Neutron Transmission Experiments Research Articles

Determination of crystallographic texture in polycrystalline materials from wavelength-resolved neutron transmission experiments: application to high-symmetry crystals

Wavelength-resolved neutron transmission experiments are useful for characterizing the microstructure of macroscopic specimens with 2D spatial resolution perpendicular to the beam direction. The crystallographic texture can affect the neutron transmission in the thermal neutron energy range, which manifests as changes in the shape and height of Bragg edges as a function of neutron wavelength. Models have been proposed to predict the transmission of textured polycrystalline materials from knowledge of the material texture and have proved to accurately predict the observed transmission data. In recent work, a novel method was described and tested for obtaining texture integral parameters from the combined analysis of transmission data measured along several directions of a specimen in a hexagonal crystal Zr alloy. However, this procedure has limitations when dealing with high-symmetry crystal structures. In this work, a generalization of such a method based on the expansion of the orientation distribution function (ODF) in symmetric generalized spherical harmonics that is applicable to all crystal and sample symmetries is presented. Using this method, the low-order Fourier coefficients of the ODF can be estimated by analyzing transmission data obtained for a reduced set of beam directions. This method was verified using a cubic Cu sample, for which transmission data were available along five different directions. Two sample symmetries were assumed to reduce the number of Fourier coefficients of the ODF. In the case of cylindrical symmetry (fiber-type texture), the results were good; but in the case of orthorhombic symmetry, some bias was observed which was attributed to the reduced number of beam directions used to perform the evaluation.

A new method to obtain the crystallographic texture of polycrystalline materials in Bragg edge imaging experiments

In polycrystalline materials, the shape and height of Bragg edges observed in wavelength resolved neutron transmission experiments depend on the crystallographic texture. Previous works have proved the capacity of actual models to predict the transmission spectra once the texture of the material is known. In this work, we summarize the recent advances done trying to obtain information of the texture from the experimental transmission data. The basis of this method resides in a compact expression of the coherent elastic cross section in terms of the Fourier coefficients of the orientation distribution function, which describes the crystallographic texture. The validity of this method is restricted to the applicability of the kinematic approximation of diffraction (polycrystalline materials with grain sizes below 10μm) and to materials with spatially uniform texture.

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.

Generation of the Thermal Scattering Law of Uranium Dioxide with Ab Initio Lattice Dynamics to Capture Crystal Binding Effects on Neutron Interactions

Scattering of thermal neutrons and Doppler broadening of epithermal neutron resonances in uranium and its compounds may be sensitive to crystal binding. The thermal scattering law (TSL) for uranium dioxide, which captures crystal binding effects, has been reevaluated for ENDF/B-VIII.0. Phonon spectra were generated using ab initio lattice dynamics for the paramagnetic phase and validated against experiment. Improved agreement with the Debye-Waller coefficient as a function of temperature is found relative to the spectrum used for the ENDF/B-VII.1 evaluation. The TSL was generated using the phonon expansion method within the NJOY nuclear data processing package and was found to be in reasonable agreement with inelastic neutron scattering measurements. The present evaluation predicts a reduction in the inelastic scattering cross section relative to ENDF/B-VII.1 and a total scattering cross section consistent with neutron transmission experiments.

Measurement of neutron total cross sections at the VESUVIO spectrometer

In this article we examine the procedures to determine total cross sections from neutron transmission experiments performed at the time-of-flight spectrometer VESUVIO (ISIS Pulsed Neutron and Muon Source, at the Rutherford Appleton Laboratory, UK). On the one hand we present a newly developed Python-based algorithm, running within the Mantid analysis environment, to obtain neutron total cross sections as a function of the incident neutron energy, employing the information recorded by the transmission monitor. We describe its basic operating principles and parameters, as well as its user interface, which is applied to a specific example of data processing on a set of graphite samples of different manufacturing processes. On the other hand, we discuss a series of features and recent upgrades that make VESUVIO the instrument of choice for accurate and broad-band thermal neutron cross section measurements.

Energy-selective neutron imaging by exploiting wavelength gradients of double crystal monochromators—Simulations and experiments

Abstract The potential of wavelength resolved neutron transmission experiments is well known. This paper is focused on the performance of the double crystal monochromator which is widely used at steady state neutron sources and compares simulation results based on neutron ray tracing with experimental results in order to provide a better understanding of the device. The influences of crystal mosacities on the neutron beam is reported for the utilised setup and the resulting wavelength gradients along one direction are determined. For the neutron imaging geometry applied, a wavelength gradient of about 0.005 A/cm at the sample position was found. Moreover, a new neutron radiography technique for Bragg edge mapping in imaging experiments utilising a neutron wavelength gradient at the sample position was developed and is reported. Experiments and simulations are found to be in good agreement.

Preparation and characterization of Portland cement pastes with Sm2O3 microparticle additions for neutron shielding applications

Mechanical, microstructural and neutron attenuation properties of white ordinary Portland cement (WOPC) pastes with Sm2O3 additions ranging from 2.5 to 20 wt% are reported. Scanning electron microscopy (SEM) micrographs revealed that incorporation of Sm2O3 above 10 wt% enhance the formation of ettringite and increase the porosity of the cement composites. Microcracks were also observed in SEM images for compositions containing 20 wt% Sm2O3. X-ray diffraction (XRD) analysis determined that samarium oxide particles did not interact chemically with the cement matrix to form new crystalline phases. On the other hand, neutron transmission experiments and Monte Carlo simulations showed that the attenuation of thermal and epithermal neutrons increased significantly with the addition of Sm2O3. Maximum enhancement in neutron shielding was obtained for specimens loaded with 20 wt% Sm2O3, which have an effective removal cross section of ∑eff=4.018cm-1 compared to ∑eff=0.515cm-1 for plain WOPC paste.

Doppler broadening of neutron-induced resonances using ab initio phonon spectrum

Neutron resonances observed in neutron cross section data can only be compared with their theoretical analogues after a correct broadening of the resonance widths. This broadening is usually carried out by two different theoretical models, namely the Free Gas Model and the Crystal Lattice Model, which, however, are only applicable under certain assumptions. Here, we use neutron transmission experiments on UO2 samples at $T=23.7$ K and $T=293.7$ K, to investigate the limitations of these models when an ab initio phonon spectrum is introduced in the calculations. Comparisons of the experimental and theoretical transmissions highlight the underestimation of the energy transferred at low temperature and its impact on the accurate determination of the radiation widths $\Gamma_{\gamma_{\lambda}}$ of the 238U resonances $\lambda$ . The observed deficiency of the model represents an experimental evidence that the Debye-Waller factor is not correctly calculated at low temperature near the Neel temperature ( $ T_{N}=30.8$ K).

A new method to quantify multiple elements by pulsed epithermal neutron transmission spectroscopy

To perform quantitative analysis of constituent elements, including light atoms, by time-of-flight (TOF) type neutron imaging, a new analysis method using epithermal neutrons excluding resonance dips has been proposed. The method is based on fitting the theoretical neutron transmission spectrum calculated from evaluated nuclear data to a measured neutron transmission spectrum. Compared to conventional imaging methods such as those using neutron radiography or neutron resonance analysis, it has the advantage of being able to evaluate multiple/light elements. We applied this technique to a pulsed neutron transmission experiment at the Hokkaido University Neutron Source. A stack of iron and carbon plates was measured as a model sample consisting of heavy and light elements. The evaluated elemental densities show reasonable agreements within an error of 10%.

Measurement of the para-hydrogen concentration in the ISIS moderators using neutron transmission and thermal conductivity

We present an experimental study to determine the para-hydrogen concentration in the hydrogen moderators at the ISIS pulsed neutron and muon source. The experimental characterisation is based on neutron transmission experiments performed on the VESUVIO spectrometer, and thermal conductivity measurements using the TOSCA para-hydrogen rig. A reliable estimation of the level of para-hydrogen concentration in the hydrogen moderators is of crucial importance in the framework of a current project to completely refurbish the first target station at ISIS. Moreover, we report a new measurement of the total neutron cross section for normal hydrogen at 15 K on the broad energy range 3 meV –10 eV suggesting a revision of the most recent nuclear libraries for incident neutron energies lower than 10 meV. Finally, we characterise systematic errors affecting the para-hydrogen level estimation due to conversion from para to ortho hydrogen, as a function of the time a batch of gas spends in every component of our gas panel and apparatus.

Inverse pole figure mapping of bulk crystalline grains in a polycrystalline steel plate by pulsed neutron Bragg-dip transmission imaging

A new mapping procedure for polycrystals using neutron Bragg-dip transmission is presented. This is expected to be useful as a new materials characterization tool which can simultaneously map the crystallographic direction of grains parallel to the incident beam. The method potentially has a higher spatial resolution than neutron diffraction imaging. As a demonstration, a Bragg-dip neutron transmission experiment was conducted at J-PARC on beamline MLF BL10 NOBORU. A large-grained Si–steel plate was used. Since this specimen included multiple grains along the neutron beam transmission path, it was a challenging task for existing methods to analyse the direction of the crystal lattice of each grain. A new data-analysis method for Bragg-dip transmission measurements was developed based on database matching. As a result, the number of grains and their crystallographic direction along the neutron transmission path have been determined.

Examination of total cross section resonance structure of niobium and silicon in neutron transmission experiments

The neutron transmission experiments are one of the main sources of information about the neutron cross section resonance structure and effect in the self-shielding. Such kind of data for niobium and silicon nuclides in energy range 7 keV to 3 MeV can be obtained from low-resolution transmission measurements performed earlier in Russia (with samples of 0.027 to 0.871 atom/barn for niobium and 0.076 to 1.803 atom/barn for silicon). A significant calculation-to-experiment discrepancy in energy range 100 to 600 keV and 300 to 800 keV for niobium and silicon, respectively, obtained using the evaluated nuclear data library ROSFOND, were found. The EVPAR code was used for estimation the average resonance parameters in energy range 7 to 600 keV for niobium. For silicon a stochastic optimization method was used to modify the resolved resonance parameters in energy range 300 to 800 keV. The improved ROSFOND evaluated nuclear data files were tested in calculation of ICSBEP integral benchmark experiments.

Full-pattern analysis of time-of-flight neutron transmission of mosaic crystals

The energy-resolved neutron transmission of mosaic crystals contains a series of dips in intensity, at specific neutron wavelengths defined by the orientation of the specimen in the neutron beam. This article presents a Rietveld type full-pattern analysis of neutron transmission experiments on mosaic crystals performed at spallation pulsed neutron sources. The proposed analysis provides precise and simple determination of lattice parameters, mosaicity, extinction factors and crystal orientation, and is especially suited to investigate the spatial variation of such microstructural information across macroscopic specimens with ∼1 mm resolution. The effect of extinction on the intensity of Bragg reflections has been successfully accounted for by a parameter measuring the ratio of the beam attenuation due to Bragg reflection to the combined attenuation due to absorption and scattering processes. Experiments were performed at the ENGIN-X beamline, ISIS Facility, UK, on several naturally occurring and man-made mosaic crystals, including a copper monochromator at temperatures between 55 and 300 K, an iron–nickel meteorite, and a natural pyrite crystal. Typical experimental resolutions found for lattice parameters and mosaicity are 0.03 and 7%, respectively. The possibilities of the technique for quantitative phase and/or texture analysis of specimens composed of several grains or phases are discussed.

Bragg Edge Neutron Transmission Strain Tomography in Granular Systems

AbstractThe advent of pixelated detectors for time‐of‐flight neutron transmission experiments has raised significant interest in terms of the potential for tomographic reconstructions of triaxial strain distributions. A recent publication by Lionheart and Withers [WRB Lionheart and PJ Withers, “Diffraction tomography of strain”, Inverse Problems, v31:045005, 2015] has demonstrated that reconstruction is not possible in the general sense; however, various special cases may exist. In this paper, we outline a process by which it is possible to tomographically reconstruct average triaxial elastic strains within individual particles in a granular assembly from a series of Bragg edge strain measurements. This algorithm is tested on simulated data in two and three dimensions and is shown to be capable of rejecting Gaussian measurement noise. Sources of systematic error that may present problems in an experimental implementation are briefly discussed.

Model for neutron total cross-section at low energies for nuclear grade graphite

At subthermal neutron energies, polycrystalline graphite shows a large total cross-section due to small angle scattering processes. In this work, a new methodology to determine pore size distributions through the neutron transmission technique at subthermal energies is proposed and its sensitivity is compared with standard techniques. A simple model based on the form factor for spherical particles, normally used in the Small Angle Neutron Scattering technique, is employed to calculate the contribution of small angle effect to the total scattering cross-section, with the width and center of the radii distributions as free parameters in the model. Small Angle X-ray Scattering experiments were performed to compare results as a means to validate the method. The good agreement reached reveals that the neutron transmission technique is a useful tool to explore small angle scattering effects. This fact can be exploited in situations where large samples must be scanned and it is difficult to investigate them with conventional methods. It also opens the possibility to apply this method in energy-resolved neutron imaging. Also, since subthermal neutron transmission experiments are perfectly feasible in small neutron sources, the present findings open new possibilities to the work done in such kind of facilities.

Imaging of Crystalline Structural Information of Japanese Swords by Pulsed Neutron Transmission Spectroscopy

Japanese swords are peculiar ones forged by a unique method developed in Japan. The forging methods are different each other, depending on age and area. Therefore, the crystalline structure of a Japanese sword depends on the age and the area. In this study, we obtained crystallographic information of Japanese swords and investigated the difference in texture and crystallite size from Japanese swords by using the pulsed neutron transmission spectroscopy. The neutron transmission experiment was carried out at HUNS (Hokkaido University Neutron Source). The samples were two types of Japanese swords. We analyzed the position-dependent neutron transmission spectra and obtained quantitative microstructural information in two-dimensional real space. We found that there were differences of the microstructure between edge and back of each Japanese sword and the microstructure was different between two Japanese swords.

Neutron Radiation Tests about FeCr Slag and Natural Zeolite Loaded Brick Samples

Neutron shielding performances of new brick samples are investigated. Brick samples including 10, 20, and 30 percentages of ferrochromium slag (FeCr waste) and natural zeolite are prepared and mechanical properties are obtained. Total macroscopic cross sections are calculated by using results of 4.5 MeV neutron transmission experiments. As a result, neutron shielding capacity of brick samples increases with increasing FeCr slag and natural zeolite percentages. This information could be useful in the area of neutron shielding.

Neutron transmission and capture of241Am

A set of neutron transmission and capture experiments based on the Time Of Flight (TOF) technique, were performed in order to determine the 241 Am capture cross section in the energy range from 0.01 eV to 1 keV. The GELINA facility of the Institute for Reference Materials and Measurements (IRMM) served as the neutron source. A pair of C6D6 liquid scintillators was used to register the prompt gamma rays emerging from the americium sample, while a Li-glass detector was used in the transmission setup. Results from the capture and transmission data acquired are consistent with each other, but appear to be inconsistent with the evaluated data files. Resonance parameters have been derived for the data up to the energy of 100 eV.

Characterization and Neutron Shielding Behavior of Dehydrated Magnesium Borate Minerals Synthesized via Solid-State Method

Magnesium borates are one of the major groups of boron minerals that have good neutron shielding performance. In this study, dehydrated magnesium borates were synthesized by solid-state method using magnesium oxide (MgO) and boron oxide (B2O3), in order to test their ability of neutron shielding. After synthesizing the dehydrated magnesium borates, characterizations were done by X-ray Diffraction (XRD), fourier transform infrared (FT-IR), Raman spectroscopy, and scanning electron microscopy (SEM). Also boron oxide (B2O3) contents and reaction yields (%) were calculated. XRD results showed that seven different types of dehydrated magnesium borates were synthesized. 1000°C reaction temperature, 240 minutes of reaction time, and 3 : 2, 1 : 1 mole ratios of products were selected and tested for neutron transmission. Also reaction yields were calculated between 84 and 88% for the 3 : 2 mole ratio products. The neutron transmission experiments revealed that the 3 : 2 mole ratio of MgO to B2O3neutron transmission results (0.618–0.655) was better than the ratio of 1 : 1 (0.772–0.843).

Validation of Bragg edge experiments by Monte Carlo simulations for quantitative texture analysis

The ability to resolve the wavelength in neutron transmission experiments has opened up a wide range of applications investigating crystallographic structures and properties. The investigations presented in this paper apply the Bragg edge transmission technique to the study of texture in a cylindrical aluminium specimen. The feasibility of this method is the main focus of this work; hence, conventional pole-figure measurements have been conducted with neutron diffraction to provide reference data for the evaluation of the transmission results. Furthermore, a Monte Carlo simulation has been developed to emulate the experimental conditions and allow investigations of instrument effects. The results have been compared and are discussed with respect to quantitative texture analysis.