Online program Overview Session: NSS-05

Abstract

Neutron imaging arrays are critical tools in nuclear non-proliferation, with applications in neutron source localization in wide-area source searches and cargo inspections. However, they commonly have large form factors due to needing large data acquisition systems, complicating deployment. A solution with simplified data acquisition is needed to make portable and field-deployable neutron imaging arrays. Organic scintillator-based neutron imaging arrays typically require pulse shape discrimination (PSD) to differentiate between neutron and gamma-ray events. This method requires full waveform data acquisition, which results in a high data throughput, and utilizes expensive analog-to-digital converters. In order to alleviate the need for full waveform acquisition, this work seeks to develop a system that instead leverages time-of-flight particle discrimination. To discriminate neutrons from gamma events using time-of-flight particle discrimination, a Segmented Organic Neutron Imager Cell, SONIC, has been constructed of 64 5x0.6x0.6 cm3 bars of EJ200. 14 of the 16 tested bars show typical pulse-integral spectra for Cs-137. This work details the introductory characterization of this imager array and evaluates its validity for future work in achieving a field-deployable, handheld neutron scatter imager.

Abstract

In areas where radiation sources are diffused three-dimensionally, it is important to develop a compact imaging sensor that can detect radiation distributions in the 4π direction. A radiation imager that detects the distribution of radiation sources with a fractal Sierpinski tetrahedron shape was developed before. The imager has 16 sensors arranged in a Sierpinski tetrahedron shape, with the space between the sensors filled with a shielding material. We have been developing a neutron imager to measure radiation distribution within the site of the Fukushima Daiichi Nuclear Power Plant (FDNPP) and in the environment contaminated by the FDNPP accident. The reactor buildings contain large amounts of radiation sources. Since the nuclear fuel and debris contained in the reactor pressure vessel (RPV) will be removed in the future, it is extremely important to identify neutron sources originating from nuclear fuel in high dose rate areas for criticality management and radiation protection of workers. Therefore, we developed a compact omnidirectional neutron imager to locate neutron sources in the reactor building. This sensor consists of thin triangular scintillators (⁶LiF/ZnS:Ag) for neutron detection attached to a tetrahedral acrylic surface and arranged in a Sierpinski tetrahedron shape, with B₄C mixed with resin for neutron shielding filled between the sensors. In this presentation, we report the results of performance evaluations of the imager using a neutron source and research reactor.

Abstract

This work presents a He-3 based slab detector for neutron spectroscopy that leverages spatially resolved detector response to infer incident neutron energy. In moderated detector systems, neutron energy information is typically degraded prior to detection, complicating spectrum unfolding. The proposed detector addresses this limitation through channel-by-channel list-mode acquisition, enabling analysis of spatial response variations correlated with neutron energy. Additional moderating and absorbing configurations are investigated to improve spectral sensitivity. A model of the detector is developed using Monte Carlo simulation and benchmarked against experimental measurements with monoenergetic neutron sources. This model will be used to generate energy-dependent response functions for use in spectral unfolding. Results indicate that combining spatial response information with multiple detector configurations will improve the conditioning of the neutron spectrum unfolding problem and enhance reconstruction accuracy. This approach provides a pathway toward information-optimized neutron spectrometry for nuclear security and safeguards applications.

Abstract

We report residual 14 MeV neutron emission between pulses of a Thermo Fisher Scientific P 211 DT neutron generator operated at 100 Hz with a 0.1% duty cycle. Using a liquid organic scintillator with pulse-shape discrimination, we detect a significant population of neutron events with light output consistent with DT neutrons. The interpulse time structure follows a two-component exponential decay with time constants of 0.8 ms and 4.7 ms, inconsistent with ¹⁷N β-delayed neutron emission. The interpulse yield represents 0.1% of the total emission (~10^-6 of instantaneous pulse intensity). These results demonstrate that the assumption of zero interpulse emission can be violated even when both the ion source and the high voltage are nominally pulsed, with direct implications for low-SNR delayed-signature measurements.

Abstract

The European Spallation Source (ESS) requires advanced neutron-beam monitors capable of stable operation under high-rate conditions while maintaining low beam attenuation. To address these requirements, a Micromegas-based Neutron Flux Monitor (NFM) has been developed by the ESS Detector Group in collaboration with DEDIP (IRFU) and CEA Saclay. The detector employs a novel bulk Micromegas implementation directly on AlMg3 alloy substrates, minimizing the use of hydrogen-rich materials and therefore reducing neutron attenuation and moderation effects along the beamline. Initial detector characterization using X-ray sources demonstrated good gain uniformity, stable high-gain operation, and good energy resolution. Subsequent neutron measurements performed with different gas mixtures and front-end electronics confirmed stable operation under representative ESS-like conditions. Measurements performed at the EMMA beamline at ISIS investigated detector performance at high neutron rates and provided important input for the optimisation of the final NFM readout architecture for future ESS deployment.

Abstract

Fast neutron spectroscopy has the potential to offer critical information for safeguards, nonproliferation, and field radiation detection applications where neutron energy distributions provide source characteristics inaccessible through conventional moderated neutron counters. Traditional portable neutron detection systems primarily rely on moderation and thermal neutron counting, which remove incident neutron energy information and limit spectroscopic capability. In this work, the nSpec system is developed as a handheld fast neutron spectrometer utilizing four novel deuterated polystyrene (dPS) scintillator detectors coupled to photomultiplier tubes in a compact, field-deployable architecture. By leveraging neutron--deuteron elastic scattering interactions, the system preserves energy-dependent neutron response while maintaining portability suitable for operational environments. Pulse shape discrimination is employed to separate neutron and gamma interactions, while detector response matrices (DRMs) derived from high-flux neutron characterization are used to reconstruct incident neutron spectra through maximum likelihood expectation maximization (MLEM) unfolding.  Prototype evaluations using representative neutron sources, including AmBe and Cf-252, demonstrate successful spectral unfolding and source discrimination capability. Ongoing development focuses on onboard electronics integration, firmware implementation, shielding optimization, and validation against simulated and experimental DRMs.  These results establish nSpec as a promising pathway toward practical handheld fast neutron spectroscopy for safeguards and field applications.

Abstract

Neutron production cross-sections are critical for dose assessment in particle therapy and space radiation protection, yet double-differential data remain scarce. We present a consistent framework for Time-Of-Flight (TOF)-based measurement built around the use of Low Gain Avalanche Detectors (LGADs) segmented into strips on the primary beam, enabling large statistical samples and precise time measurement, and a secondary neutron detector split into layers to accurately reconstruct neutron production as a function of kinetic energy and angular direction. A preliminary beam test was conducted to validate the methodology. The setup comprises an 8-strip beam monitor for primary particle tagging, a LaBr₃(Ce) scintillator with Silicon Photomultiplier (SiPM) readout as secondary radiation detector, and a plastic veto for charged-particle rejection. Neutron kinetic energy is reconstructed via relativistic TOF analysis from approximately 16 MeV to several hundred MeV. A novel Beer-Lambert fitting approach enables efficiency-independent neutron flux estimation from the attenuation pattern along the detection depth, overcoming systematic uncertainties of conventional scaling methods. Monte Carlo (MC) simulations validate both TOF distributions and flux reconstruction at the few-percent level, with computed cross-sections in good agreement with MC truth values. Combined with a rotary-translation stage enabling systematic angular scanning for full double-differential cross-section mapping, the proposed approach provides a robust and scalable framework for neutron production measurements in fragmentation events, with direct application to out-of-field dose characterization in particle therapy and astronaut radiation exposure assessment.

Abstract

Novel Photon-to-Digital Converters (3D-PDCs) are arrays of Single Photon Avalanche Diodes (SPADs) vertically-integrated with low power CMOS digital readouts, offering high timing precision ($<200$ ps FWHM) and individual-pixel control.  As single-photon counters, 3D-PDCs can be coupled to scintillators for radiation detection.  This talk will discuss the first 3D-PDC measurements of fast neutrons.  The 3D-PDCs are used to read out an organic plastic scintillator with Pulse Shape Discrimination (PSD) capabilities, making incident neutrons and gamma rays discernible by the pulse shape of the induced scintillator signal.  First PSD results will be shown and compared to standard commercial silicon photomultipliers.