Low-energy Neutrons Research Articles

Comparison of Results from Recent NNSA and CEA Measurements of the 239Pu(n, f) Prompt Fission Neutron Spectrum

The National Nuclear Security Administration (NNSA)/DP French Alternative Energies and Atomic Energy Commission (CEA)/DAM agreement on cooperation on fundamental science is a U.S.-French collaborative effort to combine intellectual and experimental resources and further the relevant nuclear science. Recently, both the NNSA and CEA experimental teams performed high-statistics measurements of the 239Pu(n, f) prompt fission neutron spectrum (PFNS) at the Los Alamos Neutron Science Center, both of which were recently published in the journal Physical Review C. These separate measurements used the same experimental area and a common neutron detector array, but differ in many aspects, including background assessments, data acquisition systems and philosophies, fission detectors, and PFNS extraction techniques. Hence, some aspects of the experimental methods and associated uncertainties are highly correlated while others are independent. The results from both measurements broke new ground for PFNS measurements given their higher accuracy and more detailed study of corrections necessary for the measured quantity compared to existing literature measurements, and both will significantly impact PFNS nuclear data evaluations for the foreseeable future. The focus of this work is to document a comparison of the results from these distinct measurements in terms of the acquired data, the PFNS results, and the measured average PFNS energies. While systematic differences between the PFNS results are present on the 1–3% level, the acquired data relative to each respective measurement at low incident neutron energies are in remarkable agreement, as are the conclusions regarding the magnitude and position of features in the PFNS relating to second-chance fission, third-chance fission, and pre-equilibrium neutron emission.

Evidence of improvement in thermoelectric parameters of n-type Bi2Te3/graphite nanocomposite

Improvement in thermoelectric parameters is reported with graphite incorporation in n-type Bi2Te3/graphite nanocomposite systems. In-depth thermoelectric properties of nanostructured Bi2Te3/graphite composites are probed both microscopically and macroscopically using x-ray diffraction, Raman spectroscopy, inelastic neutron scattering, and measurement of the temperature dependence of thermal conductivity κ, Seebeck coefficient S, resistivity ρ, and carrier concentration nH. Raman spectroscopic analysis confirms that graphite introduces defects and disorder in the system. Graphite addition induces a large (∼17%) decrease of κ, originating from a strong phonon scattering effect. A low lattice thermal conductivity, κL, value of 0.77 W m−1 K−1, approaching the κmin value, estimated using the Cahill–Pohl model, is reported for Bi2Te3 + 1.0 wt. % graphite sample. Graphite dispersion alters the low-energy inelastic neutron scattering spectrum providing evidence for modification of the Bi2Te3 phonon density of states. Improvement in other thermoelectric parameters, viz., Seebeck coefficient and resistivity, is also reported. Theoretical modeling of electrical and thermal transport parameters is carried out and a plausible explanation of the underlying transport mechanism is provided assuming a simple model of ballistic electron transport in 1D contact channels with two different energies.

THERMAL SCATTERING LAW ENDF LIBRARIES FOR LIQUID FLIBE

Several advanced nuclear reactor concepts have been proposed in the past few years where FLiBe molten salt represents a major constituent of the core. In this case, neutrons produced in fission slow down and moderate in FLiBe (a eutectic with a mixture of 2:1 ratio of LiF and BeF2) until they reach low energies (i.e, below 1 eV). At that stage, the thermalization process becomes dominant and the neutrons achieve a quasi-equilibrium energy state that is dependent on the temperature of the moderator. In neutronic simulations, the description of neutron thermalization is captured using the thermal scattering law (TSL), i.e., S(α,β), of the material in which low energy neutrons are interacting. S(α,β) defines the energy-momentum phase space that is available for an incoming low energy neutron. In addition, it is directly proportional to the double differential thermal neutron scattering cross section. In this work, the TSL of molten salt FLiBe is developed based on a generalized density of excitation states (GDOS) derived from atomic trajectories generated using classical molecular dynamics (MD) simulations that were performed with the LAMMPS code. The MD simulations utilized a Born-Mayer type atomic potential function that was verified to reproduce the properties of FLiBe including density and viscosity. The FLASSH code was used to evaluate the TSL’s ENDF File 7 in a temperature range extending from 773 K to 1673 K. In addition, ACE type cross section libraries are produced and tested with the objective of contributing the data to the National Nuclear Data Center for inclusion in the ENDF/B-VIII database.

Pulsed neutron imaging for differentiation of ice and liquid water towards fuel cell vehicle applications.

This study is the first report on liquid water and ice imaging conducted at a pulsed spallation neutron source facility. Neutron imaging can be utilised to visualise the water distribution inside polymer electrolyte fuel cells (PEFCs). Particularly, energy-resolved neutron imaging is a methodology capable of distinguishing between liquid water and ice, and is effective for investigating ice formation in PEFCs operating in a subfreezing environment. The distinction principle is based on the fact that the cross sections of liquid water and ice differ from each other at low neutron energies. In order to quantitatively observe transient freezing and thawing phenomena in a multiphase mixture (gas/liquid/solid) within real PEFCs with high spatial resolution, a pulsed neutron beam with both high intensity and wide energy range is most appropriate. In the validation study of the present work, we used water sealed in narrow capillary tubes to simulate the flow channels of a PEFC, and a pulsed neutron beam was applied to distinguish ice, liquid water and super-cooled water, and to clarify freezing and thawing phenomena of the water within the capillary tubes. Moreover, we have enabled the observation of liquid water/ice distributions in a large field of view (300 mm × 300 mm) by manufacturing a sub-zero environment chamber that can be cooled down to -30 °C, as a step towards in situ visualisation of full-size fuel cells.

Neutron radiative capture cross section for sodium with covariance analysis

The neutron radiative capture cross sections measurement has been carried out for the $$^{23}$$ Na nucleus in the neutron energy region from 0.6 to 3.2 MeV using the neutron activation technique followed by off-line $$\gamma $$ -ray spectrometry. The measurement was made relative to the $$^{115}$$ In(n,n $$\prime $$ $$\gamma $$ ) $$^{115}\hbox {In}^{m}$$ reference monitor reaction cross section. The neutrons were produced via the $$^{7}$$ Li(p,n) $$^{7}$$ Be reaction. Detailed uncertainty propagation has been performed using the covariance analysis, and the measured cross sections are being reported with their uncertainties, covariance, and correlation matrix. The necessary corrections have been made for the low background neutron energy contribution, $$\gamma $$ -ray true coincidence summing, and self-attenuation process. The obtained neutron spectrum averaged cross sections of $$^{23}$$ Na(n, $$\gamma $$ ) $$^{24}$$ Na are discussed and compared with the existing cross sections data retrieved from the EXFOR database. EMPIRE-3.2 and TALYS-1.9 calculations were performed in order to determine the radiative capture cross section in this energy region. The present results are also compared with the evaluated nuclear data from ENDF/B-VIII.0, TENDL-2019, IRDFF-1.05, JENDL-4.0, and JEFF-3.3. The obtained cross section results are in good agreement with existing experimental data, evaluated libraries, and reaction models for the highest energy points (2.11 and 3.13 MeV), while the lowest-energy point at 0.61 MeV underestimates them.

Unified description of the coupled-channels and statistical Hauser-Feshbach nuclear reaction theories for low energy neutron incident reactions

We incorporate the coupled-channels optical model into the statistical Hauser-Feshbach nuclear reaction theory, where the scattering matrix is diagonalized by performing the Engelbrecht-Weidenmüller transformation. This technique has been implemented in the coupled-channels optical model code ECIS by J. Raynal, and we extend this method so that all the open channels in a nucleon-induced reaction on a deformed nucleus can be calculated consistently.

Discovery of a new low energy neutron resonance of $$^{89}$$Y

Discovery of a new low energy neutron resonance of $$^{89}$$Y

Preliminary analysis of neutron time-delay histograms from Changvan latitude surveys

The name “Changvan” is given to a transportable neutron monitor, modernized by researchers in Thailand and housed inside a standard-size shipping container. It contains three neutron-sensitive proportional counters, which contain enriched 10BF3 gas, to detect cosmic ray showers. Primary cosmic rays are high-energy particles arriving from outer space. By colliding with atoms in the atmosphere, they produce a shower of sub-atomic particles, called secondary particles. Some particles can reach the Earth’s surface, including neutrons. The Changvan is designed to measure atmospheric neutrons during a latitude survey. The side counter tubes are ringed with lead producer to produce evaporation neutrons when secondary particles collide with it. The center counter lacks the lead producer but can receive neutrons from nearby counters. The reflector and moderator made from polyethylene thermalize the evaporation neutrons and protect against disturbance by external low-energy neutrons. In this work, we pursue methods to determine spectral changes in Galactic cosmic rays using data from the Changvan, avoiding the systematic inconsistencies of cross-station comparisons. We examine neutron time delay histograms from the three counter tubes, as recorded by specially designed readout electronics, where the time delay refers to the time interval between consecutive neutron detections in one counter. We perform a preliminary analysis of such histograms to measure the leader fraction (L) of neutrons that do not follow a previous neutron from the same primary cosmic ray. The electronic dead time will be considered in the L calculation.

IMPROVED THERMAL SCATTERING DATA PREPARATION

Convenient access to accurate nuclear data, particularly data describing low-energy neutrons, is crucial for trustworthy simulations of thermal nuclear systems. Obtaining the scattering kernel for thermal neutrons (i.e., neutrons with energy ~1 eV or less) can be a difficult problem, since the neutron energy is not sufficient to break molecular bonds, and thus the neutrons must often interact with a much larger structure. The “scattering law” S(α; β), which is a function of unitless momentum α and energy β transfer, is used to relate the material’s phonon frequency distribution to the scattering kernel. LEAPR (a module of NJOY) and GASKET are two nuclear data processing codes that can be used to prepare the scattering law and use different approaches to approximate the same equations. LEAPR uses the “phonon expansion method” which involves iterative convolution. Iteratively solving convolution integrals is an expensive calculation to perform (to ease this calculation, LEAPR uses trapezoidal integration for the convolution). GASKET uses a more direct approach that, while avoiding the iterative convolutions, can become numerically unstable for some α; β combinations. When both methods are properly converged, they tend to agree quite well. The agreement and departure from agreement is presented here.

Investigations on neutron attenuation properties of poly-boron materials using Am-Be and 252Cf sources neutron spectra

The neutron attenuation characteristics of polyboron materials was studied using Am-Be (α,n) and 252Cf (Spontaneous Fission) sources. Neutron Attenuation Factor (NAF) is obtained for poly boron materials with addition of different concentrations of boron by neutron transmission experiments. Neutron transport calculations are carried by simulating the transmission of neutron from Am-Be and 252Cf sources through polyboron materials using Monte Carlo based simulations. The experimental results are fairly in good agreement with transport calculations. The results showed that Neutron Attenuation Factor (NAF) mainly depend on the incident neutron energy and boron number density present in HDPE shielding material. The effect of boron in HDPE on neutron shielding is only marginal (2%) because of low value of neutron cross section at fast energy region. However, in the low neutron energy region, it varies with wt % of boron and 20% boron as additive in HPDE is an optimum value for neutron attenuation. The details of experimental observations and theoretical investigations are discussed in this work.

H2 production in the 10B(n,α)7Li reaction in water

We demonstrate a method for measuring the H2 produced in water from the 10B(n,α)7Li fission reaction. Low energy neutrons from the NIST Center for Neutron Research interact with borate-containing water in a temperature-controlled high pressure cell made from titanium. After exposure for one to several hours, the water is extracted and sparged with argon. H2 entrained in the sparging gas is sampled with a small mass spectrometer. To determine the neutron exposure, a small amount of sodium is included in the borate solution. The water is collected and 24Na activation is measured in a counting apparatus on the following day. The G-value for H2 at room temperature is found to be (1.18 ± 0.10) molecules H2/100eV, in good agreement with previous estimates and recent modeling calculations.

A Boron-10 nitride nanosheet for combinational boron neutron capture therapy and chemotherapy of tumor

Combination cancer therapy (e.g., radiochemotherapy) is widely used to enhance the therapeutic effects and prevent the recurrence of cancer. However, the side effects of monotherapy are also amplified when treating cancer with combination therapy. A locally activated drug delivery strategy that can release the payload in a tumor-selective manner is greatly needed to overcome the side effects of combination therapy. Here, we explore the potential of combining boron neutron capture therapy and chemotherapy as a new type of radiochemotherapy. Two-dimensional (2D) boron-10-rich nanosheets (BNNSs) were fabricated as a dual-functional delivery system: targeted boron-10 delivery systems for boron neutron capture therapy (BNCT) and drug delivery vehicles to load doxorubicin for chemotherapy. Irradiated by low-energy thermal neutron, BNNSs can produce high linear energy transfer (LET) particles to kill tumor cells, and the loaded doxorubicin can be released in situ at the same time. This neutron-triggered radiochemotherapy shows noteworthy efficacy in suppressing tumor growth in triple-negative breast cancer. To the best of our knowledge, this is the first report to combine BNCT with chemotherapy as a new type of radiochemotherapy. We hope this study could inspire additional BNCT-induced combination cancer therapies and provide insight for the further clinical translation of BNCT.

Study of spin-isospin response of 11Li neutron-drip-line nucleus with PANDORA

The spin-isospin responses of the 11Li drip-line nucleus has been measured. Preliminary results of the 11Li(p, n)11Be experiment in inverse kinematics at RI Beam Factory (RIBF) of RIKEN Nishina Center are presented including the observation of 1n, 2n, t, d, 2α and 6He+α decay channels of 11Be reaction product. Details of the experimental setup based on PANDORA (Particle Analyzer Neutron Detector Of Real-time Acquisition) low-energy neutron detector and the SAMURAI large-acceptance magnetic spectrometer are described.

Study of the 16-Channel Scintillation Detector Prototype with Silicon Photomultiplier Readout

In the Laboratory of High-Energy Physics, 16-channel prototypes of detectors (with SiPM readout produced by Ketek and Hamamatsu) were developed for their application in zero-degree calorimetry and for measuring the profile of a low-energy neutron beam. This paper presents the method for determining the voltage of the breakpoint of the noise characteristic of silicon photomultipliers in the multichannel detector and its temperature dependence in the range from 27.7°C to 39.1°C. The response of SiPM to the LED source of calibrated light pulses has been studied and the time resolution has been obtained.

Verification and Validation of the Geant4 Monte Carlo Code Toolkit for DEMO TBR Evaluations

Verification and Validation analyses of the Monte Carlo code toolkit Geant4 were conducted with nuclear data from the JEFF-3.3 library to show the suitability for fusion applications. We present benchmarking against data of the engineering design oriented HCPB (Helium Cooled Pebble Bed) Tritium Breeder Module (TBM) mock-up experiment taken from the SINBAD compilation and nuclear analyses in comparison with MCNP of the DEMO reactor equipped with a HCPB breeding blanket. Missing functionalities for Geant4 were developed such as a track length detector with multiple energy bins and tally multiplication, reflective boundaries, and statistical checks in a post-processing step. The activation foil measurements in the TBM mock-up revealed Geant4 underestimates the uncollided and fast neutron flux increasingly with penetration depth, while being consistent with MCNP for the low energy neutron flux. Geant4 underestimates the experimental tritium activity in the breeder pellets by up to 2.7% more than MCNP, which is within the experimental uncertainty. The DEMO analyses yielded a 0.97% lower Tritium Breeding Ratio (TBR) of Geant4 than MCNP for a homogenized breeder blanket, and a 1.46% higher TBR of Geant4 than MCNP for a mostly heterogenic blanket. This good agreement with MCNP demonstrates Geant4’s suitability for DEMO TBR evaluations.

Combining density functional theory and Monte Carlo neutron transport calculations to study the phonon density of states of UO2 up to 1675 K by inelastic neutron scattering

Significant advances in the use of atomistic simulation techniques, such as ab initio density functional theory and the molecular dynamics method, made it possible to predictively calculate properties of materials. In parallel, low-energy neutron scattering instruments and data analysis tools available in different institutes become mature for providing high-quality data for experimental validation purposes. Despite such experimental and theoretical improvements, the accurate modeling of experimental neutron-weighted multiphonon spectra for $\mathrm{U}{\mathrm{O}}_{2}$ over a broad temperature range still remains an issue. Combining prior phonon density of states (PDOS) from density functional theory and Monte Carlo inelastic neutron scattering calculations in a Bayesian fitting procedure is a valuable alternative approach to assess the partial PDOS of uranium and oxygen in $\mathrm{U}{\mathrm{O}}_{2}$ from 294 to 1675 K for improving the comparison to experiment and exploring the first-principles calculation hypothesis.

Neutron instrumentation research at the Low Energy Neutron Source

Particle accelerators have been used to produce neutrons at relatively small-scale facilities for many decades, but historically the vast majority of the work performed at such facilities has focus...

A Gadolinium Metal-Organic Framework Film as a Converter Layer for Neutron Detection.

Metal-organic frameworks (MOFs) are known for their versatility in terms of their crystalline structure, porosity, resistance to temperature, radiation damage, and luminescence among others. Gadolinium (Gd) is one of the elements with the highest reported cross-section for low energy neutron capture, producing internal conversion electrons and γ rays as a result of the neutron absorption. The development of Gd-BTC films (BTC=1,3,5-benzenetricarboxylate) is shown that were deposited on Si and Al substrates by airbrushing, and characterized by profilometry, Raman, EDX and X-ray diffraction. Radiation damage, thermal decomposition and neutron absorption of these films were studied as well. Gd-BTC films were attached to CMOS devices (Complementary Metal-Oxide-Semiconductor), which are sensible to the internal conversion electrons, in order to build a neutron detector. The devices Gd-BTC/CMOS could selectively detect neutrons in the presence of γ rays with a thermal neutron detection efficiency of 3.3±0.1 %, a signal to noise ratio of 6 : 1, and were suitable to obtain images.

Signatures of minimal length from Casimir–Polder forces with neutrons

In this paper, dispersion forces between neutrons are suggested as a probe of the fundamental structure of spacetime. Corrections to standard expressions for the interparticle potentials are obtained through computer algebra system strategies and a novel heuristic argument for comparison with field theory computations. It is confirmed that, to first order in the deformation parameter, the unretarded ([Formula: see text]) van der Waals potential is unchanged. The modified retarded Casimir–Polder potential, obtained from the minimal length zero-point field, compares satisfactorily in both sign and magnitude with rigorous calculations. It is shown that low energy neutron scattering can provide a gain in excess of [Formula: see text] orders of magnitude over atomic physics experiments in constraining corrections due to the existence of a minimal length.

A method to calibrate the n-γ discrimination property of scintillators in low energy region

Scintillators with n-γ discrimination property are widely used in the measurement of neutrons. In sub MeV region, the n-γ discrimination property will be greatly deteriorated so that the neutron and γ-ray events can't be accurately discriminated. As a result, the determination of the proportions of neutron and γ-ray events beyond the discrimination threshold is important if scintillators are applied to measure the low energy neutrons. In the present work, a method based on the time-of-flight technique is proposed to calibrate the proportions of neutron and γ-ray events beyond the discrimination threshold. After the calibration, the numbers of the measured neutron and γ-ray events can be separately determined even if the neutron and γ-ray events can't be perfectly discriminated.