 4096 silicon photomultipliers (SiPMs), each with a size of 3 mm × 3 mm. To achieve optimal imaging performance, each SiPM is individually read out using a newly custom-designed 64-channel application-specific integrated circuit (ASIC), designated CPRE64. A total of 64 CPRE64 chips are integrated into the system to read out the 4096 SiPM channels. This paper introduces the overall design of the two-dimensional (2D) neutron and gamma imager. A 2D neutron and gamma imager prototype, using a 25 mm × 25 mm × 5 mm scintillator coupled to an 8 × 8 SiPM array (3 mm × 3 mm pixel size), has been fabricated and characterized. The scintillator exhibits a light yield of approximately 4700 photon/MeV and an energy resolution of 14.5% for 662 keV gamma rays. Meanwhile, it provides a high light yield of roughly 7058 photon/neutron, which is approximately 1.18 times that of the reference GS20 glass. The baseline variation across CPRE64 channels was measured to be ± 10%, spanning from 1.3V to 1.7V, whereas the channel uniformity of the CPRE64 channels exceeded 90%. The prototype has been successfully used for gamma and neutron imaging, achieving an experimental neutron spatial resolution better than 1mm.

Abstract

Nuclear energy remains a pivotal low-carbon alternative for mitigating climate change, yet the management of radioactive waste presents significant technical and economic challenges. Optimizing waste characterization is essential to minimize disposal volumes and ensure safe decommissioning. This work evaluates a high-efficiency, portable Compton camera designed for the visualization and quantification of low- and intermediate-level waste (LILW).

The system was validated through three extensive field campaigns at the El Cabril disposal facility and the Santa María de Garoña nuclear power plant in Spain. These evaluations encompassed diverse waste containers and isotopic inventories to assess characterization performance in real industrial environments. Our findings demonstrate the robustness of Compton imaging, particularly regarding detection efficiency and reduced acquisition times. The system’s performance was enhanced by integrating computer-vision techniques with GPU-accelerated Maximum-Likelihood Expectation Maximization (MLEM) algorithms, enabling real-time radiological-visual fusion.

Furthermore, we demonstrate the feasibility of 3D tomographic reconstructions derived from multi-pose measurements. A significant highlight is the first field deployment of GNVision, a novel dual-particle imager for simultaneous neutron and gamma-ray mapping, aimed at identifying actinide-bearing residues. These results underscore the potential of advanced imaging to streamline waste classification and enhance safety in decommissioning workflows.

Abstract

Gamma-ray imaging is an emerging technology to identify individuals smuggling radioactive materials at international points of entry. However, imaging moving subjects requires high efficiency due to the limited data acquisition time as sources pass the imaging system. We developed a high-sensitivity gamma camera named the portable fast gamma imager (pFGI). The pFGI integrates an imaging module, an RGB-D camera, and other components into an enclosure. The imaging module combines Compton and coded aperture modalities using two position-sensitive detectors and a MURA coded aperture mask. Each detector comprises a monolithic NaI(Tl) scintillator (14.6×14.6×2.5 cm³) coupled to a 3×3 square-type photomultiplier tube array. The detectors are placed 16 cm apart, with the mask 6 cm in front of the front detector. To achieve superior imaging performance across a broad energy range, a hybrid imaging method was employed, which acquires coded aperture and Compton images, respectively, and then fuses them via pixel-wise multiplication. Furthermore, an AI-based motion-compensated method was used to integrate data along the moving subject’s trajectory. The YOLOv11 and BoT-SORT algorithms were applied to the RGB images acquired from the RGB-D camera to extract displacement vectors for each subject; then, the radiation image for each subject was translated and fused according to the trajectories. The pFGI achieved a 7° angular resolution and high sensitivity, imaging the location of a 137Cs source, producing an incremental dose rate of 0.1 µSv/h within 1 s. Additionally, in a dynamic multi-person scenario, the imager successfully and continuously imaged the moving source and identified the source carrier among multiple people. These results highlight the pFGI as a robust solution for national security.

Abstract

Radioactive materials released during the accident at the Fukushima Daiichi Nuclear Power Station were deposited not only on land surfaces but also in aquatic environments such as irrigation ponds. Monitoring radiation levels in these environments is important for environmental assessment and decontamination activities. In this study, a waterproof gamma-ray spectrometer based on a Ce-doped Gd₂Si₂O₇ (Ce:GPS) scintillator coupled with a photomultiplier tube was developed for underwater radiation measurements. The detector employed a 10 mm × 10 mm Ce:GPS scintillator and an FPGA-based data acquisition system for event-by-event signal processing and energy spectrum acquisition. To enable pond-bottom measurements, the detector was integrated into a pipe structure approximately 4 m in length. Field measurements were conducted in an irrigation pond in Fukushima Prefecture, where the detector was submerged and placed on the pond bottom. A clear 662 keV photopeak originating from cesium-137 was observed, and the measured energy resolution was 9.8% FWHM. Energy spectra were also measured at different heights above the pond bottom. The cesium-137 photopeak was clearly detected at the bottom and at 50 cm above the bottom, whereas scattered gamma-ray components became dominant at greater heights because of attenuation and scattering in water. The results demonstrate that the developed waterproof Ce:GPS spectrometer is effective for in-situ underwater gamma-ray measurements and radionuclide identification in aquatic environments contaminated by radioactive cesium.