Neutron-gamma Pulse Shape Discrimination Research Articles

Performance stability of plastics for neutron-gamma pulse shape discrimination

The introduction of the first commercially available plastic scintillators with pulse shape discrimination (PSD) offered by Eljen Technology marked progress towards potentially replacing liquid scintillators in neutron detection. However, use of these plastics over several recent years has revealed an important flaw in these materials: the eventual degradation of scintillation light output and PSD performance. Studies described in this paper considered possible reasons for this degradation. Experiments conducted with numerous lab-prepared and commercial EJ-276 samples showed that the main factor affecting the instability is oxidation that involves highly reactive radicals generated during the polymerization process or from the further breakdown of polymer chains under oxygen/air exposure. Based on the obtained results, the stability of scintillation performance for PPO (2,5-Diphenyloxazole)-based PSD plastics has been improved through modifications of the composition via the utilization of scintillation dyes and compounds with antioxidant properties that diminish the effects of oxidation. Elements of these studies were used in the commercial production of the most recent EJ-276D PSD plastic version for fast neutron detection and 6Li-loaded plastics for thermal neutron and antineutrino detection applications. Performance projections of the new PSD plastics indicate a likely degradation of less than 10 % over 5–10 years in comparison to previous EJ-276 that might lose up to 30–40 % of the scintillation light during 1–2 years of storage or deployment under ambient conditions.

Optimization of pulse shape discrimination based on frequency domain for SiPM-based NaIL scintillator detector

Frequency gradient analysis (FGA) neutron-gamma pulse shape discrimination (PSD) based on Fourier transform has proved to be an effective method for discrimination between two types of pulses by simply constructing a discrimination parameter by subtracting the amplitude of the 1st frequency component from the 0th frequency component. However, the discrimination parameter constructed by the above method could not meet the requirements at some lower sample rates, and the dark noise caused using optoelectronic devices such as miniaturized SiPMs array further erodes the difference between the two frequency components. This paper proposes a new discrimination parameter construction method based on FGA for neutron-gamma discrimination, which is referred as mFGA (modified FGA). This discrimination parameter uses the 0th frequency component and other frequency components to enhance the discrimination performance. A neutron detector module consisting of a 25.4 mm× 25.4 mm× 50.8 mm NaI:Tl,6Li (NaIL) scintillator and an array of 8 × 8 SiPMs was established to verify the method of mFGA. The module measured a 252Cf neutron source under three shielding conditions: no shield, 5 cm high-density polyethylene (HDPE), and 10 cm lead brick. The proposed mFGA method was used to calculate the figure of merit (FoM) under each shielding condition and compared to the traditional frequency gradient method (FGA) and the traditional charge comparison method (CCM). The results show that the discrimination performance with the mFGA method is significantly improved. Taking the HDPE shielding group as an example, the performances of the three methods are FoM-FGA =1.14, FoM-mFGA =2.75, FoM-CCM =0.92. At the same time, with the increase of energy, due to the neutron gamma pulse becoming similar, and the discrimination performance of FGA and CCM decrease significantly. The discrimination performance of the mFGA method is relatively stable in all energy regions. Therefore, the proposed method is superior to the traditional FGA of the 0th frequency minus the 1st frequency and has a good application prospect for neutron-gamma discrimination with detectors of poor signal quality.

Neutron-gamma pulse shape discrimination for EJ301 liquid scintillator based on machine learning

Neutron-gamma pulse shape discrimination for EJ301 liquid scintillator based on machine learning

The Neutron-Gamma Pulse Shape Discrimination of CLLB Detector

The Neutron-Gamma Pulse Shape Discrimination of CLLB Detector

Neutron resonance fission neutron analysis for nondestructive fissile material assay

We propose neutron resonance fission neutron analysis (NRFNA), an active nondestructive assay (NDA) technique, to improve the capability to identify and quantify a small amount of fissile material in a sample. NRFNA uses pulsed neutrons to induce fission reactions in the sample. Fission neutrons are detected by a neutron-gamma pulse shape discrimination (PSD) scintillation detector with time-of-flight (TOF) technique. The obtained nuclide-specific resonance peaks in the neutron energy spectrum provide information to identify and quantify a fissile material in the sample. The possibility of using PSD for NRFNA was confirmed through a test experiment using a natural uranium sample. We successfully observed the resonance peaks from 235U(n,f) reaction and showed that NRFNA would be useful for measuring a small amount of fissile material in a sample.

Fast-, Light-Cured Scintillating Plastic for 3D-Printing Applications

Additive manufacturing techniques enable a wide range of possibilities for novel radiation detectors spanning simple to highly complex geometries, multi-material composites, and metamaterials that are either impossible or cost prohibitive to produce using conventional methods. The present work identifies a set of promising formulations of photocurable scintillator resins capable of neutron-gamma pulse shape discrimination (PSD) to support the additive manufacturing of fast neutron detectors. The development of these resins utilizes a step-by-step, trial-and-error approach to identify different monomer and cross-linker combinations that meet the requirements for 3D printing followed by a 2-level factorial parameter study to optimize the radiation detection performance, including light yield, PSD, optical clarity, and hardness. The formulations resulted in hard, clear, PSD-capable plastic scintillators that were cured solid within 10 s using 405 nm light. The best-performing scintillator produced a light yield 83% of EJ-276 and a PSD figure of merit equaling 1.28 at 450–550 keVee.

Measuring and unfolding fast neutron spectra using solution-grown trans-stilbene scintillation detector

We propose an overall procedure for measuring and unfolding fast neutron spectra using a trans-stilbene scintillation detector. Detector characterization was described, including the information on energy calibration, detector resolution, and nonproportionality response. The digital charge comparison method was used for the investigation of neutron-gamma Pulse Shape Discrimination (PSD). A pair of values of 600 ns pulse width and 24 ns delay time was found as the optimized conditions for PSD. A fitting technique was introduced to increase the trans-stilbene Proton Response Function (PRF) by 28% based on comparison of the simulated and experimental electron-equivalent distributions by the Cf-252 source. The detector response matrix was constructed by Monte-Carlo simulation and the spectrum unfolding was implemented using the iterative Bayesian method. The unfolding of simulated and measured spectra of Cf-252 and AmBe neutron sources indicates reliable, stable and no-bias results. The unfolding technique was also validated by the measured cosmic-ray induced neutron flux. Our approach is promising for fast neutron detection and spectroscopy.

Achieving Efficient Neutron and Gamma Discrimination in a Highly Stable 6Li-Loaded Cs3Cu2I5 Perovskite Scintillator.

Low-dimensional Cu(I) perovskite halides with efficient exciton emissions have recently emerged as promising scintillation materials for X-ray and gamma-ray detection applications. Here, we demonstrate the possibility of using zero-dimensional Cs3Cu2I5 for the sensitive detection of thermal neutrons and neutron-gamma discrimination enabled by Li doping. Single crystals of Cs3Cu2I5 doped with 95% enriched 6Li were grown by the Bridgman method. Cs3Cu2I5:6Li offers a compelling combination of high stability against moisture and oxygen, a decent energy resolution of 4.8% for 662 keV 137Cs gamma-rays, a high light yield of 30 000 photons/MeV for gamma-rays, and 96 000 photons/neutron for thermal neutron, and a good neutron-gamma pulse shape discrimination figure of merit of 2.27. Our discovery of 6Li-doped low-dimensional perovskite halides opens up a new horizon for stable and high-performance neutron-gamma scintillator design.

Evaluation of the Constant Fraction Time-Over-Threshold (CF-TOT) method for neutron-gamma pulse shape discrimination

The use of Time-over-Threshold (TOT) for the discrimination between fast neutrons and gamma-rays is advantageous when large number of detection channels are required due to the simplicity of its implementation. However, the results obtained using the standard, Constant Threshold TOT (CT-TOT) are usually inferior to those obtained using other pulse shape discrimination (PSD) methods, such as Charge Comparison or Zero-Crossing approaches, especially for low amplitude neutron/gamma-ray pulses. We evaluate another TOT approach for fast neutron/gamma-ray PSD using Constant-Fraction Time-over-Threshold (CF-TOT) pulse shape analysis. The CT-TOT and CF-TOT methods were compared quantitatively using digitized waveforms from a liquid scintillator coupled to a photomultiplier tube as well as from a stilbene scintillator coupled to a photomultiplier tube and a silicon photomultiplier. The quality of CF-TOT neutron/gamma-ray discrimination was evaluated using Receiver Operator Characteristics curves and the results obtained with this approach were compared to that of the standard CT-TOT method. The CF-TOT PSD method results in > 99.9% rejection of gamma-rays with > 80% neutron acceptance, much better than CT-TOT.

EJ-276 based neutron spectrometer with neutron–gamma pulse shape discrimination capability

This article demonstrates the performance of the EJ-276 plastic scintillator-based neutron radiation spectrometer with neutron–gamma separation capability. This work continues the earlier research devoted to the demonstration of the EJ-276 spectrometric capabilities. An NG-150M generator located at RFNC-VNIIEF (Sarov) was used as a neutron source. Neutron energy distribution was unfolded for several angles between neutron generator and the target-scintillator direction using a Geant4 simulated response matrix and a set of experimental response spectra. The experimental neutron spectra are compared with the calculated ones. DT neutron unfolded energy uncertainty is 2%. The accompanying gamma radiation spectra were unfolded with the same uncertainty.

Detection of Fission Coincidences With Plastic Scintillators for the Characterization of Radioactive Waste Drums

This work addresses the use of plastic scintillators as an alternative to ³He neutron detectors for radioactive waste drum characterization. The time response of scintillators is three orders of magnitude faster than that of gas proportional counters, and they offer similar neutron detection efficiency at lower cost. However, they are sensitive to gamma rays and the neutron-gamma pulse shape discrimination (PSD) is not possible with standard PVT scintillators. The proposed approach uses the neutron and gamma times of flight in triple coincidences recorded with ²⁵²Cf, AmBe, and ⁶⁰Co sources. A 2-D histogram of time delays between the second and first detected pulses, on the <inline-formula> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula>-axis, and between the third and second pulses, on the <inline-formula> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula>-axis, evidences a specific region of interest for spontaneous fission coincidences. MCNPX-PoliMi simulations are performed, which are in good agreement with previous experiments and allow for investigating the types of coincidences (<inline-formula> <tex-math notation="LaTeX">$\gamma \gamma \gamma $ </tex-math></inline-formula>, nnn, <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>nn, <inline-formula> <tex-math notation="LaTeX">$\gamma \gamma \text{n}$ </tex-math></inline-formula>) and optimizing the rejection of neutron and gamma scattering crosstalk. The method was also experimentally tested with a 118-L mock-up drum filled with a metallic or an organic matrix, showing a correct estimation of the net fission signal up to an &#x201C;alpha ratio&#x201D; of 12 between fission and (<inline-formula> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>,n) neutron emissions. Matrix and localization effects were also measured, showing a sensitivity (useful signal per gram of ²⁴⁰Pu_eq) for a homogeneous distribution of plutonium in the iron matrix about three times larger than in the wood matrix, due to neutron slowing-down in the latter. This difference can be taken into account by the prior knowledge of the matrix characteristics or by neutron and gamma attenuation measurements. In contrast, in case of a point source with an unknown position in the matrix, the relative localization uncertainty is 26% for the metallic drum, and 41% for the organic one.

First measurement of neutrons produced by deuterium fusion reactions in SPIDER

The inaugural deuterium acceleration campaign in the SPIDER negative ion source facility in Padua has recently taken place. The first neutrons generated by deuterium-deuterium beam-target fusion reactions (2.45 MeV) have been recorded, occurring from the collision of accelerated deuterium with deuterium absorbed by the calorimeter of SPIDER. A neutron detector based on a novel EJ276 plastic scintillator has been employed to successfully measure the neutron flux, which shows strong agreement with the extracted current of the acceleration grid. We have performed neutron-gamma pulse shape discrimination with the EJ276 device at SPIDER combined with direct spectroscopic comparisons of the D-D neutrons with data from the Frascati neutron generator (241AmB quasi-monoenergetic 2.5 MeV). Despite the low statistics produced in this first campaign, both pulse shape discrimination and spectral analysis of the fusion neutrons was viable. The success of these first measurements has led to the installation of an array of 6 new scintillators to be used for further physical studies in future campaigns.

Calibration of EJ-276 plastic scintillator for neutron–gamma pulse shape discrimination experiments

Calibration of the scintillator detector based on a plastic scintillator EJ-276 for neutron–gamma pulse shape discrimination (PSD) experiments was carried out. Optical characteristics of electrons as the function of their deposited energy were experimentally studied. The calibration was implemented by comparing the experimental response from gamma sources with that obtained by means of GEANT4 simulation. It is shown that a similar relationship for protons built on the basis of the published data on EJ-299-33, is also applicable to EJ-276 which confirms the information about the equivalence of these scintillators. The experimental response of the EJ-276 to 14 MeV neutrons is compared with Geant4 simulation data. The experimental and simulated data were also presented as the function of light output in electronic equivalent (MeVee) obtained by both exponential and Birks’ laws.An experimental setup was constructed with Ø51 × 51 mm EJ-276 scintillator. The CAEN DT5730B digitizer was used for PSD signal discrimination. Based on the current experiments and the earlier published data a calibration relation between the setup channels and the light output was obtained in electronic equivalent (MeVee).

Pulse shape discrimination and exploration of scintillation signals using convolutional neural networks

We demonstrate the use of a convolutional neural network to perform neutron-gamma pulse shape discrimination, where the only inputs to the network are the raw digitised silicon photomultiplier signals from a dual scintillator detector element made of 6Li F:ZnS(Ag) scintillator and PVT plastic. A realistic labelled dataset was created to train the network by exposing the detector to an AmBe source, and a data-driven method utilising a separate photomultiplier tube was used to assign labels to the recorded signals. This approach is compared to the charge integration and continuous wavelet transform methods and a simpler artificial neural net. It is found to provide superior levels of discrimination, achieving an area under the curve of 0.996 ± 0.003. We find that the neural network is capable of extracting interpretable features directly from the raw data. In addition, by visualising the high-dimensional representations of the network with the t-SNE algorithm, we discover that not only is this method robust to minor mislabeling of the training dataset but that it is possible to identify an underlying substructure within the signals that goes beyond the original labelling. This technique could be utilised to explore and cluster complex, raw detector data in a novel way that may reveal more insights than standard analysis methods.

A functional form for liquid scintillator pulse shapes

Digitization of detector signals enables analysis of the original waveform to extract timing, particle identification, and energy deposition information. Here we present the use of analytical functions based on sigmoids to model and fit such pulse shapes from liquid organic scintillators, though the method should also be applicable to other detector systems. Neutron and gamma interactions in NE213 detectors were digitized from the phototube anode and fit using a sigmoid-based function. The acuity of the fit in extracting timing information and performing neutron-gamma pulse-shape discrimination are presented and discussed.

Testing of Scintillation Detectors in Quasi-Monoenergetic Neutron Spectra in a Silicon Filtered Neutron Beam at the LVR-15 Research Reactor

With increasing needs of neutron detection in energy and homeland security sectors, there is an aim in the development of new suitable detection materials with possible n/g separation and also satisfactory resolution. This issue is connected with a well-defined neutron field. The neutron standards 252Cf(spontaneous fission) and 235U(nth fission) have smooth spectra; thus, the tests in these fields can be used for testing of neutron gamma pulse shape discrimination (PSD), but cannot reveal possible problems in the deconvolution. New neutron field which is formed from fission spectrum filtered by 1 m thick silicon block was developed in Research Center Rez. Due to the course of the silicon cross section, the well-distinguished neutron peaks occur in filtered neutron spectrum. In this field, various scintillation materials, HIDEX Aqualight, and EJ299-33A were tested.

A neutron-gamma pulse shape discrimination method based on pure and mixed sources

We present a novel trainable approach to distinguish neutrons from gammas using a particle detector. Traditionally, Pulse Shape Discrimination (PSD) methods for this problem utilize an ad-hoc computation of tail signal energy to perform the detection. Our first contribution is a rigorous analysis of the performance of this existing approach on gold standard Time of Flight (TOF) data. While this approach performs well for high energy pulses, its accuracy drops dramatically as the pulse energy decreases. Our second contribution is a novel data driven classifier that is trained from two readily available sources: one that emits gamma particles (Cs-137), and another that emits a mixture of gamma and neutron particles (Cf-252). We test our approach using TOF experiments and show a marked improvement in accuracy over the traditional method for low false positive rates and low energies.

Improved neutron–gammadiscrimination at low-light output events using conical trans-stilbene

Scintillator geometry affects light-collection efficiency (LCE) and, as a consequence, detector-performance metrics that depend on LCE, such as particle-type identification. This work compares the neutron–gammapulse-shape discrimination (PSD) performance of two trans-stilbene crystals of different geometries: a cylinder and cone. Both geometries have a base diameter and height of 50 mm. Results demonstrate that the cone achieved a higher figure of merit over the cylinder by approximately 22.6% for pulses ranging from 25 keVee to 100 keVee. This improvement can be especially helpful for applied detection systems to better resolve neutrons and gamma rays at low-light output events (<100 keVee), where PSD can suffer.

Study of sampling rate influence on neutron–gamma discrimination with stilbene coupled to a silicon photomultiplier

Choosing a digitizer with an appropriate sampling rate is often a trade-off between performance and economy. The influence of sampling rates on the neutron-gamma Pulse Shape Discrimination (PSD) with a solid stilbene scintillator coupled to a Silicon Photomultiplier was investigated in this work. Sampling rates from 125MSPS to 2GSPS from a 10-bit digitizer were used to collect detector pulses produced by the interactions of a Cf-252 source. Due to the decreased signal-to-noise ratio (SNR), the PSD performance degraded with reduced sampling rates. The reason of PSD performance degradation was discussed. Then, an efficient combination of filtering and digital signal processing (DSP) was then applied to suppress the timing noise and electronic background noise. The results demonstrate an improved PSD performance especially at low sampling rates, down to 125MSPS. Using filtering and DSP, the ascribed Figure of Merit (FOM) at 125keVee (± 10keVee) increased from 0.95 to 1.02 at 125MSPS. At 300keVee and above, all the FOMs are better than 2.00. Our study suggests that 250MSPS is a good enough sampling rate for neutron-gamma discrimination in this system in order to be sensitive to neutrons at and above ~ 125keVee.

The efficient neutron-gamma pulse shape discrimination with small active volume scintillation detector

A small detector with EJ-301 liquid scintillation was manufactured for the study on the neutron-gamma pulse shape discrimination. In this research, four algorithms, including Threshold crossing time (TCT), Pulse gradient analysis (PGA), Charge comparison method (CCM), and Correlation pattern recognition (CPR) were developed and compared in terms of their discrimination effectiveness between neutrons and gamma rays. The figures of merits (FOMs) obtained for 100 ÷ 2000 keVee (keV energy electron equivalent) neutron energy range show the charge comparison method was the most efficient of the four algorithms.