Neutron Scattering Cross Sections Research Articles

Pseudofermion functional renormalization group study of dipolar-octupolar pyrochlore magnets

Motivated by recent experiments on Ce2Zr2O7 that reveal a dynamic, liquidlike ground state, we study the nearest-neighbor XYZ Hamiltonian of dipolar-octupolar pyrochlore magnets with the pseudofermion functional renormalization group (PFFRG), which is numerically implemented by the software. Taking the interaction between the octupolar components to be dominant and antiferromagnetic, we map out the phase diagram demarcating the quantum disordered and magnetically ordered states. We identify four distinct phases, namely, the 0-flux and π-flux quantum spin ices and the all-in-all-out magnetic orders along the local z and x axes. We further use the static two-spin correlation output by the PFFRG algorithm to compute the polarized neutron scattering cross sections, which are able to capture several qualitative features observed experimentally, in the materially relevant parameter regime that stabilizes the π-flux quantum spin ice. Our results provide support for a quantum spin-liquid ground state in Ce2Zr2O7. Published by the American Physical Society 2024

Signature of surface anisotropy in the spin-flip neutron scattering cross section of spherical nanoparticles: Atomistic simulations and analytical theory

Signature of surface anisotropy in the spin-flip neutron scattering cross section of spherical nanoparticles: Atomistic simulations and analytical theory

Determining the Composition and Structure of a High-Performing SEI Layer for Lithium-Mediated Nitrogen Reduction to Ammonia

The industrial synthesis of ammonia (NH3) via the Haber-Bosch process has enabled mass production of fertilizer and the human population growth to 8 billion.1 NH3 also shows promise as a long-term, energy-dense fuel amenable to widespread distribution.2 However, the Haber-Bosch process operates at high temperatures and pressures in centralized facilities and accounts for 1.3% of global carbon emissions.1,2 To ensure a food-secure future and realize NH3 as a truly carbon-free fuel, we must develop an NH3 synthesis alternative that operates at ambient conditions and can be driven by renewables.One of the most promising NH3 synthesis method under mild conditions is the electrochemical, Li-mediated process (Li-NRR). In this process, Li is electrodeposited from solution and reacted with N2 to form Li3N. A proton shuttle, e.g., ethanol, protonates Li3N to form NH3.3 The electrodeposited Li forms a solid-electrolyte interphase (SEI) layer that controls N2, Li+, and H+ transport and is key to system performance. The use of fluorinated Li salts, e.g., LiBF4, have recently advanced Li-NRR system performance. Improved selectivity has been attributed to changes in the SEI layer, but evaluating SEI structure and composition remains a significant challenge.3 Neutron-based characterization methods, complementary to X-ray methods, offer unique benefits to study SEI layers containing light elements such as Li- and H; for example, an element’s neutron scattering cross section does not depend on its atomic number.4 In this work, we studied the formation of the SEI layer in-situ before and after applying -0.3 mA/cm2 in galvanostatic experiments (chronopotentiometry, CP) on a copper (Cu) working electrode. We utilized a combination of deuterated and non-deuterated solvents and proton donors to distinguish their contributions to the SEI. We found that the SEI forms two layers: a compact, lithium rich layer, and a diffuse layer with carbonaceous species (Figure 1).4 We also identified the presence of boron in the layers, indicated by neutron absorption. Finally, we observed more proton-rich species in experiments containing the non-deuterated H+ donor, corroborating previous studies that indicate that Li-ethoxide may be a major component of the diffuse SEI (Figure 1b-c).5 Compared to LiClO4-based electrolytes, the structure of these LiBF4-derived layers is consistent even after repeated current cycling, indicating a more stable SEI structure and suggesting a relationship between SEI stability and increased Li-NRR performance.References Nørskov, J. et al. DOE Roundtable Report (2016).Chang, F. et al. Advanced Materials 33, 2005721 (2021).Li, S. et al. Joule 6, 2083–2101 (2022).Blair, S.J., Doucet, M. et al. ACS Energy Lett., 7, 6, 1939-1946 (2022).Steinburg, K. et al. Nature Energy, 8, 138-148 (2023). Figure 1: (a) reflectivity curve before and after 4 mins constant applied current at -0.3 mA/cm2 (chronopoteniometry, CP) to plate Li using 1 M LiBF4 with 1 vol% ethanol, (b) resulting scattering length density (SLD) profile, and (c) schematic of layers indicated in SLD profile Figure 1

Tetrahedral triple-Q magnetic ordering and large spontaneous Hall conductivity in the metallic triangular antiferromagnet Co1/3TaS2

The triangular lattice antiferromagnet (TLAF) has been the standard paradigm of frustrated magnetism for several decades. The most common magnetic ordering in insulating TLAFs is the 120° structure. However, a new triple-Q chiral ordering can emerge in metallic TLAFs, representing the short wavelength limit of magnetic skyrmion crystals. We report the metallic TLAF Co1/3TaS2 as the first example of tetrahedral triple-Q magnetic ordering with the associated topological Hall effect (non-zero σxy(H = 0)). We also present a theoretical framework that describes the emergence of this magnetic ground state, which is further supported by the electronic structure measured by angle-resolved photoemission spectroscopy. Additionally, our measurements of the inelastic neutron scattering cross section are consistent with the calculated dynamical structure factor of the tetrahedral triple-Q state.

Influence of neutron irradiation on the electronic properties of hexagonal boron nitride measured by terahertz time-domain spectroscopy

Due to the low atomic number of B, hexagonal boron nitride (hBN) has a large neutron scattering cross section and, therefore, is an ideal material for the realization of solid-state neutron detector. Here we apply the THz time-domain spectroscopy to study the effect of neutron irradiation on electronic properties of pyrolytic (PBN) and hot-pressed boron nitride (HBN). The key electronic parameters of these samples, such as the static dielectric constant ε b , the effective carrier density N*, the carrier relaxation time τ, and the electronic localization factor α, are determined optically, and their dependences upon the neutron irradiation fluence (NIF) are examined. We find that for hBN,N* and ε b decrease while τ and |α| increase with increasing NIF. These results can be used to further understand the neutron irradiation effects on the basic physical properties of hBN material. We believe that the results obtained from this work can benefit to the design and application of hBN material for neutron detectors.

Characterization of aluminum and boron carbide based additive manufactured material for thermal neutron shielding

We characterize the suitability of two additive manufactured materials for use in shielding thermal neutrons for neutron scattering instrumentation. We compare two different boron based additive manufactured masks to a traditionally manufactured boron nitride mask in a real-use case scenario. The first additive manufactured mask is a composite of aluminum and boron carbide. The second additive manufactured mask is a composite of boron carbide and cyanoacrylate. We report the thermal conductivity of the aluminum boron carbide mask in comparison to other relevant materials. The aluminum boron carbide material may find use in shielding applications where thermal conductivity, a low total neutron scattering cross section, and low toxicity are criteria.

Effect of annealing on the magnetic microstructure of high-pressure torsion iron: the relevance of higher-order contributions to the magnetic small-angle neutron scattering cross section.

The development of higher-order micromagnetic small-angle neutron scattering theory in nanocrystalline materials is still in its infancy. One key challenge remaining in this field is understanding the role played by the microstructure on the magnitude and sign of the higher-order scattering contribution recently observed in nanocrystalline materials prepared by high-pressure torsion. By combining structural and magnetic characterization techniques, namely X-ray diffraction, electron backscattered diffraction and magnetometry with magnetic small-angle neutron scattering, this work discusses the relevance of higher-order terms in the magnetic small-angle neutron scattering cross section of pure iron prepared by high-pressure torsion associated with a post-annealing process. The structural analysis confirms: (i) the preparation of ultra-fine-grained pure iron with a crystallite size below 100 nm and (ii) rapid grain growth with increasing annealing temperature. The analysis of neutron data based on the micromagnetic small-angle neutron scattering theory extended to textured ferromagnets yields uniaxial magnetic anisotropy values that are larger than the magnetocrystalline value reported for bulk iron, supporting the existence of induced magnetoelastic anisotropy in the mechanically deformed samples. Furthermore, the neutron data analysis revealed unambiguously the presence of non-negligible higher-order scattering contributions in high-pressure torsion iron. Though the sign of the higher-order contribution might be related to the amplitude of the anisotropy inhomogeneities, its magnitude appears to be clearly correlated to the changes in the microstructure (density and/or shape of the defects) induced by combining high-pressure torsion and a post-annealing treatment.

On the angular anisotropy of the randomly averaged magnetic neutron scattering cross section of nanoparticles.

The magnetic small-angle neutron scattering (SANS) cross section of dilute ensembles of uniformly magnetized and randomly oriented Stoner-Wohlfarth particles is calculated using the Landau-Lifshitz equation. The focus of this study is on the angular anisotropy of the magnetic SANS signal as it can be seen on a two-dimensional position-sensitive detector. Depending on the symmetry of the magnetic anisotropy of the particles (e.g. uniaxial, cubic), an anisotropic magnetic SANS pattern may result, even in the remanent state or at the coercive field. The case of inhomogeneously magnetized particles and the effects of a particle-size distribution and interparticle correlations are also discussed.

Micromagnetic simulation of neutron scattering from spherical nanoparticles: Effect of pore-type defects

We employ micromagnetic simulations to model the effect of pore-type microstructural defects on the magnetic small-angle neutron scattering cross section and the related pair-distance distribution function of spherical magnetic nanoparticles. Our expression for the magnetic energy takes into account the isotropic exchange interaction, the magnetocrystalline anisotropy, the dipolar interaction, and an externally applied magnetic field. The signatures of the defects and the role of the dipolar energy are highlighted and the effect of a particle-size distribution is studied. The results serve as a guideline to the experimentalist.

The Neutron Scattering Cross Section and Angular Distribution Measurement Program at LANL

Neutron scattering cross sections and angular distributions are leading terms in descriptions of neutron transport through any system. Despite the fundamental importance of nuclear data on these quantities, significant gaps in understanding and lack of experimental data persist in heavier elements and down to lighter structural materials, such as iron and aluminum. Recent measurements on carbon have also shown definitive proof that the neutron angular distribution can change with respect to the emission angle of γ-rays from inelastic scattering, thereby complicating γ-tagged measurements of inelastic neutron scattering. In this work, we describe the emerging program at Los Alamos National Laboratory for measurements of neutron scattering cross sections and neutron, γ-ray, and correlated n-γ angular distributions utilizing liquid scintillator detectors and the Correlated Gamma-Neutron Array for Scattering (CoGNAC) of CLYC scintillators. Currently, these detectors are operated simultaneously with the in-progress array of CLYC detectors in an inverted position, and data analysis techniques are being developed to span scattering measurements from light nuclei up through actinides. Preliminary results for measurements on 12C, 27Al, and 56Fe are presented here with a description of the analysis methods applied and anticipated capabilities of the full-scale detection system.

Theoretical calculations of neutron scattering cross sections for tetrahydrofuran-containing clathrate hydrates at low temperature

Tetrahydrofuran-containing clathrate hydrate is considered as a potential cold and very cold neutron moderator material. The fully deuterated form is more promising because of the lower neutron absorption cross section of deuterium. In this work we present theoretical calculations of neutron scattering cross sections for hydrogenated and deuterated clathrates at low temperature. The scattering cross sections are generated by using the crystalline structure and phonon density of states obtained from density functional theory calculations. The theoretical neutron scattering cross sections serve to compare against existing or future experimental data.

Evaluation of Thermal Neutron Scattering Law and Cross Sections for Calcium Hydride

Presented here are the calculated thermal scattering law (TSL) and thermal neutron scattering cross sections for Calcium Hydride, hereafter referred to by its chemical symbol CaH2. The only other such data prior to this evaluation are thermal neutron scattering libraries in the JEFF database, which do not fully capture the scattering physics of the CaH2 system. The data in this evaluation are calculated from first principles; Density Functional Theory (DFT) is used to calculate the phonon density of states (DOS), which is the primary input required to derive the TSL. The TSL and cross sections have been evaluated for the three non-equivalent atom cites in the CaH2: Ca, H1, and H2. Each evaluation has been submitted to the NNDC for consideration in the next ENDF/B database release.

A benchmark study of large-angle neutron scattering cross section of tungsten using two shadow bars technique at 14 MeV

In fusion reactor design, neutron leaks intensively from blanket material through a gap. In this streaming phenomenon, backscattering cross section is known to be very crucial. In the present study, the author's team carried out a new experiment for benchmarking the large angle scattering cross section of tungsten using a DT neutron source of OKTAVIAN facility, Osaka University, Japan. Tungsten-containing material is under consideration as the radiation shield in a fusion reactor. The experimental geometry consists of a DT neutron source, two shadow bars, niobium foil, and a tungsten target. Four irradiations were performed at a neutron energy of 14 MeV using DT neutrons to extract only the contribution of large angle scattering cross section. By using two shadow bars, room return contribution was effectively suppressed. Consequently, only backscattering neutrons were measured by using a niobium foil. In the present benchmark study, obtained experimental data were compared with numerical calculations by MCNP6 using various nuclear data libraries, including JENDL-4.0, JENDL-5, JEFF-3.3, and ENDF/B-VIII.

Neutron thermal cross sections of 3D-printing organic polymers using the Average Functional Group Approximation

We provide a worked example on how to obtain the total neutron scattering cross section of organic polymers at thermal neutron energies by means of the Average Functional Group Approximation. Within this framework, a polymer is rationalised as composed of hydrogen atoms belonging to specific functional groups,therefore taking into account the material-specific incoherent inelastic scattering contributions to the thermal cross section. Such simplified model allows the calculation of thermal neutron cross sections for a broad variety of materials of interest in neutron applications, e.g., sample containers and shielding components. We discuss in detail the case of ABS, often available as a 3D-printing material, whereby three polymers (acrylonitrile, butadiene, and styrene) can be combined in several mass percentages depending on the final application. Results are obtained using the NCrystal module already featuring the Average Functional Group Approximation.

Magnetic neutron scattering from spherical nanoparticles with Néel surface anisotropy: atomistic simulations.

A dilute ensemble of randomly oriented non-interacting spherical nanomagnets is considered, and its magnetization structure and ensuing neutron scattering response are investigated by numerically solving the Landau-Lifshitz equation. Taking into account the isotropic exchange interaction, an external magnetic field, a uniaxial magnetic anisotropy for the particle core, and in particular the Néel surface anisotropy, the magnetic small-angle neutron scattering cross section and pair-distance distribution function are calculated from the obtained equilibrium spin structures. The numerical results are compared with the well known analytical expressions for uniformly magnetized particles and provide guidance to the experimentalist. In addition, the effect of a particle-size distribution function is modelled.

Magnetic neutron scattering from spherical nanoparticles with Néel surface anisotropy: analytical treatment.

The magnetization profile and the related magnetic small-angle neutron scattering cross section of a single spherical nanoparticle with Néel surface anisotropy are analytically investigated. A Hamiltonian is employed that comprises the isotropic exchange interaction, an external magnetic field, a uniaxial magnetocrystalline anisotropy in the core of the particle and the Néel anisotropy at the surface. Using a perturbation approach, the determination of the magnetization profile can be reduced to a Helmholtz equation with Neumann boundary condition, whose solution is represented by an infinite series in terms of spherical harmonics and spherical Bessel functions. From the resulting infinite series expansion, the Fourier transform, which is algebraically related to the magnetic small-angle neutron scattering cross section, is analytically calculated. The approximate analytical solution for the spin structure is compared with the numerical solution using the Landau-Lifshitz equation, which accounts for the full nonlinearity of the problem. The signature of the Néel surface anisotropy can be identified in the magnetic neutron scattering observables, but its effect is relatively small, even for large values of the surface anisotropy constant.

Z+: Neutron cross section separation from wide-angle uniaxial polarization analysis.

We introduce a simple method to extract the nuclear coherent and isotope incoherent, spin incoherent, and magnetic neutron scattering cross section components from powder scattering data measured using a single neutron beam polarization direction and a position-sensitive detector with large out-of-plane coverage. The method draws inspiration from polarized small-angle neutron scattering and contrasts with conventional so-called "xyz" polarization analysis on wide-angle instruments, which requires measurements with three orthogonal polarization directions. The viability of the method is demonstrated on both simulated and experimental data for the classical "spin ice" system Ho2Ti2O7, the latter from the LET direct geometry spectrometer at the ISIS facility. The cross section components can be reproduced with good fidelity by either fitting the out-of-plane angle dependence around a Debye-Scherrer cone or grouping the data by angle and performing a matrix inversion. The limitations of the method and its practical uses are discussed.

Correlated n−γ angular distributions from the Q=4.4398 MeV C12(n,n′γ) reaction for incident neutron energies from 6.5 MeV to 16.5 MeV

Neutron scattering cross sections and angular distributions represent one of the most glaring sources of uncertainty in calculations of nuclear systems. Even simple nuclei like $^{12}\mathrm{C}$ show indications of errors in nuclear databases for scattering reactions. Measurements of inelastic neutron scattering have historically measured either the scattered neutrons or the nuclear deexcitation $\ensuremath{\gamma}$ emission. Only a very small number of experiments attempted correlated measurements of both the neutron and $\ensuremath{\gamma}$ data simultaneously, even though these $n\text{\ensuremath{-}}\ensuremath{\gamma}$ correlations could be essential for understanding particle transport in nuclear systems. In this work we describe a measurement of the $n, \ensuremath{\gamma}$, and correlated $n\text{\ensuremath{-}}\ensuremath{\gamma}$ angular distributions from the $Q=4.4398 \mathrm{MeV}$ $^{12}\mathrm{C}(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction in a single experiment using an EJ-309 liquid scintillator detector array with wide angular coverage, and with a continuous incident neutron energy range from 6.5 to 16.5 MeV. We also provide a thorough covariance description of these results, including normalization of the probability distribution. While the measured $n$ distributions agree well with the relatively large number of available literature measurements, there are comparatively very few measurements of the $\ensuremath{\gamma}$ distributions from this reaction. However, our data support the presence of a nonzero ${a}_{4}$ Legendre polynomial component of the $\ensuremath{\gamma}$ angular distribution suggested in past measurements, which is currently not incorporated in the ENDF/B-VIII.0 library despite the use of these same literature data for evaluation of the $^{12}\mathrm{C}(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ cross section. The correlated $n\text{\ensuremath{-}}\ensuremath{\gamma}$ distribution measurements are limited to three measurements at incident neutron energies near 14 MeV. Our results do not generally agree with any of these literature measurements. We observe clear indications of significant changes in the $n$ distribution for specific $\ensuremath{\gamma}$-detection angles and vice versa especially near thresholds for other reaction channels, which shows the potential for significant bias in experiments that, for example, tag on inelastic scattering using a single or small number of $\ensuremath{\gamma}$-detection angles and could impact particle transport calculations.

Study of the influence of MoO3 concentration on the chemical structure, physical properties, and radiation absorption prowess of alumino lead borate glasses

The present study presents the preparation of a glass system with composition of 55B2O3 - 30Pb3O4- Al2O3- MoO3, where by melt quenching conventional method. The structure of the synthesized samples was examined by XRD and FT-IR techniques. It was found that the molybdenum acts as a modifier and enhances the change between BO3 and BO4 structural units. Increasing MoO3 in the sample improved the glass network compactness. The influence of MoO3 content was studied on several physical parameters such as inter-ionic distance (Ri), the polaron radius (rp) and internuclear distance (ri), B—B separation (dB-B) average coordination number, number of bonds, field strength of (Mo+3), and the floppy modes. Increasing MoO3 doping concentration from 0–5 mol % produced corresponding increase in fast neutron effective removal cross section from 0.07127–0.10825 cm−1, total cross section for thermal neutrons from 68.35875–105.7526 cm−1, and an increment in the cold neutron scattering cross section. Furthermore, the influence of MoO3 doping in the glasses was such that the stopping powers and ranges RCSDA /Rp of electrons, proton, alpha particles, and carbon ion follows the trend: BPAM-G1 > BPAM-G2 > BPAM-G3 > BPAM-G4 > BPAM-G5, and (RCSDA /Rp)BPAM-G1 > (RCSDA /Rp)BPAM-G2 > (RCSDA /Rp)BPAM-G3 > (RCSDA /Rp)BPAM-G4 > ((RCSDA /Rp)BPAM-G5 respectively. On the other hand, the doping produced no noticeable differences in the equivalent atomic number and the exposure buildup factor of the glasses.

Unraveling the magnetic softness in Fe-Ni-B-based nanocrystalline material by magnetic small-angle neutron scattering.

Magnetic small-angle neutron scattering is employed to investigate the magnetic interactions in (Fe0.7Ni0.3)86B14 alloy, a HiB-NANOPERM-type soft magnetic nanocrystalline material, which exhibits an ultrafine microstructure with an average grain size below 10 nm. The neutron data reveal a significant spin-misalignment scattering which is mainly related to the jump of the longitudinal magnetization at internal particle-matrix interfaces. The field dependence of the neutron data can be well described by micromagnetic small-angle neutron scattering theory. In particular, the theory explains the 'clover-leaf-type' angular anisotropy observed in the purely magnetic neutron scattering cross section. The presented neutron data analysis also provides access to the magnetic interaction parameters, such as the exchange-stiffness constant, which plays a crucial role towards the optimization of the magnetic softness of Fe-based nanocrystalline materials.