Neutron Counting Research Articles

Power law quantities based on the distributions of time intervals between successive neutron counts

Power law quantities based on the distributions of time intervals between successive neutron counts

A detection system for accurate ([formula omitted], n) neutron counting measurements of low-rate ([formula omitted], n) neutron sources

The determination of the (α, n) neutron yield of (α, n) neutron sources with low absolute total neutron emission rates is a challenging task given the complexity of the (α, n) neutron production mechanism, experimental limitations involved with measurements of weak neutron sources, and, for sources containing special nuclear material, concurrence of the production of (α, n) and fission neutrons. However, (α, n) neutron sources are prevalent in a variety of nuclear engineering disciplines – such as nuclear fuel cycle engineering, nuclear safeguards and nonproliferation, and nuclear waste management – and as such, characterization of physical (α, n) neutron sources and accurate representation of (α, n) neutron production in neutron transport codes are critical aspects of these disciplines. In this work, a moderated 3He neutron detector array with high fidelity listmode data acquisition capability was developed for measurements of low-rate (α, n) neutron sources to support (α, n) reaction nuclear data validation needs for nonproliferation applications as well as to provide experimental data for benchmarking (α, n) neutron source modeling methodologies in Monte Carlo neutron transport codes. Measurements were performed of a calibrated Eckert & Ziegler AmBe (α, n) neutron source individually and simultaneously with 252Cf sources of different strengths to simulate pure (α, n) and mixed (α, n) + fission source conditions. Total neutron counting and coincidence analysis methods developed for system data analysis were applied to the measured data, and deduced (α, n) neutron yield values agreed to within 1% of the with the vendor-calibrated AmBe source (α, n) neutron yield traceable to international standards for all measurements. Total (α, n) neutron yield uncertainties for source conditions dominated by (α, n) neutron emission did not exceed 3%.

Feasibility of irrigation monitoring with cosmic-ray neutron sensors

Abstract. Accurate soil moisture (SM) monitoring is key in irrigation as it can greatly improve water use efficiency. Recently, cosmic-ray neutron sensors (CRNSs) have been recognized as a promising tool in SM monitoring due to their large footprint of several hectares. CRNSs also have great potential for irrigation applications, but few studies have investigated whether irrigation monitoring with CRNSs is feasible, especially for irrigated fields with a size smaller than the CRNS footprint. Therefore, the aim of this study is to use Monte Carlo simulations to investigate the feasibility of monitoring irrigation with CRNSs. This was achieved by simulating irrigation scenarios with different field dimensions (from 0.5 to 8 ha) and SM variations between 0.05 and 0.50 cm3 cm−3. Moreover, the energy-dependent response functions of eight moderators with different high-density polyethylene (HDPE) thickness or additional gadolinium thermal shielding were investigated. It was found that a considerable part of the neutrons that contribute to the CRNS footprint can originate outside an irrigated field, which is a challenge for irrigation monitoring with CRNSs. The use of thin HDPE moderators (e.g. 5 mm) generally resulted in a smaller footprint and thus stronger contributions from the irrigated area. However, a thicker 25 mm HDPE moderator with gadolinium shielding improved SM monitoring in irrigated fields due to a higher sensitivity of neutron counts with changing SM. This moderator and shielding set-up provided the highest chance of detecting irrigation events, especially when the initial SM was relatively low. However, variations in SM outside a 0.5 or 1 ha irrigated field (e.g. due to irrigation of neighbouring fields) can affect the count rate more than SM variations due to irrigation. This suggests the importance of retrieving SM data from the surrounding of a target field to obtain more meaningful information for supporting irrigation management, especially for small irrigated fields.

Optimal Temporal Filtering of the Cosmic-Ray Neutron Signal to Reduce Soil Moisture Uncertainty

Cosmic ray neutron sensors (CRNS) are increasingly used to determine field-scale soil moisture (SM). Uncertainty of the CRNS-derived soil moisture strongly depends on the CRNS count rate subject to Poisson distribution. State-of-the-art CRNS signal processing averages neutron counts over many hours, thereby accounting for soil moisture temporal dynamics at the daily but not sub-daily time scale. This study demonstrates CRNS signal processing methods to improve the temporal accuracy of the signal in order to observe sub-daily changes in soil moisture and improve the signal-to-noise ratio overall. In particular, this study investigates the effectiveness of the Moving Average (MA), Median filter (MF), Savitzky-Golay (SG) filter, and Kalman filter (KF) to reduce neutron count error while ensuring that the temporal SM dynamics are as good as possible. The study uses synthetic data from four stations for measuring forest ecosystem-atmosphere relations in Africa (Gorigo) and Europe (SMEAR II (Station for Measuring Forest Ecosystem-Atmosphere Relations), Rollesbroich, and Conde) with different soil properties, land cover and climate. The results showed that smaller window sizes (12 h) for MA, MF and SG captured sharp changes closely. Longer window sizes were more beneficial in the case of moderate soil moisture variations during long time periods. For MA, MF and SG, optimal window sizes were identified and varied by count rate and climate, i.e., estimated temporal soil moisture dynamics by providing a compromise between monitoring sharp changes and reducing the effects of outliers. The optimal window for these filters and the Kalman filter always outperformed the standard procedure of simple 24-h averaging. The Kalman filter showed its highest robustness in uncertainty reduction at three different locations, and it maintained relevant sharp changes in the neutron counts without the need to identify the optimal window size. Importantly, standard corrections of CRNS before filtering improved soil moisture accuracy for all filters. We anticipate the improved signal-to-noise ratio to benefit CRNS applications such as detection of rain events at sub-daily resolution, provision of SM at the exact time of a satellite overpass, and irrigation applications.

A Feynman-α analysis ranging over time constants of delayed neutrons based on time-sequence counting data consecutively acquired for a very long time

ABSTRACT Time-sequence counting data of a neutron counter were acquired for 17 consecutive days at shutdown state of UTR-KINKI reactor and then a Feynman-α analysis over a very wide gate width range was carried out to give light on a delayed-neutron contribution of all precursors, where statistical error of the correlation amplitude Y was estimated using the bootstrap method. A first plateau of the Y could be observed in a gate width range of several hundred milliseconds. With an enlargement of gate width beyond a time constant of prompt neutrons, the Y monotonically increased above the first plateau of prompt neutrons. Enlarging gate width beyond 100 s, the Y seemed to approach a second plateau of delayed neutrons. The second plateau originated from the delayed neutrons released from the longest-life 87Br. In the gate width range beyond 100 s, however, the statistical error increased sharply. The increasing error indicates a statistical limit resulting from the present measurement time. A value Y p of the first plateau of prompt neutrons was determined from another Feynman- analysis confined to a short gate width range and the Y was divided by the plateau’s value Y p to eliminate an unknown detection efficiency. The Y-to-Y p ratio was calculated using several sets of delayed-neutron group parameters for thermal fission of 235U nuclide, to validate these sets by comparing with the measured ratio to which all precursors contributed. The ratios calculated using Keepin’s 6-group and Spriggs’ 8-group parameters agreed very well with the measured ratio in a gate width range up to 100 s and consequently these parameters could be validated. However, the ratios calculated using the other sets were in a very poor agreement with the measured ratio in the gate width range.

Dual discrimination of fast neutrons from strong γ noise using organic single-crystal scintillator

<h2>Summary</h2> Extensive fast-neutron-based applications and radiation monitoring are inseparable from fast-neutron detection, but the interference of accompanying γ rays requires fast-neutron/γ discrimination (FNGD). Organic single-crystal scintillators (OSCSs) generate fast-neutron-induced delayed fluorescence and γ-induced prompt fluorescence (PF), which can be discriminated by one-step pulse-shape discrimination (PSD). However, PSD cannot identify neutron-induced PF, introducing errors in neutron counting. Here, we report a dual-discrimination approach for accurate fast-neutron detection using γ-ray-insensitive tetraphenylethylene (TPE). In contrast to previous OSCSs, TPE realizes FNGD by PSD and pulse-height discrimination (PHD), since it simultaneously exhibits disparities in shape and pulse height under neutron and γ irradiation. PHD enables secondary FNGD from PF, significantly increasing neutron counts. The improved FNGD method based on TPE reveals a high figure of merit (FoM = 2.4) and count rate of neutrons (35.5%). Our research provides new ideas and available materials for high-efficiency FNGD, promoting the applications of fast neutrons.

Delayed neutron counting of uranium pins using moderated neutrons from portable DD neutron generator in high frequency mode

Delayed neutron counting is often used to verify the characteristics of nuclear material. Use of portable neutron generators in high frequency pulsing mode enables effective analysis with higher counting efficiency and lower radiation protection demands. The paper deals with delayed neutron counting of uranium pins with P385 portable DD neutron generator in polyethylene-based setup. Counting is performed in high frequency mode of neutron generator. The viability of such measurement in the frequency range from 100 Hz to 250 Hz was demonstrated and optimal pulsing parameters for P385 neutron generator were found. Delayed neutron counting was then performed for two types of uranium rods. Delayed neutrons were counted both inbetween neutron pulses during neutron generation and once the emission of neutrons stopped. The results were further validated by Monte Carlo (MCNP) simulations. For the geometry of studied rods, the MCNP calculation were done to calculate the dependence of the response to delayed neutrons on rod enrichment, to show the viability to use the method for rod enrichment verification. An option of using cadmium inset in irradiation channel to overcome the effect of self-shielding for samples with higher enrichment was proposed, experimentally tested, and evaluated through MCNP calculations.

A new porosity evaluation parameter based on neutron porosity logging

The neutron porosity logging is one of the main methods for measuring the formation porosity. However, for the formation with complex mineral compositions and fluid properties, it is difficult to calculate the formation porosity accurately by the neutron porosity logging. In this paper, the relationship between the neutron slowing-down length and the neutron diffusion length and the formation hydrogen index is studied. Based on the thermal-neutron flux formula, a new parameter, apparent formation hydrogen index, is constructed by the neutron counting ratio and the formation density and the capture cross section. The apparent formation hydrogen index is close to the formation hydrogen index and is not affected by the excavation effect. The response characteristic of the apparent formation hydrogen index is more consistent with the volume physical model. The porosity calculated by the new parameter is closer to the formation porosity, especially in the gas-bearing formation.

Long-term evolution of the neutron rate at the Canfranc Underground Laboratory

We report results on the long-term variation of the neutron counting rate at the Canfranc Underground Laboratory, of importance for several low-background experiments installed there, including rare-event searches. The measurement campaign was performed employing the High Efficiency Neutron Spectrometry Array (HENSA) mounted in Hall A and lasted 412 live days. The present study is the first long-term measurement of the neutron rate with sensitivity over a wide range of neutron energies (from thermal up to 0.1 GeV and beyond) performed in any underground laboratory so far. Data on the environmental variables inside the experimental hall (radon concentration, air temperature, air pressure and humidity) were also acquired during all the measurement campaign. We have investigated for the first time the evolution of the neutron rate for different energies of the neutrons and its correlation with the ambient variables.

Fission chamber data acquisition system for neutron flux measurements on the Mega-Amp Spherical Tokamak Upgrade.

Neutron flux measurements are important in fusion devices for both safety requirements and physics studies. A new system has been built for the Mega-Amp Spherical Tokamak Upgrade (MAST Upgrade) that provides neutron count, DC, and Campbell mode measurements for a 1 µs period at 1MHz. The acquisition system uses a Red Pitaya board to sample current from two fission chambers mounted on the side of the MAST-U vessel. The system-on-chip design of the Zynq-7020 on the Red Pitaya also allows a web server implementation using Flask for data retrieval and diagnostic configuration over the MAST Upgrade network.

Mitigation of long-lived modes by lower hybrid wave on the HL-2A tokamak

Mitigation of long-lived mode (LLM) by lower hybrid wave (LHW) is recently achieved on the HL-2A tokamak. The LLM changes from a typical steady-state to a fishbone-like frequency-chirping characteristic, and its higher poloidal harmonics disappear when LHW is injected into toroidal plasma. It is found that density fluctuation declines during this process while the total neutron count increases gradually. Those evidences indicate there is a mitigation effect of LHW on LLM, and the underlying mechanism can be explained as follows. On one hand, high power LHW firstly causes a drop in toroidal rotation and then results in decline of E × B shear, which is unfavorable for the maintenance of the internal transport barrier. The resulting relaxation of ion temperature gradient enhances thermal transport and leads to a drop of ion temperature at the core region. Thus, the plasma pressure becomes flattened and finally contributes to the mitigation of highly saturated internal mode. On the other hand, hybrid simulation by M3D-K suggests that the off-axis LHW can also reduce the grower rate of LLM via changing safety factor and magnetic shear.

Real-time pulse shape discrimination for neutron scattering science using EJ-270

A detector for neutron scattering applications utilizing the 6Li loaded pulse shape discrimination scintillator EJ-270 was developed. The detector utilises a pulse shape discrimination algorithm developed for high-count rate applications implemented in FPGA to enable real time operation. The implemented algorithm we call Tail Sum was specifically developed for high-rate thermal neutron detection and implemented into a 2 channel 500 MSps 14-bit ADC with a Xilinx Kintex 7 K325T FPGA. The detector consisted of a 48 mm diameter × 15 mm thick cylinder of EJ-270 mounted on a Hamamatsu R6231-100 PMT and an E1198-2 base at -1000 V. The performance of the system was evaluated using a moderated 241Am-Be mixed radionuclide source, a figure of merit of 1.605 ± 0.007 was measured. The γ-ray sensitivity of the system was measured using a 60Co source and found to be 3.23 × 10-6 ± 7 × 10-8. The high neutron count rate capability of the system was tested on the Emma instrument at the ISIS pulsed neutron and muon source based at the Rutherford Appleton Laboratory, a peak count rate of 5 × 105 neutrons per second was measured on a single channel and was found to have no statistically significant deviation from the expected count-rate.

Advancements in Gd-based neutron detection (2): Proton–gamma correlation approach

The paper introduces new developments in the conception, optimization, and principle validation of a neutron counter, based on the insertion of gadolinium in plastic scintillators, that is under work at CEA LIST since 2017. The radiation sensor, comprising two volumes of scintillating polymers, it is, by nature, sensitive to both neutron and gamma radiations. This allows the design of a versatile n/γ detector, provided that it is possible to algorithmically separate the signature of both types of radiations. After amplitude and gamma–gamma coincidence filtering, the authors investigate a third n/γ discrimination mode by proton–gamma temporal correlation filtering. After a bibliographic study, the article details the estimation of the temporal parameter useful for such a discrimination, firstly by explicit calculations, secondly by means of a MCNP6.1-based, dedicated simulator. A time-based-processing firmware, embedded into a pulse acquisition card, was then created to compare these estimates with experimental distributions. A 50-μs temporal gate was identified as relevant to discriminate temporal correlations in presence of a neutron emitter. A compensation technique was eventually implemented in order to obtain first experimental estimates of neutron sensitivity, and neutron detections limits in a mixed field. The study ends up providing a maximum reachable sensitivity in the order of 0.3 c.n−1.cm 2 under a fast neutron flux (AmBe), with two 1-L plastic scintillators separated by a 250-μm Gd converter, and the associated temporal correlation and compensation algorithm, as well as neutron detection limits in the same order as those of a He-3 Bonner sphere over acquisition times between 1 and 10 min.

Assay of an alpha solid waste drum using passive neutron counting

Assay of an alpha solid waste drum using passive neutron counting

Demonstration of power distribution estimation using ex-core detectors by reactor experiment at UTR-KINKI

ABSTRACT A novel core power distribution reconstruction method based on ex-core detectors is demonstrated at the Kindai University Reactor UTR-KINKI. Technology to monitor the interior of the core is critical for assuring nuclear safety. Typical core monitoring systems utilize in-core detectors. However, the harsh environment inside the core degrades detector maintainability and reliability. On the other hand, monitoring using ex-core detectors is difficult due to the lack of information of neutrons. A previous study, therefore, proposed a new power distribution reconstruction method using power correlation between fuel regions, PHOEBE. However, PHOEBE has not been verified experimentally. Thus, the present study demonstrates PHOEBE at UTR-KINKI. The experiment measures the neutron counts using four detectors outside the reflector, and the power distribution was found to be well reconstructed. Furthermore, the present study estimates the power distribution using only two detectors for confirming the effect of power correlation. When the control rod had a large impact on detectors, estimated power distribution was emphasized. By contrast, small control rod effect cases suggested the advantage of PHOEBE. However, a more complex experimental geometry, such as a multi-layered fuel region, is required to more clearly evaluate the effect of considering the power correlation between fuel regions.

Ultradian Near 2-4-h Rhythms of Body Temperature in Laboratory Rodents Depend on External Environmental Heliogeophysical Factor Reflected by Neutron Monitor Count Rate.

The study collated oscillations of body temperature of С57BL/6 mice and Roborovski hamsters (Phodopus roborovskii) in the period range of 100-240 min and fluctuations of the secondary cosmic rays reflected by neutron count rate of a neutron monitor at the Earth's surface. Cross-correlation of body temperature and neutron count rate data revealed their strong association only for simultaneous measurements. Collation of the phase of neutron count rate fluctuations with maxima of body temperature oscillations revealed the coincidence of the phases of both processes. There was a temporary connection between the periods of body temperature elevation and neutron count rate rise. Thus, not only the spectral parameters of biological and physical processes, but also their phases coincided, which corroborates the hypothesis on stimulating effect of increasing neutron count rate on activity of the animals.

Reactivity estimation based on the linear equation of characteristic time profile of power in subcritical quasi-steady state

ABSTRACT The reactivity was estimated from a time profile of neutron count rate or simulated data in a quasi-steady state (QSS) after a sudden status change of reactivity or external neutron source strength. The estimation was based on the equation of power in subcritical QSS. The purpose of the study is to develop the method of timely reactivity estimation from complicated time profile of neutron count rate. The developed method was applied to the data simulating neutron count rate created by using one-point kinetics code, AGNES, and Poisson-distributed random noise. It was also applied to the transient subcritical experiment data measured by using transient experiment criticality facility. The result shows that the difference of the estimated and reference values was within about 5% or less for for simulated data and within about 7% or less for ρ$ ≃1.4 and for the experimental data. The possibility of the reactivity estimation several 10 seconds after the status change was also shown.

Theoretical Derivation of a Unique Combination Number Hidden in the Higher-Order Neutron Correlation Factors Using the Pál-Bell Equation

The Pál-Bell equation is a backward-type master equation that describes the probability generating function (PGF) of neutron counts in a neutron multiplication system. Thanks to the Pál-Bell equation with the two-forked and the fundamental mode approximations, an analytical solution of PGF of neutron counts in a source-driven subcritical system can be theoretically derived. This theoretical derivation clarifies that the unique combination number of double factorial (2n−3)!! does exist in the ratio of the higher-order neutron correlation factors measured in a critical state even for any kind of fissile and moderator materials. Additionally, the unique combination numbers are experimentally validated for the order 3 ≤ n ≤ 6 through reactor noise measurements in actual subcritical systems. This knowledge can be used to judge whether a target system is in a deep subcritical state or to roughly estimate the magnitude of subcriticality, based on the factorial moments of the measured reactor noise in a zero-power state.

Cosmic-ray atmospheric cutoff energies of polar neutron monitors

The atmospheric cutoff, similarly to the geomagnetic cutoff, is the lower energy limit for cosmic ray particles that can reach a given location on the ground and be registered by a detector there, e.g., by a neutron monitor. It is caused by the decreasing intensity of a cosmic-ray cascade in the lower atmosphere. Although the geomagnetic cutoff is higher than atmospheric over the most of the Earth’s surface, the latter is dominant and therefore defines the neutron monitor count rate in the polar regions. The atmospheric cutoff decreases with the altitude, and this provides additional sensitivity of high-altitude polar neutron monitors to low-energy particles, mainly solar energetic protons during the so-called ground-level enhancement events.In this work, we quantitatively estimated the atmospheric cutoff energies for 21 polar neutron monitor stations in conditions without a significant solar energetic particle event. The cutoff value can be as low as about 300 MeV for protons (VOSTOK and DOMC/DOMB stations), which is notably lower than about 430 MeV at sea level. In addition to that, we estimated the worst case scenario of the strongest-ever observed ground-level enhancement event GLE#05, occurred on the 23rd of February, 1956, and showed that the atmospheric cutoff becomes as low as about 100 MeV. In other words, some neutron monitor stations can register particles with energies of even about 100 MeV during an exceptionally strong solar particle event. It is explained by the highly intensive and soft spectrum of the event in its early delayed phase.

The Mini Labyrinth Benchmark for Radiation Protection and Shielding Analysis

The Mini Labyrinth experiment is a neutron and gamma shielding experiment currently being developed at the Slovak University of Technology, Bratislava (STU). The STU Mini Labyrinth consists of NEUTRONSTOP shielding blocks, water tank, PuBe neutron source, and graphite prism. This article describes the second construction version of the Mini Labyrinth experiment and presents the newest results of the neutron and gamma fields’ simulation and measurement. The PuBe neutron source with the emission rate of 1.06E7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{n}\cdot \text{s}^{-1}$ </tex-math></inline-formula> was used in the experiment. The measurement of gamma ambient dose equivalent H*(10) and neutron count rates is performed by the Thermo Scientific RadEye portable survey meter and the SNM-11 BF3-filled corona detector. The simulation part was carried out using the state-of-the-art MCNP6 and SCALE6 MONACO stochastic calculation tools, considering the detailed geometry of the Mini Labyrinth and a combined neutron and gamma source of particles. The comparisons were performed between codes and the experiment. The propagation of cross-sectional uncertainties was investigated through the shielding analysis. Almost perfect agreements between simulation codes were achieved. The comparison with measurement suggests the further needs of room effect assessment.