Neutron Counting Research Articles

Mesoscale soil moisture survey by mobile cosmic-ray neutron sensor across various landscapes in the Heihe River Basin

The cosmic-ray neutron technology is a novel method for upscaling soil moisture, holding significant importance in drought monitoring, soil water storage estimation, calibration of remote sensing products, and data assimilation of land surface models. However, most studies conducted soil moisture measurement experiments using static Cosmic-Ray Neutron Sensors (CRNS) and are often limited to a single landscape, lacking in-situ measurement for the cosmic-ray neutron rover across various landscapes. This study carried out soil moisture observation experiments using a cosmic-ray neutron rover in nine grassland plots, nine farmland plots, and nine desert plots of the Heihe River Basin, Northwest China. The neutron counts displayed clear variations among different plots, and an equation could represent the relationship between neutron intensity and multiple variables near the ground. The parameter N0 was the smallest for grassland and greatest for desert, and there was a significant negative correlation between Normalized Difference Vegetation Index and N0 (P <0.05). Furthermore, the performance of CRNS notably improved after considering soil lattice water and soil organic matter. The Root Mean Square Error (RMSE) between the measured and oven-dried soil moisture increased with radius. Soil moisture measured by CRNS strongly supported the 100 m diameter footprint assumption (RMSE <0.044 g/g). The study offers valuable insights into the application and promotion of cosmic-ray neutron technology across various landscapes within a basin.

A low-power neutron and gamma-ray detector for environmental radiation monitoring using CLYC scintillator

This paper describes a low-power neutron and gamma-ray detector based on a CLYC scintillator enriched with 96% 6Li for environmental radiation monitoring. The detector is designed in compliance with the IP66 standard having water and dust release capability. The power consumption of the detector is 3.5 W, and the operating temperature is from -15° to +40°. The energy response range for gamma rays is 40 keV to 3 MeV, while the energy resolution of the detector is 4.8% (662 keV). A standard figure of merit (FOM) is used to evaluate neutron-gamma discrimination, achieving a value of 3.42. The mixed neutron-gamma environment was simulated at the Back-n neutron beam of the China Spallation Neutron Source (CSNS). The test results show that the detector can detect the changes in the gamma dose and neutron count rates when the beam is turned on and off in real-time. The proposed detector is suitable for the application of long-term environmental radiation monitoring for nuclear facilities or buildings housing radioactive sources.

High-precision digital time-amplitude analysis system for the neutron Time-Of-Flight spectrometer in HL-2M

Neutron time-of-flight (TOF) spectrometers are widely used for neutron diagnostics in large-scale fusion facilities. However, traditional amplitude and time analysis methods are limited in terms of resolution, flexibility, and other factors. To overcome these limitations, we propose the Digital Time-Amplitude Analysis (DTAA) system, which combines the advantages of both methods by using a high-performance FPGA for real-time time and amplitude analysis of particle pulses. The DTAA system provides accurate timing and pulse shape information, fast processing speed, and low implementation cost. We have also developed a multi-mode data joint time-of-flight correction algorithm and verified its performance through simulation. The DTAA system passed laboratory and tokamak tests, achieving a time measurement precision of 122 ps and accurately measuring neutron counts in tokamak experiments. This work represents a key step towards comprehensive high-performance integrated particle pulse analysis.

Preliminary results of the 105GHz collective Thomson scattering system on HL-2A.

A 105GHz collective Thomson scattering (CTS) diagnostic has been successfully developed for fast-ion measurements on the HL-2A tokamak, and it has been deployed during an experimental campaign. Enhanced signals exhibiting synchronous modulation characteristics have been observed across all CTS channels upon the launch of a modulated probe wave. Results show that the intensity of the CTS signal increases with Neutral Beam Injection (NBI) power and is proportional to neutron count, indicating that the scattering signal contains a contribution from fast ions. Compared with the signal without NBI, the enhanced scattering spectrum due to NBI is slightly wider than the predicted fast ion range. Such broadening might be attributed to the heating effects of the gyrotron.

Prospect for measurements of (γ, n) reaction cross-sections of p-nuclei at ELI-NP

The gamma beam system under construction at the ELI-NP facility in Romania is projected to give the nuclear physics community access to an experimental system providing a high-intensity, narrow bandwidth photon beam at variable energy. With high-efficiency detector systems in place, the experimental programme will have a strong potential for in-depth studies of rare stable isotopes originating from the astrophysical p-process. In particular, the neutron detection systems are already implemented through a dedicated 3He long neutron counter array, called ELIGANT-TN, that is completed and in use. In this mini-review, we will give a summary of the current status of existing (γ, n) cross-section data, as well as the methods to obtain them, and highlight the future potential to expand and improve such data using the ELI-NP instrumentation and beam-lines.

Modelling of Tarapur BWR lattice and core to estimate the required number of SB-BE startup neutron sources

During the refuelling and criticality of Boiling Water Reactors (BWR), fission counters are used as Source Range neutron Monitors (SRMs) to get the count rates. The study shows that the neutron counts due to spontaneous fission sources are insufficient to meet the SRM's limitations of 3 counts per second (cps). In this work, TAPS fuel assembly is modelled using the lattice transport code DRAGON5, and lattice parameters are generated as a function of fuel depletion. These parameters have been further used to model the reload-2 of the BWR core. 3D diffusion theory code KINFIN is used to simulate the BWR core with external neutron sources to estimate the neutron flux at three SRM locations. Thermal fluxes are then used to compute the neutron count rates using the sensitivity of SRMs for different combinations of five Sb-Be neutron sources.

An integrated density correction method of four-detector density logging in cased holes

Density logging is critical to shale oil and gas exploration. In density logging of cased holes, the inelastic gamma count and hydrogen index are closely related to the correction of formation density. Aimed at the problems of obtaining the pure inelastic gamma count and correcting the hydrogen index in density logging, the relevant research is implemented in the four detector logging tool. Firstly, a technique to separate the pure inelastic gamma count for density logging is provided. It is based on the pure count coefficient. Then on the basis of determining the density through the pure inelastic count ratio, the hydrogen index is corrected by the epithermal neutron and thermal neutron count ratio. Finally, the integrated density correction equation is proposed. The accuracy of this method is checked using a Monte Carlo model, which provides a strong experimental foundation. By examining the influencing factors, the results show that while the density after Halliburton correction is significantly impacted by borehole size, lithology, salinity, and borehole fluid, the density after integrated correction is least affected by these variables. Besides, the integrated density correction method is more favorable for large borehole size, low salinity and sandstone formation. Through logging examples, it is discovered that the overall uncertainty of the density log after the integrated density correction is lower than the Halliburton density correction log, with a maximum of only 0.013 g/cm3, indicating that the density results are more reliable. The study's findings offer a correction technique for precisely estimating the density of cased holes.

Proof-of-concept study on a water phantom-based neutron spectrometer: Experimental test with 252Cf and 241Am-Be sources

Boron neutron capture therapy (BNCT) is a promising cancer treatment that uses energetic ions released from 10B(n, α)7Li reactions. Accurate assessment of neutron energy spectra is important for simulation-based evaluation of neutron doses during BNCT. In this study, a proof-of-concept study was conducted for a neutron spectrometry technique that involves the use of a water phantom, which is commonly used for quality assurance in BNCT, as a moderator. The technique involves applying unfolding to the count rate distribution of the thermal neutron counter measured within the phantom to derive the energy spectrum. We performed experiments using a spherical 3He proportional counter in neutron fields generated by 252Cf and 241Am-Be sources. The results demonstrated that the spectrometer reasonably reproduced neutron spectra and showed the potential of using a water phantom as a moderator for such a technique.

Evaluation of neutron attenuation properties using helium-4 scintillation detector for dry cask inspection

In this paper, we demonstrate the neutron attenuation of dry cask shielding materials using the S670e helium-4 detector manufactured by Arktis Radiation Ltd. In particular, two materials expected to be applied to the TN-32 dry cask manufactured by ORANO Korea and KORAD-21 by the Korea Radioactive Waste Agency (KORAD) were utilized. The measured neutron attenuation was compared with our Monte Carlo N-Particle Transport simulation results, and the difference is given as the root mean square (RMS). For the fast neutron case, a rapid decline in neutron counts was observed as a function of increasing material thickness, exhibiting an exponential relationship. The discrepancy between the experimentally acquired data and simulation results for the fast neutron was maintained within a 2.3% RMS. In contrast, the observed thermal neutron count demonstrated an initial rise, attained a maximum value, and exhibited an exponential decline as a function of increasing thickness. In particular, the discrepancy between the measured and simulated peak locations for thermal neutrons displayed an RMS deviation of approximately 17.3–22.4%. Finally, the results suggest that a minimum thickness of 5 cm for Li-6 is necessary to achieve a sufficiently significant cross-section, effectively capturing incoming thermal neutrons within the dry cask.

Improved fast neutron detection using CNN-based pulse shape discrimination

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.

Nondestructive millimeter-scale void detection for thick steel-shell–concrete interface of immersed tube tunnel: case study

The structural form of sandwich-structured immersed tunnel (SSIT) can be complex. During the casting of self-compacting concrete, creating void defects between the steel shell and concrete interface is not difficult, which can adversely affect the overall safety and service life of the structure. However, detecting millimeter-scale voids covered by a thick steel plate is a technical challenge for current engineering industries. In this study, we proposed a nondestructive millimeter-scale void detection method for SSITs with thick steel shells by combining impact imaging and neutron methods. First, based on the near-source wavefield theory and count rate of thermal neutrons, the void area and depth calculation methods were derived theoretically, and then the coupling detection method and grading criteria for void severity were proposed. Additionally, the void detection performance was validated for a full-scale SSIT model test by blind detection. Finally, the proposed method was applied to the SSIT of the Shenzhen–Zhongshan bridge. The results showed that the proposed method could quantitatively determine the location and distribution pattern of a void; however, it could not accurately determine the void depth. In contrast, the neutron method could accurately calculate the void depth but had a large minimum detectable unit area. The proposed method could effectively compensate for the limitations of both methods. Statistically, the coincidence rate of the model test was 95%, 89%, and 87.5% for the void location, void area, and void depth, respectively, when the error range was ±2 mm. Using this method, 30 tubes in the Shenzhen–Zhongshan bridge were inspected, and by summarizing the void law, suggestions to improve the casting process were proposed, such as adjusting the casting speed. Meanwhile, the void probability decreased significantly. The proposed method provides an important basis for high-quality construction in SSIT projects.

Investigation of Campbell mode with automatic calibration operated in fission chamber-based neutron flux monitoring system on EAST

To satisfy the wide-range requirement for the fission chamber-based neutron flux monitoring (NFM) system on experimental advanced superconducting tokamak (EAST), the pulse-counting and Campbell modes were combined to expand the measurement range. However, to obtain the neutron yields, the results of the Campbell mode should first be converted into neutron count rates via linear calibration. During the plasma experiment, the non-neutron backgrounds vary with respect to different experimental conditions, which leads to a baseline dynamic change in the Campbell mode. In future research, NFM system need provide real-time feedback parameters regarding the operational status of the fusion device. Therefore, an automatic calibration method was developed to automatically deduct various baselines of the Campbell mode and obtain the neutron yields in real time. This method continuously updates the average value of each sub-interval (multiple sub-intervals are set in the calibration region) using a linear superposition averaging method. In addition, to optimize the dual-mode switching threshold, ensure the calibration region and define the upper limit of Campbell mode, a simulated neutron signal generator was designed and applied. The switching threshold of the dual modes was set to 400 kcps, the calibration region of the Campbell mode was selected as 50–500 kcps, and the upper limit of the equivalent count rate in the Campbell mode reached to 108cps with a relative error of<4%. Finally, the automatic calibration method was tested using experimental data from the NFM system on EAST. The automatic calibration results basically overlapped with the reference results and superior to the offline fixed calibration results. Under different experimental conditions, the relative root mean square error (RRMSE) values of the automatic calibration results were lower than those of the offline fixed calibration results. Therefore, the proposed automatic calibration method is accurate and reliable, and can satisfy the real-time measurement requirements of wide-range NFM system.

Development of a compact fast-neutron spectrometer for nuclear emergency response applications

We have developed a Compact Fast Neutron Spectrometer (CFNS) for passive assay of special nuclear material (SNM) through the observation of fast neutrons. The CFNS consists of eight organic glass scintillators (OGS) coupled to silicon photomultipliers and a waveform digitizer, which are integrated within a human-portable box. The CFNS determines the neutron energy profile by spectrum unfolding using the Maximum-Likelihood Expectation Maximization method. The detector acquisition system was optimized to have a dynamic range of up to 10 MeV neutron energy. Bulk special nuclear material (SNM) measurements from the National Criticality Experiments Research Center were analyzed for SNM validation/examination. The results show that the CFNS can be used to distinguish between fission and (α, n) neutron emitters, regardless of intervening material type (Cu and polyethylene) and thickness, by taking the ratio of neutron counts at different regions in the unfolded energy spectrum. Additionally, by fitting an exponential curve to the unfolded energy spectrum of PuO2 and Pu neutron emitters, the CFNS showed the ability of distinguishing between pure Pu oxide, pure Pu metal and mixed oxide-metal configurations.

Time-Dependent Experiment on Reactor Power Distribution Estimation by Ex-Core Detectors at UTR-KINKI

An experiment was conducted that demonstrates a novel core power distribution reconstruction method based on ex-core detectors using time-dependent measurement at the University Teaching and Research Reactor of Kindai University (UTR-KINKI). Although the proposed method PHOEBE was able to identify the power distribution change caused by control rods under static conditions in a previous experiment, time-dependent experiments were not conducted. Hence, the present study measured time-dependent neutron counts using ex-core detectors to reconstruct the power distribution based on PHOEBE. Extraction of the control rods was expected to cause a shift in the reactor power distribution from the north side to the south, and the results of the power distribution reconstruction also demonstrated this power shift. This result experimentally and qualitatively demonstrated the detection of time-dependent power shifts based on PHOEBE. However, quantitative verification was difficult in this study because there are no verified time-dependent three-dimensional neutronics codes available. This issue will be addressed in a future study when a code becomes available.

Neutron Measurements and Monte Carlo Simulations of Spent Nuclear Fuel in Dry Cask Storage Using a New Remote Monitoring System Prototype

Pending the availability of an operational long-term spent nuclear fuel (SNF) repository or other disposal methods, SNF will be increasingly stored in interim dry casks. Casks loaded with commercial SNF may contain several significant quantities of plutonium, so appropriate nuclear material safeguards monitoring is in order. An external remote monitoring system (RMS) developed by researchers at Texas A&M University is proposed to further the current dry cask safeguards regime, which is limited to containment and surveillance mechanisms. In this study, neutron measurements of SNF in dry cask storage were performed with the external RMS at a commercial interim spent fuel storage installation. Corresponding neutron transport simulations using MCNP were conducted with two types of detector responses (tallies) and the results were compared with measurements. The objectives of the study were to add dry cask measurement data to the literature, to assess the performance of the external RMS in full-scale dry cask measurements, and to investigate the degree to which measurements could be estimated with high-fidelity radiation transport simulations. The study demonstrated that the external RMS can acquire neutron count rate measurements with a relative error of less than 0.5% in 5 min or less through the shielding of a dry cask lid. Additionally, the developed simulation model matched trends in the measurement data to a degree that exceeds results in current literature, and normalization factors were calculated to better estimate the magnitude of neutron count rates.

Pulse height estimation and pulse shape discrimination in pile-up neutron and gamma ray signals from an organic scintillation detector using multi-task learning

We developed a multi-tasking deep learning model for simultaneous pulse height estimation and pulse shape discrimination for pile-up n/γ signals. Compared with single-tasking models, our model showed better spectral correction performance with higher recall for neutrons. Further, it achieved more stable neutron counting with less signal loss and a lower error rate in the predicted gamma ray spectra. Our model can be applied to a dual radiation scintillation detector to discriminatively reconstruct each radiation spectrum for radioisotope identification and quantitative analysis.

Benchmarking Neutron Counting System for Passive Measurements and Active Interrogation of Unknown Objects for Fissile and Fissionable Materials Determination

The low rate at which some fissile isotopes, such as 235U, 233U, and 239Pu, undergo spontaneous fission leads to a weak signal, resulting in a high-uncertainty in applying passive neutron counting techniques. Stimulating fission through active neutron interrogation can overcome this issue. At Canadian Nuclear Laboratories, a 252Cf and a deuterium-deuterium neutron source are available. In this study, a neutron counting system was designed to perform passive measurements and active neutron interrogation for a search of special nuclear material. The detection system consists of a cylindrical cavity surrounded by a polyethylene moderator with 3He detectors interspersed throughout. When used for passive measurements, the sample is placed in the cylindrical cavity, whereas in active interrogation mode, the 252Cf neutron source and the sample are placed in close proximity to each other in the cylindrical cavity. Measurements that actively interrogated samples, notably containing (among other isotopes) either 235U or 239Pu whose mass was on the order of fractions of a gram, carried out using the 252Cf neutron source found that the average delayed neutron count rate was on the same order of magnitude as those obtained from passive measurements using several kilograms of natural uranium. The Monte Carlo N-Particle 6 version 2.0 radiation transport code was used to simulate the aforementioned active interrogations and to inform the experimental results. Results showed that, due to the close proximity of the polyethylene moderator to the 252Cf source, the neutron energy spectrum traversing the fissile sample has a significant thermal component that maximizes the fission reaction rate in the interrogated fissile samples, thereby allowing for successful measurements.

Charge-Sign Dependent Cosmic-Ray Modulation Observed with the Calorimetric Electron Telescope on the International Space Station.

We present the observation of a charge-sign dependent solar modulation of galactic cosmic rays (GCRs) with the Calorimetric Electron Telescope onboard the International Space Station over 6yr, corresponding to the positive polarity of the solar magnetic field. The observed variation of proton count rate is consistent with the neutron monitor count rate, validating our methods for determining the proton count rate. It is observed by the Calorimetric Electron Telescope that both GCR electron and proton count rates at the same average rigidity vary in anticorrelation with the tilt angle of the heliospheric current sheet, while the amplitude of the variation is significantly larger in the electron count rate than in the proton count rate. We show that this observed charge-sign dependence is reproduced by a numerical "drift model" of the GCR transport in the heliosphere. This is a clear signature of the drift effect on the long-term solar modulation observed with a single detector.

Performance optimization of the scintillator neutron detectors for EMD in CSNS

Abstract Chinese Spallation Neutron Source (CSNS) has successfully produced its first neutron beam in 28th August 2017. It has been running steadily from March, 2018. According to the construction plan, the Engineering Materials diffractometer (EMD) will be installed between 2019- 2023. This instrument requires the neutron detectors with the cover area near 3m2 in two 90° neutron diffraction angle position, the neutron detect efficiency better than 40%@1Å, and the spatial resolution better than 4mm×200mm in horizontal and vertical directions respectively. We have developed an one-dimensional position sensitive neutron detector based on the oblique 6LiF/ZnS(Ag) scintillators, wavelength shifting fibers and SiPMs (Silicon Photomultipliers) readout. The inhomogeneity of the neutron detection efficiency between each pixel and each detector module, which caused by the inconsistency of the wave-length shifting fibers in collecting scintillation photons, need to be mitigated before the installation. A performance optimization experiment of the detector modules was carried out on the BL20 (Beam Line 20) of CSNS. Using water sample, the neutron beam with Φ5mm exit hole was dispersed relate evenly into the forward space. According to the neutron counts of each pixel of the detector module, the readout electronics threshold of each pixel is adjusted. Compared with the unadjusted detector module, the inhomogeneity of the detection efficiency for the adjusted one is improved from 69% to 90%. The test result of the diffraction peak of the standard sample Si showed that the adjusted detector module works well.

A new neutron-gamma porosity measurement method for pulsed neutron logging tools

As an alternative method for the compensated neutron porosity measurement method, the neutron-gamma porosity measurement method is widely used in cased holes. However, the neutron-gamma porosity measurement method has some problems, such as the lack of detailed theoretical analysis and low sensitivity in high-porosity formations. The purpose of this paper is to clarify the theory of neutron-gamma porosity measurement, improve the measurement sensitivity, and eliminate the effects of environmental factors. Based on the neutron diffusion theory and the gamma transport theory, the distribution of capture gamma rays is described. A new neutron-gamma porosity measurement method is developed based on inelastic and capture gamma-ray distribution. The applicability of the new method in different environments is analyzed using the Monte Carlo method, and the effectiveness of this method is verified using the synthetic model. The results indicate that the capture gamma-ray ratio is only related to the source-detector distance, gamma-ray attenuation length, and neutron migration length. By combining density, different neutron cross sections, inelastic gamma-ray count ratio, and capture gamma-ray count ratio, the parameter secondary gamma-ray hydrogen index (SGH) related to hydrogen index can be extracted. The dynamic range of SGH is much higher than thermal neutron count ratio and the capture gamma-ray count ratio. The errors in porosity calculation by this method under different borehole and lithologic conditions are generally less than 2 pu. The calculation error of this method is close to that of the compensated neutron porosity measurement method. In addition, the applicability of the new method is analyzed without density data, and the results indicate that the calculation results are basically consistent with the results of calculation with density data. The new method has great potential for extensive applications in cased holes or open holes as an alternative for the compensated neutron porosity logging measurement method.