Neutron Counting Research Articles

Design and Implementation of Matryoshka-type Neutron Spectrometer.

Aiming at portability and mobility, the design and implementation of a portable neutron spectrometer, namely the Matryoshka-type Neutron Spectrometer, was completed while considering the idea of a Bonner Sphere Spectrometer. The Matryoshka-type Neutron Spectrometer used a spherical 3He proportional counter as a thermal neutron detector. A number of cylindrical acrylic cups with diameters from 4 cm to 40 cm were made as containers for water as a moderator. The Monte Carlo method was adopted in order to obtain the energy response matrix of the Matryoshka-type Neutron Spectrometer. During the measurement, each acrylic cup contained water as a moderator, and the 3He proportional counter was set to the geometric center to perform neutron counting. With the energy response matrix obtained above, the neutron spectrum was resolved by a classical inversion algorithm. After the measurement, water was drained and the acrylic cups were placed one inside the other, like a Matryoshka doll. The design of a Matryoshka-type Neutron Spectrometer has reduced the mass and volume of the Bonner Sphere Spectrometer and was not difficult to carry around. Comparison tests acquiring background and Am-Be source neutron spectra between the Matryoshka-type Neutron Spectrometer and Bonner Sphere Spectrometer proved the effectiveness of the Matryoshka-type Neutron Spectrometer.

DSP Implementation of Neutron Radiation Detection Based on Kalman Filter

Aiming at the situation of nuclear neutron pulse count distortion and low measurement accuracy, a digital processing chip and Kalman filter are designed to replace the traditional hardware filtering method. The system uses TMS320F28335 as the main control chip, and takes advantage of the DSP-specific digital signal processing chip. After using the extended Kalman filter to reduce the noise and filter the pulse neutron count, the experiment proves that the nuclear pulse neutron count measurement after the extended Kalman filter process is stable, and the measurement accuracy has been significantly improved. The overall structure and working principle of the system are introduced, and specific hardware and software design schemes are given.

Neutron flux and spectrum in the Dresden Felsenkeller underground facility studied by moderated He3 counters

Ambient neutrons may cause significant background for underground experiments. Therefore, it is necessary to investigate their flux and energy spectrum in order to devise a proper shielding. Here, two sets of altogether ten moderated $^3$He neutron counters are used for a detailed study of the ambient neutron background in tunnel IV of the Felsenkeller facility, underground below 45 meters of rock in Dresden/Germany. One of the moderators is lined with lead and thus sensitive to neutrons of energies higher than 10 MeV. For each $^3$He counter-moderator assembly, the energy dependent neutron sensitivity was calculated with the FLUKA code. The count rates of the ten detectors were then fitted with the MAXED and GRAVEL packages. As a result, both the neutron energy spectrum from 10$^{-9}$ MeV to 300 MeV and the flux integrated over the same energy range were determined experimentally. The data show that at a given depth, both the flux and the spectrum vary significantly depending on local conditions. Energy integrated fluxes of $(0.61 \pm 0.05)$, $(1.96 \pm 0.15)$, and $(4.6 \pm 0.4) \times 10^{-4}$ cm$^{-2}$ s$^{-1}$, respectively, are measured for three sites within Felsenkeller tunnel IV which have similar muon flux but different shielding wall configurations. The integrated neutron flux data and the obtained spectra for the three sites are matched reasonably well by FLUKA Monte Carlo calculations that are based on the known muon flux and composition of the measurement room walls.

Long-Time Correlations in Single-Neutron Interferometry Data

We present a detailed analysis of the time series of time-stamped neutron counts obtained by single-neutron interferometry. The neutron counting statistics display the usual Poissonian behavior, but the variance of the neutron counts does not. Instead, the variance is found to exhibit a dependence on the phase-shifter setting which can be explained by a probabilistic model that accounts for fluctuations of the phase shift. The time series of the detection events exhibit long-time correlations with amplitudes that also depend on the phase-shifter setting. These correlations appear as damped oscillations with a period of about 2.8 s. By simulation, we show that the correlations of the time differences observed in the experiment can be reproduced by assuming that, for a fixed setting of the phase shifter, the phase shift experienced by the neutrons varies periodically in time with a period of 2.8 s. The same simulations also reproduce the behavior of the variance. Our analysis of the experimental data suggests that time-stamped data of singleparticle interference experiments may exhibit transient features that require a description in terms of non-stationary processes, going beyond the standard quantum model of independent random events.

Development and evaluation of a list mode neutron coincidence collar for spatial response measurements of fresh fuel assemblies

The capabilities of current safeguards neutron instrumentation can be expanded by acquiring and analyzing data in new ways. A new List Mode Collar (LMCL) was created by retrofitting a traditional safeguards neutron coincidence counting system, the Neutron Coincidence Collar, with newly developed preamplifiers on each of its 18 3He tubes in active interrogation mode. By applying list mode data acquisition (LMDA) and analysis, a signal from each 3He tube can now be recorded and analyzed to determine the neutron count rate measured in each individual 3He tube. Further, the count rate distribution across all 3He tube locations in the system can be determined. This is the first time that a full list mode method has been implemented using a traditional neutron coincidence collar and spatial response data has been obtained.The objective of this research was to develop and evaluate a laboratory demonstration list mode neutron coincidence collar (LMCL) that can extract a greater number of useful signatures than are currently generated using the current collar. A new analysis method called the “List Mode Response Matrix” has also been developed for use in combination with these measurements to improve the detection of missing fuel pins (partial defects) during the nondestructive assay of fresh nuclear fuel assemblies, although analysis algorithms will be reported separately from this experimental evaluation. In this paper, 18 channel spatial response data are presented for a 252Cf source moving within the LMCL detector cavity. LMDA, combined with the addition of multiple preamplifiers, facilitates this capability by increasing the number of signals that can be measured simultaneously; allowing for in-depth analysis of neutron coincidence events to determine the location and distribution of nuclear material within the measurement geometry. This approach enables a spatial response measurement to form a “pattern” or item signature unique to the fissioning source’s distribution; in the case of the LMCL, the distribution of fresh low-enriched uranium fuel within the assembly. A major benefit provided by LMDA is that this can all be achieved from a single measurement pulse train in offline analysis.To experimentally demonstrate this concept, a Mirion Technologies (Canberra) model JCC-71 Neutron Coincidence Collar has been retrofitted with new electronics designed at Oak Ridge National Laboratory. This paper presents the results from the experimental evaluation of the laboratory demonstration system. The results indicate equivalent performance of the LMCL to the original collar in addition to improved spatial response capabilities, while maintaining the original system footprint.

Source multiplication measurements and neutron correlation analyses for a highly enriched uranium subcritical core driven by an inherent source in Kyoto University Critical Assembly

ABSTRACT For a highly enriched uranium subcritical core driven by an inherent or an external Am-Be source in Kyoto University Critical Assembly (KUCA), a series of the source multiplication measurements, the Feynman-α and the Rossi-α neutron correlation analyses was carried out to demonstrate quantitatively that the use of the former inherent source had some advantages over that of the latter external source. Under a reactor drive by the external source, the subcriticality obtained by the neutron source multiplication method (NSM) significantly depended on neutron counter position and was consequently in very poor agreement with that calculated by the continuous-energy Monte Carlo code MVP (version 3) with the nuclear data library JENDL-4.0. In contrast to the above drive, the other drive by the inherent source spectacularly reduced the counter-position dependence and lead to a good agreement with the calculation by the MVP. The prompt-neutron decay constant determined from the Feynman-α analysis under the external source was also significantly dependent on the counter position. The Rossi-α analysis had a relatively slight counter-position dependence of the decay constant to the Feynman-α analysis but had a very low S/N ratio, which indicated an inferior applicability of the Rossi-α analysis to the reactor drive and enlarged a statistical uncertainty of the decay constant. On the other hand, the reactor drive by the inherent source considerably reduced the counter-position dependence of the Feynman-α analysis and spectacularly enhanced the S/N ratio of the Rossi-α analysis. An effective strength of the inherent source could be also determined from the S/N ratio. When the prompt-neutron decay constant obtained under this reactor drive was transformed into each subcriticality by the Simmons and King method, where a subcritical dependence of the generation time and the effective delayed-neutron fraction was neglected, the subcriticality transformed seemed to agree well with that calculated by the MVP. When the decay constant was transformed by the inhour method, where the subcritical dependence of these kinetic parameters was considered, a systematic disagreement between the transformed and the calculated subcriticality appeared.

Neutron/Gamma separation in [formula omitted] thick single crystal diamonds

Single Crystal Diamond (sCD) are candidate neutron detectors for the ITER Radial Neutron Camera (RNC) due to their capability of performing both neutron counting and spectroscopy while withstanding high levels of radiation. This paper presents a study on methods of separation between neutron and gamma events, through the analysis of the pulse shapes of a 500μm thick sCD. Methods for gamma rejection may be beneficial especially during ITER deuterium–deuterium operation to extract the 2.5 MeV neutron signal. Four neutron/gamma separation algorithms for sCD neutron detectors are presented. Three algorithms rely on pulses shape information and one on the frequency analysis. The final experimental implementation based on multi-step separation approach achieved above 90% of correctly discriminated pulses.

Error Estimation for Soil Moisture Measurements With Cosmic Ray Neutron Sensing and Implications for Rover Surveys

Cosmic ray neutron (CRN) sensing allows for non-invasive soil moisture measurements at the field scale and relies on the inverse correlation between aboveground measured epithermal neutron intensity (1 eV−100 keV) and environmental water content. The measurement uncertainty follows Poisson statistics and thus increases with decreasing neutron intensity, which corresponds to increasing soil moisture. In order to reduce measurement uncertainty, the neutron count rate is usually aggregated over 12 or 24 h time windows for stationary CRN probes. To obtain accurate soil moisture estimates with mobile CRN rover applications, the aggregation of neutron measurements is also necessary and should consider soil wetness and driving speed. To date, the optimization of spatial aggregation of mobile CRN observations in order to balance measurement accuracy and spatial resolution of soil moisture patterns has not been investigated in detail. In this work, we present and apply an easy-to-use method based on Gaussian error propagation theory for uncertainty quantification of soil moisture measurements obtained with CRN sensing. We used a 3rd order Taylor expansion for estimating the soil moisture uncertainty from uncertainty in neutron counts and compared the results to a Monte Carlo approach with excellent agreement. Furthermore, we applied our method with selected aggregation times to investigate how CRN rover survey design affects soil moisture estimation uncertainty. We anticipate that the new approach can be used to improve the strategic planning and evaluation of CRN rover surveys based on uncertainty requirements.

Analysis of Active Neutron Measurements From the Mars Science Laboratory Dynamic Albedo of Neutrons Instrument: Intrinsic Variability, Outliers, and Implications for Future Investigations

AbstractThe Dynamic Albedo of Neutrons (DAN) instrument onboard Mars Science Laboratory uses neutron die‐away, an active nuclear spectroscopy technique, to measure the abundance and depth distribution of hydrogen and neutron‐absorbing elements (e.g., Fe and Cl) in the top ∼0.5 m of the Mars subsurface. We examined the intrinsic variability in neutron die‐away data using simulated DAN measurements with a range of compositions relevant to equatorial and high‐latitude environments on Mars. Our analysis shows that the total neutron counts and timing of neutron arrival explain most variability in thermal neutron die‐away curves for a DAN‐like instrument configuration over homogeneous subsurfaces. We analyzed the variability in thermal neutron die‐away across a variety of elemental compositions that might be observed by DAN or future instruments. We found that when H concentration is low (e.g., in equatorial regions including Gale crater), neutron die‐away is most sensitive to variations in H. Conversely, when H concentration is high (e.g., in poleward regions of Mars or icy bodies), neutron die‐away is most sensitive to variations in neutron absorbers. We estimated that the H and neutron absorber concentrations in DAN measurements acquired Sols 1–2080 in Gale crater and found that most measurements had low H and moderate absorber content. We presented an outlier analysis of these measurements to identify targets with high potential scientific impact and found that most outliers along the traverse corresponded with abnormally low or high neutron absorber content measurements, some of which may be associated with high‐silica fracture‐associated halos and felsic igneous material.

Applications of HPGe-detected high energy gamma rays toward quantifying neutron emission rates and 234U enrichment in UF6 cylinders

This study presents a 234U enrichment meter that uses high energy gamma and neutron count rate as measured by a portable, high-purity germanium gamma ray detector. Twenty-one 30B cylinders containing UF6 of varying origin were analyzed with an HPGe detector for high-energy gamma ray and neutron counts. Excellent correlations (R2≥ 0.95) between various regions of ≥3.0 MeV gamma rays, neutron count rate, and uranium enrichment were found. These neutron and high-energy gamma data are used to create calibration-dependent enrichment meters for 234U. The 234U enrichment calculated from the neutron count rate and 4 – 6 MeV gamma count rate is more accurate than methods that rely on the 120.9 keV peak which is strongly attenuated inside commercial sized UF6 cylinders. These neutron and high-energy gamma enrichment calculators can also be used to determine a 235U enrichment, assuming mass-based enrichment from natural uranium, but the results are not as accurate as the traditional 185.7 keV peak enrichment meter.

A mm3 Fiber-Coupled Scintillator for In-Core Thermal Neutron Detection in CROCUS

This article presents the development, full-characterization, and in-core testing of a miniature neutron detector for the investigation of highly localized in-core neutronics effects in the zero-power reactor CROCUS, operated at the Ecole Polytechnique federale de Lausanne (EPFL), Switzerland. A ZnS: 6LiF screen, mixing inorganic scintillator and neutron converter, with a surface of 1 mm2 and a thickness of 0.2 mm, was coupled with a silicon photomultiplier (SiPM) via a 10-m optical fiber. Analog readout electronics were adopted in this prototype version to process the SiPM output signal. A first campaign was carried out to assess the detection system capabilities in terms of neutron detection and counting in mixed neutron-gamma radiation fields. The miniature detector was thoroughly characterized with a Pu-Be source installed in the CARROUSEL facility and was subsequently tested inside a control rod guide tube of the CROCUS reactor in different reactor conditions, i.e., at shutdown, startup, and critical. The detector shows a linear response with a reactor power increase up to 6.5 W (i.e., around $10 {^{\mathrm{ 8}}}\,\,\text {cm}^{-2}\cdot \text {s}^{-1} $ total neutron flux) and excellent neutron counting capabilities, when compared to other localized detection systems available in CROCUS, such as the miniature fission chambers and single-crystal vapor deposited (sCVD) diamond detectors. In addition, the exposure of the miniature detector to a 60Co source with an activity of ~250 GBq, combined with the computation of the gamma flux in CROCUS with the Serpent 2 Monte Carlo code, confirmed the gamma insensitivity of the system in a mixed neutron-gamma field.

Study of neutron moderation properties of liquid moderators in room and cryogenics temperature

Neutron moderation is crucial to sustain the chain reactions in a nuclear fission reactor. To understand further basic characteristics of neutron moderation, this research investigated fundamental moderating properties of liquid moderators typically used in modern power reactors, namely water and salt solution. Specifically, we employed a small angle neutron scattering (SANS) technique at beam-port number 4 of Malaysia’s PUSPATI TRIGA Reactor to study moderating characteristics of the liquid moderators at both room and cryogenic temperatures. To achieve cryogenic temperatures, a leakage-proof liquid sample holder was placed in a cryostat cooled by liquid nitrogen. Moderated and unmoderated neutron spectra were collected using Microspec-6 for analysis, which clearly shows that peak neutron counts dropped at cryogenic from room temperatures.

Performance Assessment of Amplification and Discrimination Electronic Devices for Passive Neutron Measurements

The assessment of fissile material mass is a key requirement to enhance radioactive waste management and to ensure a high level of safety in the nuclear industry. The assessment of plutonium fissile mass by passive coincidence measurements is usually obtained by detecting the neutrons generated by the spontaneous fissions of Pu isotopes. The detection and the quantification of a fissile mass in the radioactive waste can also be carried out using active neutron interrogation with a pulsed D-T neutron generator. The 14-MeV neutrons are moderated to induce fissions in the uranium and plutonium-contaminated waste. The emitted neutron signal can be analyzed by considering their detection time after the generator pulse. Data are measured according to the principles of the neutron measurement techniques. As proportional counters filled with 3He gas display high neutron detection efficiency and good gamma-ray immunity, they are the reference detector for passive neutron coincidence counting. A charge preamplifier or a current amplifier, depending on applications, collects the electric pulse produced by neutron interaction in the 3He gas, and a threshold discriminator produces a logic pulse used for neutron counting. This article describes the performance assessment of different commercially-available electronics from Mirion Technologies, Precision Data Technology (PDT), Mesytec, as well as MONACO electronics originally developed by CEA LIST for fission chamber measurements in experimental reactors. Comparative passive neutron measurements are carried out with these electronics at CEA/DEN Nuclear Measurement Laboratory in Cadarache. Overall, PDT and Mesytec electronics show similar detection efficiency as the ACH-NA98 charge amplifier, which is commonly used in our laboratory for such applications. However, MONACO electronics have a lower detection efficiency, similar to the Mirion 7820 current amplifier used in specific high-count rate applications. An optimization of MONACO settings would probably be necessary to adapt to 3He counters instead of fission chambers.

Direct Determination of a Bare Neutron Counter Yield Function

AbstractGround‐based neutron counters are a standard tool for detecting atmospheric showers from GeV range primary cosmic rays of either solar or galactic origin. Bare neutron counters, a type of lead‐free neutron monitor, function much like standard neutron monitors but have different yield functions primarily because they are more sensitive to neutrons of lower energy. When operated together with standard monitors, the different yield functions allow estimates to be made of the energy spectrum of galactic or solar particles. In 2010 a new array of 12 bare neutron detectors was installed at the South Pole to operate together with the neutron monitor there. Prior to installation, two of the detectors were operated on a ship that traveled from Sweden to Antarctica and back from November 2009 to April 2010. The purpose of this latitude survey was to use Earth's magnetic field as a spectrometer, blocking cosmic rays below the local cutoff rigidity (momentum per unit charge), from which we determined the response function versus rigidity of these bare counters. By comparing that measured response function to direct measurements of the cosmic ray spectrum taken by the PAMELA spacecraft, we were able to make a direct determination of the yield function for these detectors.

Opportunities and challenges in using catchment-scale storage estimates from cosmic ray neutron sensors for rainfall-runoff modelling

Adequate characterization of catchment storage dynamics is crucial in hydrological models, yet scale-representative storage measurements are rare. Recent developments in Cosmic Ray Neutron Sensor (CRNS) technology and monitoring networks provide a powerful source of more scale-appropriate soil moisture data for many modelling applications. However, the potential in rainfall-runoff modelling is undeveloped. Here we present the first application of CRNS data in conceptual rainfall-runoff modelling and explore this potential in the context of a mixed-agricultural landscape in Scotland. We deployed and calibrated a CRNS in a heterogeneous soil-land use footprint over a ∼3-year period. In this generally wet environment, the CRNS shallow sensing depth and relatively high neutron count uncertainty were identified as major challenges. However, given the better spatial coverage (up to 14 ha) and ease for maintenance, CRNS was thought to represent the simplest approach for long-term monitoring of managed mixed-agricultural sites. We used CRNS-derived, as well as single point-scale estimates, of near-surface soil storage (SNS) to explore their characterisation of storage dynamics at the catchment-scale. Inter-comparison using linear regression showed that SNS related well to catchment-scale storage dynamics, however this relationship was stronger for CRNS (R2 = 0.91) compared to point-scale derived estimates (R2 = 0.76). Based on this, we evaluated the effect of using the CRNS and point scale derived SNS data to constrain storage estimates controlling runoff generation in a common rainfall-runoff model (HBV-light). Including CRNS or point-scale field SNS data alone in model calibration was especially useful for intermediate and wet periods. A combined model calibration using discharge and either SNS storage estimates provided a better representation of catchment internal dynamics, additionally reducing uncertainty during low flows. In the context of mixed-agricultural landscapes in humid environments, this study showed the potential of using CRNS over point scale data (in terms of representativeness for single point data and practicality for point sensor networks) to characterise the catchment storage-discharge relationship and inform hydrological modelling.

Nondestructive measurements of residual 235U mass of Israeli Research Reactor-1 fuel using the Advanced Experimental Fuel Counter

In 2018, a measurement campaign took place with participants from Los Alamos National Laboratory (LANL), the Nuclear Research Centre-Negev (NRCN) and Soreq Nuclear Research Center (SNRC) at the Israeli Research Reactor-1 (IRR-1) in which 14 of the reactor’s used fuel assemblies (FAs) with varied amount of depletion were measured with the nondestructive assay instrument Advanced Experimental Fuel Counter (AEFC). Designed for safeguards purposes, the AEFC measures both neutrons emitted from the FA (passive neutrons) and fission neutrons induced by an external neutron source (in this experiment, 252Cf). Signals recorded with the AEFC include total neutron count rates (Singles), time-correlated neutron count rates (Doubles), and total gamma-ray count rates. The 235U content of the FAs was previously assessed by two independent methods: (1) measurement of the transparency of the FA to low-energy gamma rays from an activated rhenium source (rhenium gamma transmission, or the RGT method) and (2) calculation of the ∼30-year burnup history of the core using detailed three-dimensional Monte-Carlo core depletion calculations. The results from the FAs that had been measured via the RGT method were used to construct the calibration curves, which translate the AEFC count rates to 235U mass. Then, the calibration was evaluated using AEFC measurements of six additional FAs that were not measured via the RGT method. From the results, it was determined the Doubles calibration curve was more reliable than that of the Singles and follows a simple second-order polynomial fit for the whole range of residual 235U mass content, albeit with larger statistical uncertainty.Detailed uncertainties quantification was conducted for both the AEFC Singles and Doubles. This includes the analysis of statistical uncertainties, calibration uncertainty, and random uncertainties due to the sensitivity of the AEFC to several sources of uncertainty, namely the FA position, FA orientation, interrogation source position, and ambient pool temperature. An overall total uncertainty of 6 g of 235U is estimated for the Singles and Doubles, which is mainly due to calibration uncertainty (for the Singles) and statistical uncertainty (for the Doubles), and which constitutes 3%–6% of the 235U total mass in the FAs, depending on their level of depletion.

A linear equation of characteristic time profile of power in subcritical quasi-steady state

ABSTRACT An equation of power in subcritical quasi-steady state has been derived based on one-point kinetics equations for the purpose of utilizing it for the development of timely reactivity estimation from complicated time profile of neutron count rate. It linearly relates power, P, to a new variable q, which is a function of time differential of the power. It has been confirmed that the points (q, P) calculated by using one-point kinetics code, AGNES, are perfectly in a line described by the new equation and that the points (q, P) calculated from transient subcritical experiment data measured by using TRACY made a line with a slope indicated by the new equation.

Assimilation of Cosmic‐Ray Neutron Counts for the Estimation of Soil Ice Content on the Eastern Tibetan Plateau

AbstractAccurate observations and simulations of soil moisture phasal forms are crucial in cold region hydrological studies. In the seasonally frozen ground of eastern Tibetan Plateau, water vapor, liquid, and ice coexist in the frost‐susceptible silty‐loam soil during winter. Quantification of soil ice content is thus vital in the investigation and understanding of the region's freezing‐thawing processes. This study focuses on the retrieval of soil ice content utilizing the in situ soil moisture (i.e., liquid phase) and cosmic ray neutron measurements (i.e., total water including liquid and ice), with Observing System Simulation Experiments. To derive the total soil water from neutron counts, different weighting methods (revised, conventional, and uniform) for calibrating the cosmic‐ray neutron probe (CRNP) were intercompared. The comparison showed that the conventional nonlinear method performed the best. Furthermore, to assimilate fast neutrons using the particle filter, the STEMMUS‐FT (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil) model was used as the physically based process model, and the COSMIC model (Cosmic‐ray Soil Moisture Interaction Code) used as the observation operator (i.e., forward neutron simulator). Other than background inputs from disturbed initializations in the STEMMUS‐FT, model uncertainties were predefined to assimilate fast neutrons. We observed that with enough spread of uncertainties, the updated states could mimic the CRNP observation. In all setups, assimilating CRNP measurements could enhance total soil water analyses, which consequently led to the improved detection of soil ice content and therefore the freezing thawing‐process at the field scale.

Subcriticality estimation by extended Kalman filter technique in transient experiment with external neutron source at Kyoto University Critical Assembly

The monitoring subcriticality by the extended Kalman filter (EKF) is evaluated in the presence of an external neutron source through transient analyses and compares with that by inverse kinetic method at Kyoto University Critical Assembly. Throughout the transient experiment, subcriticality is deduced with the use of neutron counts measured every 1 s. For ensuring the initial condition of the EKF technique, the basic transient experiment is carried out by dropping a control rod into the core at a critical state, and the result by the EKF technique shows remarkably accurate subcriticality as compared with the reference value obtained by the rod drop method with less fluctuation than by the inverse kinetic method. In the ADS transient experiment with the 14 MeV stable neutron source, the subcriticality is set as target subcriticality of the accelerator-driven system ($$k_\mathrm{eff}= 0.97$$); the external neutrons are then injected from outside the core. The EKF technique reveals significantly good agreement with MCNP6.1, whereas subcriticality by the inverse kinetic method fluctuates widely during the transient experiment. Thus transient experiment shows that the EKF technique is applicable to subcriticality monitoring with a high time resolution and a high degree of precision.

A High-Precision Tagged Neutron n-p Scattering Measurement at 14.9 MeV

The n-p scattering angular distribution was measured with 14.9 MeV incident neutrons produced at the neutron facility of Ohio University. The traditional time-of-flight technique with neutron-gamma discrimination was applied for the measurement of the number and energy of scattered neutrons. The scattering angle varied from 20 to 65 deg (laboratory system) in 5 deg incremental steps corresponding to an ejectile energy range from 13.16 to 2.66 MeV. The efficiency of the neutron detectors was measured in the energy range 2 to 9 MeV relative to the 252Cf standard and was calculated using Monte Carlo methods in the 2 to 14 MeV energy range. Two methods of analysis were applied for experimental and simulated data: a traditional approach with a fixed threshold ~0.1MeVee and a dynamic threshold approach. The efficiencies determined by both methods are in excellent agreement for simulated and experimental results within the energy interval 2 to 9 MeV. The experimental (<9 MeV) and calculated efficiencies (>9 MeV) were applied for evaluation of the n-p scattering experimental result. The corrections for neutron attenuation in the “scatter-detector” were calculated with analytical formulas and by the Monte Carlo method. Additional minor corrections for edge effect, C(n,n’)3α background and dead time were also included. The present data agree with recent evaluations for the n-p angular distribution within about 1.6%. The current state-of-the-art of experimental uncertainties that can be realized for a neutron counting experiment were reached in this investigation. An additional correlation analysis allows us to conclude that the standard deviation connected with existing correlations may be the main component of the total uncertainty.