• Nuclear Engineering and Technology
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• v.50 no.8
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• pp.1426-1432
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• 2018

This paper presents a neutron detector configuration using EJ-301 scintillation liquid, a R9420 photo-multiplier and a homemade preamplifier. The detector qualities which include the energy linearity, efficiency response and neutron/gamma discrimination are guaranteed for neutron detection in the energy range from 0 to 3000 keVee. Regarding the neutron/gamma discrimination capability, four pulse shape discrimination (PSD) methods which are the threshold crossing time (TCT), pulse gradient analysis (PGA), charge comparison (CC) and correlation pattern recognition (CPR), were evaluated and discussed; among of these, the CPR method provides the best neutron/gamma discrimination.

• Nuclear Engineering and Technology
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• v.55 no.11
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• pp.4057-4065
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• 2023

Organic scintillators are widely used for neutron/gamma detection. Pulse shape discrimination algorithms have been commonly used to discriminate the detected radiations. These algorithms have several limits, in particular with plastic scintillator which has lower discrimination ability, compared to liquid scintillator. Recently, machine learning (ML) models have been explored to enhance discrimination performance. Nevertheless, obtaining an accurate ML model or evaluating any discrimination approach requires a reference neutron dataset. The preparation of this is challenging because neutron sources are also gamma-ray emitters. Therefore, this paper proposes a pipeline to prepare clean labeled neutron/gamma datasets acquired by an organic scintillator. The method is mainly based on a Time of Flight setup and Tail-to-Total integral ratio (TTT_ratio) discrimination algorithm. In the presented case, EJ276 plastic scintillator and ²⁵²Cf source were used to implement the acquisition chain. The results showed that this process can identify and remove mislabeled samples in the entire ToF spectrum, including those that contribute to peak values. Furthermore, the process cleans ToF dataset from pile-up events, which can significantly impact experimental results and the conclusions extracted from them.

• Nuclear Engineering and Technology
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• v.51 no.1
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• pp.263-268
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• 2019

The $^3He$ proportional chamber is widely used for neutron measurement owing to its high neutron detection efficiency and simplicity for gamma-ray rejection. In general, the neutron and gamma-ray signals obtained from the $^3He$ proportional chamber can be easily separated by the difference in the pulse heights. However, for a high gamma-ray field, the gamma-ray signal cannot be precisely eliminated by the pulse height due to gamma-ray pulse pileup which causes the pulse height of gamma-ray pulse to increase and making the pulses due to neutrons and gamma rays indistinguishable. In this study, an improved algorithm for $n/{\gamma}$ discrimination using a parameter, which is the ratio of the rise time to the pulse height, is proposed. The $n/{\gamma}$ discrimination performance of the algorithm is evaluated by applying it to $^{252}Cf$ neutron signal separation from various gamma-ray exposure rate levels ranging 0.1-5 R/h. The performance is compared to that of the conventional pulse-height analysis method in terms of the gamma elimination ratio. The suggested algorithm shows better performance than the conventional one by 1.7% (at 0.1 R/h) to 70% (at 5 R/h) for gamma elimination.

• Nuclear Engineering and Technology
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• v.55 no.5
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• pp.1677-1686
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• 2023

As the demand for the detection of special nuclear materials (SNMs) increases, the use of imaging instruments that can sensitively image both gamma-ray and neutron signatures has become necessary. This study compared the pulse shape discrimination (PSD) performance of gamma/neutron events when employing either a pixelated stilbene or a plastic (EJ-276) scintillator array coupled to a silicon photomultiplier (SiPM) array in a dual-particle imager. The stilbene array allowed a lower energy threshold above which neutron and gamma-ray events can be clearly distinguished. A greater number of events can, therefore, be used when forming both gamma-ray and neutron images, which shortens the time required to acquire the images by nearly seven times.

• Journal of Radiation Protection and Research
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• v.44 no.2
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• pp.53-63
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• 2019

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 high-speed 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 $300{\pm}100keVee$, $500{\pm}100keVee$, and $700{\pm}100keVee$ 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.

• Nuclear Engineering and Technology
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• v.55 no.11
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• pp.3925-3934
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• 2023

The importance of fast neutron detection for nuclear safeguards purposes has increased due to its potential advantages such as reasonable cost and higher precision for larger sample masses of nuclear materials. Pulse-shape discrimination (PSD) is inevitably used to discriminate neutron- and gamma-ray- induced signals from organic scintillators of very high gamma sensitivity. The light output (LO) threshold corresponding to several MeV of recoiled proton energy could be necessary to achieve fine PSD performance. However, this leads to neutron count losses and possible distortion of results obtained by neutron multiplicity counting (NMC)-based nuclear material accountancy (NMA). Moreover, conventional PSD techniques are not effective for counting of neutrons in a high-gamma-ray environment, even under a sufficiently high LO threshold. In the present work, PSD performance (figure-of-merit, FOM) according to LO bands was confirmed using a conventional charge comparison method (CCM) and compared with results obtained by convolution neural network (CNN)-based PSD algorithms. Also, it was attempted, for the first time ever, to reject fake neutron signals from distorted PSD regions where neutron-induced signals are normally detected. The overall results indicated that higher neutron detection efficiency with better accuracy could be achieved via CNN-based PSD algorithms.

• Nuclear Engineering and Technology
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• v.53 no.11
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• pp.3784-3789
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• 2021

We report a simple technique for direct neutron spectroscopy using pure LaCl₃ crystals. Pure LaCl₃ crystals exhibit considerably better pulse shape discrimination (PSD) capabilities with relatively good energy resolution as compared with Ce-doped LaCl₃ crystals. Single crystals of pure and Ce-doped LaCl₃ were grown using an inhouse-developed Bridgman furnace. PSD capabilities of these crystals were investigated using ²⁴¹Am and ¹³⁷Cs sources. Fast neutron detection was tested using a²⁵²Cf source and three separate bands corresponding to electron, proton, and alpha were observed. The proton band induced by the ³⁵Cl(n,p)³⁵S reaction can be used for direct neutron spectroscopy because proton energy is proportional to incident neutron energy. Owing to good scintillation performance and excellent PSD capabilities, pure LaCl₃ is a promising candidate for space detectors and other applications that necessitate gamma/fast neutron discrimination capability.

• Journal of Radiation Protection and Research
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• v.42 no.1
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• pp.48-55
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• 2017

Background: Simultaneous detection of neutrons and gamma rays have become much more practicable, by taking advantage of good gamma-ray discrimination properties using pulse shape discrimination (PSD) technique. Recently, we introduced a commercial CLYC system in Korea, and performed an initial characterization and simulation studies for the CLYC detector system to provide references for the future implementation of the dual-mode scintillator system in various studies and applications. Materials and Methods: We evaluated a CLYC detector with 95% $^6Li$ enrichment using various gamma-ray sources and a $^{252}Cf$ neutron source, with validation of our Monte Carlo simulation results via measurement experiments. Absolute full-energy peak efficiency values were calculated for gamma-ray sources and neutron source using MCNP6 and compared with measurement experiments of the calibration sources. In addition, behavioral characteristics of neutrons were validated by comparing simulations and experiments on neutron moderation with various polyethylene (PE) moderator thicknesses. Results and Discussion: Both results showed good agreements in overall characteristics of the gamma and neutron detection efficiencies, with consistent ~20% discrepancy. Furthermore, moderation of neutrons emitted from $^{252}Cf$ showed similarities between the simulation and the experiment, in terms of their relative ratios depending on the thickness of the PE moderator. Conclusion: A CLYC detector system was characterized for its energy resolution and detection efficiency, and Monte Carlo simulations on the detector system was validated experimentally. Validation of the simulation results in overall trend of the CLYC detector behavior will provide the fundamental basis and validity of follow-up Monte Carlo simulation studies for the development of our dual-particle imager using a rotational modulation collimator.

• Nuclear Engineering and Technology
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• v.56 no.2
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• pp.745-752
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• 2024

A method for gamma/neutron event classification based on frequency-domain analysis for mixed radiation environments is proposed. In contrast to the traditional charge comparison method for pulse-shape discrimination, which requires baseline removal and pulse alignment, our method does not need any preprocessing of the digitized data, apart from removing saturated traces in sporadic pile-up scenarios. It also features the identification of neutron events in the detector's full energy range with a single device, from thermal neutrons to fast neutrons, including low-energy pulses, and still provides a superior figure-of-merit for classification. The proposed frequency-domain analysis consists of computing the fast Fourier transform of a triggered trace and integrating it through a simplified version of the transform magnitude components that distinguish the neutron features from those of the gamma photons. Owing to this simplification, the proposed method may be easily ported to a real-time embedded deployment based on Field-Programmable Gate Arrays or Digital Signal Processors. We target an off-the-shelf detector based on a small CLYC (Cs2LiYCl6:Ce) crystal coupled to a silicon photomultiplier with an integrated bias and preamplifier, aiming at lightweight embedded mixed radiation monitors and dosimeter applications.

• Nuclear Engineering and Technology
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• v.55 no.2
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• pp.648-654
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• 2023

A dual-particle imager (DPI) is configured in a hand-held form factor, then one can efficiently and conveniently deploy the DPI to detect the presence of special nuclear materials (SNM) and identify any isotopic variations that differ from their natural abundances. Here we show that by maximizing the areal coupling between a pixelated scintillator array and the partitioned photosensor readout such as a silicon photomultiplier (SiPM), the information utilization of the gamma-ray and neutron information in the radiation field can be enhanced, thus enabling one to rapidly acquire spatial maps of the distributions on gamma-ray and neutron emitters.