Background: As neutron fields are always accompanied by gamma rays, it is essential to distinguish neutrons from gamma rays in the detection of neutrons. Neutrons and gamma rays can be separated by pulse shape discrimination (PSD) methods. Recently, we performed characterization of a stilbene scintillator detector and an EJ-301 liquid scintillator detector with a highspeed digitizer DT5730 and investigated optimized PSD variables for both detectors. This study is for providing a basis for developing fast neutron/gamma-ray dual-particle imager. Materials and Methods: We conducted PSD experiments using stilbene scintillator and EJ-301 liquid scintillator and evaluated neutron and gamma ray discriminability of each PSD method with a 137 Cs gamma source and a 252 Cf neutron source. We implemented digital signal processing techniques to apply two PSD methods-the charge comparison (CC) method and the constant time discrimination (CTD) method-to distinguish neutrons from gamma rays. We tried to find optimized PSD variables giving the best discriminability in a given experimental condition. Results and Discussion: For the stilbene scintillator detector, the charge comparison method and the constant time discrimination method both delivered the PSD FOM values of 1.7. For the EJ-301 liquid scintillator detector, both PSD methods delivered the PSD FOM values of 1.79. With the same PSD variables, PSD performance was excellent in 300 ± 100 keVee, 500 ± 100 keVee, and 700 ± 100 keVee energy regions. This result shows that we can achieve an effective discrimination of neutrons from gamma rays using these scintillator detector systems. Conclusion: We applied both PSD methods to a stilbene and a liquid scintillator and optimized the PSD performance represented by FOM values. We observed a good separation performance of both scintillators combined with a high-speed digitizer and digital PSD. These results will provide reference values for the dual-particle imager we are developing, which can image both fast neutrons and gamma rays simultaneously.

A rotational modulation collimator (RMC) imager consists of two collimator masks rotating ahead of a non-position-sensitive detector. It requires fewer detectors than other radiation imagers, which offers the possibility of reducing the instrument complexity and cost. A hemispherical RMC (H-RMC) is capable of imaging radiation with a significantly better field of view (FOV) than conventional RMCs with cylindrical geometry. In this study, a novel hemispherical collimator design was developed in order to extend the FOV to nearly 2π. We designed the hemispherical mask considering several parameters, demonstrated the measured performance of the instrument, and showed the two-dimensional reconstructed images of radiation distribution. The H-RMC imager described herein featured a symmetric design which lead to degenerate reconstruction of two possible source locations for a single true source location. Nevertheless, it has been possible to establish that the maximum likelihood expectation maximization (MLEM) approach with regularization technique significantly reduced the artifact of misestimated source positions and improved the values of signal-to-noise ratio and the structural similarity index compared to the conventional MLEM method.

A dual-particle imager based on a rotational modulation collimator (RMC) utilizing a Cs ₂ LiYCl ₆ :Ce (CLYC) scintillator was developed and investigated for its dual-particle imaging performance. Design considerations and previous simulation work were verified by conducting experiments, and experimental results showing neutron and gamma-ray imaging capabilities were obtained. The imaging performance of the CLYC-based RMC system was further evaluated by investigating it in a mixed neutron and gamma-ray environment, and dual-particle imaging capability of the RMC system was demonstrated. Signal-to-noise ratio (SNR) values larger than 45 were achieved for reconstructed images of each neutron and gamma-ray source distribution, if modulation patterns were obtained with average fractional counting uncertainty of less than 3%. The structural similarity (SSIM) index of reconstructed images was calculated to be almost unity, indicating an artifact-free image.

The aim of this work is to develop a gamma-ray/neutron dual-particle imager, based on rotational modulation collimators (RMCs) and pulse shape discrimination (PSD)-capable scintillators, for possible applications for radioactivity monitoring as well as nuclear security and safeguards. A Monte Carlo simulation study was performed to design an RMC system for the dual-particle imaging, and modulation patterns were obtained for gamma-ray and neutron sources in various configurations. We applied an image reconstruction algorithm utilizing the maximum-likelihood expectation-maximization method based on the analytical modeling of source-detector configurations, to the Monte Carlo simulation results. Both gamma-ray and neutron source distributions were reconstructed and evaluated in terms of signal-to-noise ratio, showing the viability of developing an RMC-based gamma-ray/neutron dual-particle imager using PSD-capable scintillators.