Neutron Signal Research Articles

Research on data processing technology based on 6LiF/ZnO:Ga fiber neutron detector array

The 6LiF/ZnO:Ga fiber neutron detector, consisting of a 6LiF/ZnO:Ga scintillator, optical fiber, and PMT, has distinct advantages such as compactness, strong real-time online measurement capabilities, and resistance to electromagnetic interference. These characteristics make it highly suitable for neutron detection in confined spaces with significant electromagnetic interference. However, its small size limits the neutron detection efficiency of a single detector, necessitating the use of multiple fibers to construct a fiber array and enhance sensitivity. The goal is to study data processing techniques for the 6LiF/ZnO:Ga fiber detector array. Inside the FPGA, three digital signal processing channels are used to extract multiple information from signal such as the amplitude, timestamp, and measurement channel ID. The fast channel adopts triangular shaping to provide real-time baselines and timestamps for the discrimination channel and the slow channel. The slow channel adopt cascade shaping method to extract the amplitude of nuclear signal. The discrimination channel method adopt cascade discrimination method to realize the real-time discrimination between gamma signals, neutron signals and interference signals. The method has been successfully applied to 6LiF/ZnO:Ga fiber neutron detector arrays with different mass ratios, providing new solution for digital data processing in fiber neutron detector arrays.

Time-of-flight vs time-of-arrival in neutron spectroscopic measurements for high energy density plasmas.

The neutron time-of-flight (nToF) diagnostic technique has a lengthy history in Inertial Confinement Fusion (ICF) and High Energy Density (HED) Science experiments. Its initial utility resulted from the simple relationship between the full width half maximum of the fusion peak signal in a distant detector and the burn averaged conditions of an ideal plasma producing the flux [Lehner and Pohl, Z. Phys. 207, 83-104 (1967)]. More recent precision measurements [Gatu-Johnson etal., Phys. Rev. E 94(8), 021202 (2016)] and theoretical studies [Munro, Nucl. Fusion 56, 035001 (2016)] have shown the spectrum to be more subtle and complicated, driving the desire for an absolute calibration of the spectrum to disambiguate plasma dynamics from the conditions producing thermonuclear reactions. In experiments where the neutron production history is not well measured, but the neutron signal is preceded by a concomitant flux of photons, the spectrum can be in situ calibrated using a set of collinear detectors to obtain a true "time-of-flight" measurement. This article presents the motivation and overview of this technique along with estimates of the experimental precision needed to make useful measurements in existing and future nToF systems such as the pulsed power Z-machine located in Albuquerque, NM, at Sandia National Laboratories.

Characterization of diamond and organic scintillation detectors utilizing radiation sources for continuous plasma operation.

This paper provides a comprehensive study of neutron calibration methodologies, specifically highlighting the capabilities for n-γ discrimination in diamond and EJ-309, and stilbene scintillation detectors. The calibration process detailed in this study includes pulse height analysis and pulse shape discrimination, relying on the analysis of charge deposition resulting from both γ and neutron interactions. Utilizing 60Co and 252Cf radiation sources, the energy spectra of these sources are obtained. The characterized detectors were used in ST40 experiments and allowed acquiring neutron signal during a plasma shot with good agreement among diamond and scintillation detectors. Then, the diamond detector was cross-calibrated against indium activation foils placed at the same location in proximity to the ST40 during plasma shots: both detectors measured a neutron flux of ≈106 cm-2 s-1 at ≈1 m distance from the tokamak center, and the discrepancy between the diamond detector and the activation foils is ≈25%.

Generation of neutron spectra and emission yields from proton-induced reactions on natC and natLi targets using CR-39 detectors

Fast neutron spectrometry and dosimetry at a specified location in a compact radiation environment is difficult due to large size of the conventional neutron spectrometers. In such situations, a small-size bare CR-39 detector can be used as a neutron detector to quantify the fast neutron fraction. In the present study, CR-39 detectors were placed very close to natural lithium and carbon targets during the proton irradiations to acquire the emission neutron spectra. The neutron spectra and the ambient dose equivalents in the forward and lateral directions with respect to the incident proton beam were estimated for both targets at incident proton energies between 8 and 20 MeV. The measured neutron spectra using CR-39 detectors could identify the quasi mono-energetic neutron features from the Li(p,n) and 13C(p,n) reactions effectively. An important observation of the present study is the identification of the fast neutron signature from the 13C(p,n) system from the discrete state de-excitations of excited 14N composite nuclei. The theoretical evaluation of the neutron spectral features and relative neutron energy distributions were performed using the FLUKA: FLUktuierende KAskade, a Monte Carlo simulation package, and the estimates agreed with the experimental results for both systems. The neutron ambient dose equivalents were also estimated from the measured spectra. These neutron fluence and dose estimates at close vicinity to the target can serve as an essential basis for shielding calculations and planning the pertinent radiation protection strategies.

Optimization of fast output from SiPM array for pulse shape discrimination techniques at high counting rates

Plastic scintillators with pulse shape discrimination (PSD) capability, when combined with silicon photo-multipliers (SiPMs), enable the development of compact neutron detectors with lower power consumption and insensitivity to magnetic fields. However, the SiPM array has a significantly large pulse width of the output signal, which leads to discrimination difficulties at high counting rates. There are two outputs of a SiPM array: a standard output for energy measurement and a fast output for timing measurement. The pulse width of the standard output signal is in the range of hundreds nanoseconds, while the pulse width of the other one is in the range of tens nanoseconds. Fast output signals have traditionally been considered suitably for fast timing, however utilizing them for PSD has been little explored. In our previous work, we successfully implemented PSD by using fast output signals from a SiPM. Despite the obvious negative overshoot in fast signals, we achieved PSD of fast signals by using longer sampling times. However, this approach was still limited at high counting rates. This study proposes a compensation network for fast output signals, which is composed of a compensation unit consisting of a resistor and a capacitor. This design effectively compensates the overshoot of the fast output and shortens the pulse width from 60 ns to 44 ns. Additionally, it highlights the frequency amplitude differences between the neutron and gamma signals from fast output in the high-frequency component, thus improving the PSD capability of the fast output.

A scintillating-fiber detector for making high-time-resolution secondary D-T neutron measurements in KSTAR.

A scintillating fiber (Sci-Fi) detector for the middle neutron flux range was installed in KSTAR as part of a collaboration between the National Institute for Fusion Science and the Korea Institute of Fusion Energy. The detector could make relatively high-time-resolution measurements of secondary deuterium (D)-tritium (T) neutron fluxes to investigate the degradation of D-D-born triton confinement, which is crucial for demonstrating alpha particle confinement, particularly above 0.9 MA in KSTAR. The pulse-height spectrum of the Sci-Fi detector exhibited two peaks, the higher of which corresponded to D-T neutrons. A discrimination technique was applied to extract the D-T neutron signal, revealing the time evolution of the D-T neutron flux during relatively high plasma current discharges with a 50ms temporal resolution. Future research will involve investigating the causes of the degradation of the triton burnup ratio above 0.9 MA in KSTAR.

Investigating the use of pixel scrambling and diffusion in secure radiographic inspections

In warhead verification, physical encryption technology could play a critical role in protecting confidential information on geometric structure and isotopic composition of a true warhead. As an important supplement to physical encryption, algorithmic encryption still has great potential in improving defense-in-depth security for nuclear arms control verification. To further supplement feasible nuclear arms control verification technologies, we propose a verification method based on neutron induced fission reactions employing both physical field flux encryption and algorithm encryption. Physical encryption processes the classified geometry or composition information by encrypting the fission neutron signal of the tested item with a randomly shielded mask. Algorithm encryption adopts pixel scrambling, pixel diffusion for secondary encryption. To verify the robustness and security of this new verification method, numerical simulations are performed using the Monte Carlo toolkit Geant4. Verification results indicate a high level of robustness and security with a low level of noise (∼<0.5%).

Observation of d(t,n)α Neutrons Following d(d,p)t Reactions in a Deuterium Gas Cell: An Attempt to Repeat Ruhlig’s 1938 Observation of Secondary Reactions

In order to benchmark methods used to calculate reaction-in-flight fusion reactions in inertial confinement fusion and address issues related to the first claimed observation of d(t,n)α reactions in 1938, secondary d(t,n)α reactions have been observed following d(d,p)t reactions in deuterium gas. A pulsed 200-nA, 2.2-MeV deuterium beam from the Triangle Universities Nuclear Laboratory FN tandem accelerator was injected into a cylindrical multiatmosphere deuterium gas target. The incident beam traversed along the target cylinder’s 3-cm symmetry axis after its passage through a Havar entrance foil. Two different Havar foil thicknesses were used to obtain 1.5- and 0.6-MeV deuteron beams entering the deuterium cell. The cylinder’s radius was 2 cm to allow for d(d,p)t tritons emitted perpendicular to the beam to range out in the deuterium gas. The neutron emission from the cell was observed via its time of flight to a liquid scintillator placed at various angles to the beam direction, at a distance of 243 cm. Pulse-shape-discrimination techniques were used to separate neutron and gamma-ray signals seen in the liquid scintillator. The observed probability of ~2 × 10–4 for inducing secondary d(t,n)α fusion in the gas cell per d(d,p)t reaction is consistent with theoretical expectations.

Nanoparticle ZnS:Ag/6LiF—a new high count rate neutron scintillator with pulse shape discrimination

A neutron sensitive scintillator employing nanoparticles of ZnS:Ag has been developed for the first time. Pulse shape differences between neutron and gamma signals were observed in this material and neutron-gamma discrimination was applied. With initial signal processing parameters, gamma sensitivities of 8.5×10−6 to 60Co gammas were achieved. The average primary decay of neutron scintillation in nanoparticle ZnS:Ag/6LiF was measured to be 18ns, and afterglow was significantly suppressed in comparison to standard ZnS:Ag/6LiF scintillators that employ micron sized ZnS:Ag. Fast decay times and minimal afterglow indicate potential for use in high count rate capability applications. Prospective count rate capabilities were investigated here as proof of concept, with rates of 1.12Mcps measured for a single readout channel with less than 3.5% count loss. This is approximately 70 times greater than the count rate capability of the current standard ZnS:Ag/6LiF scintillation detectors. With improvements to signal processing and scintillator composition, nanoparticle ZnS:Ag/6LiF could be a promising candidate for future high rate capability neutron detectors.

Experimental study of plastic scintillators array for compact fast neutron-gamma dual-modality imaging system

In a compact multi-radiation imaging system, the substitution of traditional charge-coupled devices with a plastic scintillators array coupled with silicon photomultiplier detectors significantly enhances the radiation tolerance of the detection and imaging module, thereby reducing the system footprint. We present a detailed structure of a compact fast neutron/gamma-ray dual-modality imaging system based on a plastic scintillators array and evaluate its performance in imaging various samples. The detector array consists of nine 14.7 × 14.7 × 100 mm3 EJ-276 plastic scintillators coupled with corresponding silicon photomultiplier units, arranged in a 3 × 3 grid. The usage of EJ-276 plastic scintillators allows for discrimination between neutron and gamma signals through pulse shape discrimination techniques, enabling data separation between neutrons and gamma rays. In this paper, we present the experimental procedure and results of fast neutron-gamma imaging for objects containing aluminum and polyethylene materials. Fast neutrons and gammas are delivered by an Cf-252 source. The experimental results demonstrate that the system achieves dual-modality grayscale imaging and has resolution for sample thickness. Reconstructed image contrast allows qualitative differentiation of sample materials, especially detecting low-Z materials encapsulated by high-Z materials.

On the resolution of the three-axis neutron diffractometer setting optimized for some powder diffractometry studies

Presented three-axis neutron diffractometer performance documents the feasibility of using it in special cases high-resolution powder diffraction studies, namely, for elastic and plastic deformation studies of bulk polycrystalline samples when the whole powder diffraction spectrum is not required. Contrary to the conventional double-axis setting the suggested alternative consists of an unconventional three axis set-up employing a bent perfect crystal (BPC) monochromator and analyzer with a polycrystalline sample in between. The analysis of the profile of the beam diffracted by a sample is carried out by rocking the BPC-analyzer and the neutron signal is registered by a point detector. Though the diffractometer alternative is, for measurements, much more time consuming, its resolution is, however, substantially higher and permits also plastic deformation studies on the basis of analysis of the sample diffraction profiles. The so-called analyzer rocking curve then provides a sample diffraction profile and could reflect the lattice or structural changes. Moreover, much larger widths (up to 10 mm) of the irradiated gauge volumes can be investigated when just slightly affecting the resolution of the experimental setting.

Development of a silicon-based thermal neutron detection system

Neutron detection systems are of increasing importance in applications from basic science to medical applications and reactors. 3He proportional counters remain the most popular choice for monitoring thermal neutrons with a detection efficiency of around 60%, however, due to 3He global shortages, a new generation of detection technologies will be required to meet the rising demand. As a result, extensive research is being conducted to investigate alternative methods of neutron detection. This work presents such a system and demonstrates its calibration and evaluation using an AmBe neutron source. The detection system involves silicon sensors coated by converter layers to make the detectors sensitive to thermal neutrons via neutron capture and measurement of the resulting secondary charged particles. The detection system is presented in two configurations, a single and a multi-layer configuration, where the latter is used to increase the total detection efficiency. In addition, the system is capable of determining coincident signals from a single neutron capture, a feature which allows background suppression and an increase in the purity of the neutron signal which is particularly useful in mixed radiation environments.

Use of a commercial neutron counting module to validate neutron-particle transport code simulation of a new cosmic radiation neutron monitor design

This paper reports on one of several experiments used to validate MCNP models and processes employed to define a cosmic radiation (CR) neutron source and benchmark to optimise a new ground level neutron monitor (NM) design. The new NM design aims to be more compact, non-toxic and capable of producing comparable counting efficiencies to the NM-64; a monitor standardised in 1964 and which makes up most of the established global network. A broad-energy neutron monitor, the N50L, originally designed for nuclear material accountancy and safeguards, is used to provide primary data in support of our new NM design. Ambient count rate data from the N50L were acquired from it bare and inside a lead (Pb) sarcophagus to boost the neutron signal. Measured data were compared with calculations made using a Monte Carlo N-Particle (MCNP) transport code model of the setup. The calculated compared to the measured count rate for the monitor inside the Pb sarcophagus are in good agreement with a ratio close to one. However, overpredictions (by almost a factor of two) of the calculated rate for the bare monitor suggest inadequacies of the model for high-energy physics. Pressure corrected count rate data were also compared with data from several existing NM-64s over the same period. Our data are in good agreement with variations observed by these existing monitors on the global network. The N50L uses helium-3 (3He) counters and roughly equates to about 1/17th of the proposed final NM design in terms of detector volume. Scaled count rates from the N50L are within 73% of the rates derived from an NM-64 for a similar latitude, geomagnetic cutoff rigidity and altitude, and differs by <8% of the MCNP simulated response of the proposed design.

Simulation of Neutron Leakage Variations at Fuel Substitution in a Small Modular Reactor and Implications for Unattended Safeguards Monitoring

According to a recently proposed nuclear safeguards technique, monitoring the power-normalized, ex-core neutron detection rate over time could be used to detect undeclared changes to the fissile composition of a reactor core. In this study, Monte Carlo simulations have been used to verify some of the underlying assumptions of this technique and the possibilities of using it to detect undeclared fuel substitutions during the first 2-year cycle of a light water small modular reactor. Depletion calculations and neutron transport simulations were used to study the changes in the power-normalized neutron leakage rate J b / P core through the core barrel upon fuel substitutions and whether these changes are fully explained by changes in the core fissile composition. Several substitution scenarios have been studied, where partially depleted fuel assemblies were substituted with fresh fuel assemblies after 1 year of irradiation. The modeled substitution scenarios, which included substituting up to 4 out of 37 fuel assemblies in the core at a time, resulted in changes in J b / P core of up to 3.5% depending on which fuel assemblies were substituted. The results indicate that the ex-core neutron signatures are not only sensitive to core-averaged nuclide densities, fission cross sections, and neutron flux, but also the spatial distributions of these and other parameters throughout the core. Effects such as these could mean that monitoring the core fissile composition with the proposed technique might be more complex than previously suggested.

Search for a Neutron Dark Decay in ^{6}He.

Neutron dark decays have been suggested as a solution to the discrepancy between bottle and beam experiments, providing a dark matter candidate that can be searched for in halo nuclei. The free neutron in the final state following the decay of ^{6}He into ^{4}He+n+χ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity ^{6}He^{+} beam at Grand Accélérateur National d'Ions Lourds, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of Br_{χ}≤4.0×10^{-10} (95% C.L.). Using the dark neutron decay model proposed originally by Fornal and Grinstein, we translate this into an upper bound on a dark neutron branching ratio of O(10^{-5}), improving over global constraints by one to several orders of magnitude depending on m_{χ}.

Development of a neutron yield measurement system utilizing BF_3 detectors on the HL-3 tokamak

A neutron yield measurement system, comprising five BF3 proportional counters with excellent n/γ discrimination capabilities, has been meticulously developed and implemented on the HL-3 tokamak to provide time-resolved assessments of neutron emission rates. The size of moderator, optimized for the flat energy responses, was skillfully simulated and designed using the MCNP code. An in-depth analysis of measurement errors stemming from pile-up phenomena determined that the upper limit of the count rate, under the current electronics design, is 131 kcps with a relative error of less than 20%. Drawing from the pulse amplitude spectrum obtained from a 252Cf neutron source and experimentation on the HL-3 tokamak, the amplitude threshold for the BF3 neutron detector systems was judiciously established. Furthermore, it was conclusively affirmed that the neutron yield measurement system possesses the capability to effectively discriminate between neutron and gamma signals. During HL-3 discharges, deuterium-deuterium (DD) neutrons were reliably detected with a temporal resolution of 10 ms, and the variations in neutron count rates was found to be in alignment with the neutral beam injection. Ultimately, the total neutron yield was successfully estimated through the utilization of count rates from four BF3 detectors, thereby validating the robust performance of the neutron yield measurement system.

Testing a novel sensor design to jointly measure cosmic-ray neutrons, muons and gamma rays for non-invasive soil moisture estimation

Abstract. Cosmic-ray neutron sensing (CRNS) has emerged as a reliable method for soil moisture and snow estimation. However, the applicability of this method beyond research has been limited due to, among others, the use of relatively large and expensive sensors. This paper presents the tests conducted on a new scintillator-based sensor especially designed to jointly measure neutron counts, muons and total gamma rays. The neutron signal is first compared against two conventional gas-tube-based CRNS sensors at two locations. The estimated soil moisture is further assessed at four agricultural sites, based on gravimetric soil moisture collected within the sensor footprint. Muon fluxes are compared to the incoming neutron variability measured at a neutron monitoring station and total gammas counts are compared to the signal detected by a gamma ray spectrometer. The results show that the neutron dynamic detected by the new scintillator-based CRNS sensor is well in agreement with conventional CRNS sensors. The derived soil moisture also agreed well with the gravimetric soil moisture measurements. The muons and the total gamma rays simultaneously detected by the sensor show promising features to account for the incoming variability and for discriminating irrigation and precipitation events, respectively. Further experiments and analyses should be conducted, however, to better understand the accuracy and the added value of these additional data for soil moisture estimation. Overall, the new scintillator design shows to be a valid and compact alternative to conventional CRNS sensors for non-invasive soil moisture monitoring and to open the path to a wide range of applications.

Neutron-gamma discrimination with broaden the lower limit of energy threshold using BP neural network

Neutron-gamma discrimination is a tough and significative in experimental neutrons measurements procedure, especially for low-energy neutrons signal discrimination. In this work, based on the Pulse Shape Discrimination (PSD) and Back-Propagation (BP) artificial neural networks, a neutron-gamma discrimination method is developed to broaden the lower limit of energy threshold with the hidden layer of 20 neurons. Compared with neutron-gamma discrimination method based on PSD only, the developed neutron-gamma discrimination method based on the PSD and BP-ANN can discriminate neutron and gamma-ray signals with low energy threshold, which can discriminate signals up to 99.93%. Moreover, this work can reduce the energy threshold from 350 keV to 70 keV, as well as the acquired data utilization increased from 60% to more than 99.9%, which overcome the hardware limitations and distinguish neutron and gamma-ray signals, effectively. The developed neutron-gamma discrimination method and the trained neural network can be directly used to other experimental neutrons measurements.

Hydration of biologically relevant tetramethylammonium cation by neutron scattering and molecular dynamics.

Neutron scattering and molecular dynamics studies were performed on a concentrated aqueous tetramethylammonium (TMA) chloride solution to gain insight into the hydration shell structure of TMA, which is relevant for understanding its behavior in biological contexts of, e.g., properties of phospholipid membrane headgroups or interactions between DNA and histones. Specifically, neutron diffraction with isotopic substitution experiments were performed on TMA and water hydrogens to extract the specific correlation between hydrogens in TMA (HTMA) and hydrogens in water (HW). Classical molecular dynamics simulations were performed to help interpret the experimental neutron scattering data. Comparison of the hydration structure and simulated neutron signals obtained with various force field flavors (e.g. overall charge, charge distribution, polarity of the CH bonds and geometry) allowed us to gain insight into how sensitive the TMA hydration structure is to such changes and how much the neutron signal can capture them. We show that certain aspects of the hydration, such as the correlation of the hydrogen on TMA to hydrogen on water, showed little dependence on the force field. In contrast, other correlations, such as the ion-ion interactions, showed more marked changes. Strikingly, the neutron scattering signal cannot discriminate between different hydration patterns. Finally, ab initio molecular dynamics was used to examine the three-dimensional hydration structure and thus to benchmark force field simulations. Overall, while neutron scattering has been previously successfully used to improve force fields, in the particular case of TMA we show that it has only limited value to fully determine the hydration structure, with other techniques such as ab initio MD being of a significant help.

Material Discrimination Using X-Ray and Neutron

Background: A nondestructive test is commonly used to inspect the surface defects and internal structure of an object without any physical damage. X-rays generated from an electron accelerator or a tube are one of the methods used for nondestructive testing. The high penetration of X-rays through materials with low atomic numbers makes it difficult to discriminate between these materials using X-ray imaging. The interaction characteristics of neutrons with materials can supplement the limitations of X-ray imaging in material discrimination.Materials and Methods: The radiation image acquisition process for air-cargo security inspection equipment using X-rays and neutrons was simulated using a GEometry ANd Tracking (Geant4) simulation toolkit. Radiation images of phantoms composed of 13 materials were obtained, and the R-value, representing the attenuation ratio of neutrons and gamma rays in a material, was calculated from these images.Results and Discussion: The R-values were calculated from the simulated X-ray and neutron images for each phantom and compared with those obtained in the experiments. The R-values obtained from the experiments were higher than those obtained from the simulations. The difference can be due to the following two causes. The first reason is that there are various facilities or equipment in the experimental environment that scatter neutrons, unlike the simulation. The other is the difference in the neutron signal processing. In the simulation, the neutron signal is the sum of the number of neutrons entering the detector. However, in the experiment, the neutron signal was obtained by superimposing the intensities of the neutron signals. Neutron detectors also detect gamma rays, and the neutron signal cannot be clearly distinguished in the process of separating the two types of radiation. Despite these differences, the two results showed similar trends and the viability of using simulation-based radiation images, particularly in the field of security screening. With further research, the simulation-based radiation images can replace ones from experiments and be used in the related fields.Conclusion: The Korea Atomic Energy Research Institute has developed air-cargo security inspection equipment using neutrons and X-rays. Using this equipment, radiation images and R-values for various materials were obtained. The equipment was reconstructed, and the R-values were obtained for 13 materials using the Geant4 simulation toolkit. The R-values calculated by experiment and simulation show similar trends. Therefore, we confirmed the feasibility of using the simulation-based radiation image.