Dependent Neutron Research Articles

Freezing of the octahedral tilt near ferromagnetic transition and appearance of a glassy phase at low temperature driven by the tilt instabilities in SrRuO3

The emergence of low temperature glassy phase in widely known itinerant ferromagnet SrRuO3 is remotely understood. In order to understand this aspect, we have undertaken a detailed temperature dependent (5–250 K) neutron diffraction study. We observe a freezing of the octahedral tilt near the ferromagnetic transition and an unusual deviation in the octahedral tilt near the onset of low temperature spin glass like phase. A reduction of the ordered magnetic moment and a decline in the total integrated magnetic intensity is observed around the temperature where the glassy behaviour starts to appear. The magnetotransport study also reveals the possibility for an additional magnetic ordering by demonstrating a peak in magnetoresistance at the low temperature side as well. The neutron diffraction study presented here provides useful information to understand the observed unusual low temperature magnetic phenomena in SrRuO3.

Numerical solution of the time dependent neutron transport equation by the method of the characteristics

This study presents three numerical algorithms to solve the time dependent neutron transport equation by the method of the characteristics. The algorithms have been developed taking into account delayed neutrons and they have been implemented into the novel MCART code, which solves the neutron transport equation for two-dimensional geometry and an arbitrary number of energy groups. The MCART code uses regular mesh for the representation of the spatial domain, it models up-scattering, and takes advantage of OPENMP and OPENGL algorithms for parallel computing and plotting, respectively. The code has been benchmarked with the multiplication factor results of a Boiling Water Reactor, with the analytical results for a prompt jump transient in an infinite medium, and with PARTISN and TDTORT results for cross section and source transients. The numerical simulations have shown that only two numerical algorithms are stable for small time steps.

Temperature dependent neutron diffraction and Mössbauer studies in zinc ferrite nanoparticles

The micro-level magnetic alignments of zinc ferrite nanoparticles synthesized by sol–gel method were investigated using magnetization study, neutron diffraction and Mössbauer spectroscopy. Magnetization behavior of the sample clearly indicates the presence of superparamagnetism. The neutron diffraction was performed at seven different temperatures between 6K and 300K. The cation distribution is found to have a variation from the from normal spinel structure. Both A and B site magnetic moment values found to vary with temperature which is found to correlates with variations in bond lengths and lattice constants. The Mössbauer spectrum at room temperature shows typical doublet while the spectrum 5K shows well defined sextet. The two sub-spectra in the sextet resolves well by the application of 5T magnetic field and the noncollinear nature of magnetic alignment with in the tetrahedral and octahedral sites of zinc ferrite are deduced.

On the Build-Up Factor from the Multi-Group Neutron Diffusion Equation with Cylindrical Symmetry

We consider the time dependent neutron diffusion equation for one energy group in cylinder coordinates, assuming translational symmetry along the cylinder axis. This problem for a specific energy group is solved analytically applying the Hankel transform in the radial coordinate r. Our special interest rests in the build-up factor for a time dependent linear neutron source aligned with the cylinder axis, which in the limit of zero decay constant reproduces also the static case. The new approach to solve the diffusion equation by integral transform technique is presented and results for several parameter sets and truncation in the solution for the flux and build-up factor are shown and found to be compatible to those of literature [1,2].

Complex structural phase transitions in slightly Ca modified Na0.5Bi0.5TiO3

A temperature dependent neutron powder diffraction study, in conjunction with dielectric and ferroelectric characterization, of slightly Ca modified Na0.5Bi0.5TiO3 (NBT) revealed an instability with regard to a non-polar orthorhombic (Pbnm) distortion above room temperature. This intermediate orthorhombic phase has earlier been reported for unmodified NBT by electron diffraction studies, but has never been captured by global (x-ray/neutron) diffraction techniques. Calcium substitution seems to amplify the magnitude of this intermediate orthorhombic distortion thereby making the corresponding superlattice reflections become visible in the neutron diffraction pattern. The study revealed the following sequence of very complex structural evolution with temperature: .

Validation of energy moments from the one-dimensional energy dependent neutron diffusion equation, MCNP5 and Attila-7.1.0 with the GODIVA experiment

Normalized neutron energy moments (moments) from the one-dimensional energy dependent neutron diffusion equation (EDNDE), Monte Carlo N Particle 5 version 1.40 (MCNP5) and Attila-7.1.0-beta version (Attila) are validated with the GODIVA experiment (GODIVA). Energy moments 0–5 for all three methods are compared to GODIVA moments. GODIVA moments are measured with two methods. The 1st method is a time of flight (T-O-F) measurement of the average energy (moment 1) of the leaking neutrons from the surface of GODIVA and the 2nd method is from back calculating moments from foil activation analysis of various metal foils at the center of GODIVA. The error range of the EDNDE normalized moments compared to GODIVA is from 0% to 24%. The MCNP5 error range compared to GODIVA is 0–12% and the Attila error range is 0–79%. The method of moments is shown to be a fast reliable method, compared to either Monte Carlo methods (MCNP5) or 30 multi-energy group methods (Attila) with regard to the GODIVA experiment.

Verification of analytic energy moments for the one-dimensional energy dependent neutron diffusion equation with MCNP5 and Attila-7.1.0

The energy dependent neutron diffusion equation (EDNDE) is converted into a moment equation which is solved analytically for the 1-D problem of a bare sphere of pure 235U. The normalized moments 0–5 generated analytically are compared to normalized energy moments, from Monte Carlo N Particle 5 version 1.40 (MCNP5) and Attila-7.1.0-beta version (Attila). The analytic normalized neutron energy moments, fall between the results from MCNP5 (lower bound) and Attila (upper bound) and are accurate compared to MCNP5 neutron energy moments when error in this Monte Carlo simulation are considered. The error range is from 0% to 14%. The Attila moments are less accurate when compared to MCNP5 than the analytical moments derived in this work. The method of moments is shown to be a fast reliable method, compared to either Monte Carlo methods (MCNP5) or 30 multi-energy group methods (Attila).

Determination of the magnetic structure of SmFe3(BO3)4 by neutron diffraction: comparison with other RFe3(BO3)4 iron borates

Temperature dependent neutron diffraction studies were performed on SmFe3(BO3)4. The crystallographic structure was determined to stay as R32 over the whole studied temperature range of 2 K < T < 300 K. A magnetic phase transition characterized by the magnetic propagation vector κ = [0 0 3/2] takes place at TN = 34 K. The magnetic structure sees an easy-plane arrangement within the trigonal basal a–b-plane of ferromagnetic layers of iron and samarium having a canting angle of about 70° relative to each other. Neighbouring layers in the c-direction are antiferromagnetically coupled; at 2 K the magnetic moment values amount to μFe = 4.2(1) μB and μSm = 0.8(2) μB. The non-Brillouin type increase of the iron magnetic moment below TN points to a strong Fe–Sm exchange and to the simultaneous appearance of long range magnetic order on both sublattices.

Pressure and Temperature Dependent Structure Of Zircon Type ThGeO4

Herein we report the results of high pressure diffraction studies of zircon type ThGeO4. ThGeO4 exhibits anisotropic compressibility with the average compressibility along a-axis (20.8 × 10−4/GPa) larger than that along c-axis (9.98 × 10−4/GPa). Fitting the pressure dependence unit cell volume to 3rd order Birch-Murnaghan equation of states, the zero pressure bulk modulus (Ko) and volume (Vo) of 166(5) GPa and 341.6(3) Å3, respectively have been obtained. Preliminary studies on temperature dependent neutron and x-ray diffraction studies on ThGeO4 revealed anisotropic expansion behaviour with larger expansion coefficient along c-axis compared to a-axis. No structural transition under temperature or pressure is observed in between ambient pressure to 10 GPa and in the temperature range of 25 to 1273K.

On the use of different analytical solutions for recalculation of the YALINA-Booster experiment SC3A

The SC3A experiment in the YALINA-Booster facility in Belarus is described and investigated. For this investigation the very special configuration of YALINA-Booster core, consisting of a fast and a thermal zone, decoupled with a neutron ‘valve’ is analyzed in detail based on a full HELIOS model for the calculations. The two region design causes unexpected results in the experiments. The special problems for the analysis of the experiments are shown. The results for different analytical solution (one group diffusion, one group P1 transport and two group diffusion) are analyzed and compared. To model the streaming of neutrons from the thermal area into the fast area, a special two group analytical solution for the space–time dependent neutron flux with two sources is developed from the available Green’s functions for two groups. The new analytical solutions show very good agreement in the comparison with the experimental results. Especially, with the two group and two source solution the unexpected behavior at the outermost detector can be reproduced. Thus analytical solutions without separation of space and time are a very promising tool to develop a new method for the analysis of ADS experiments.

Inelastic neutron scattering an ab-initio calculation of negative thermal expansion in Ag2O

The compound Ag2O undergoes large and isotropic negative thermal expansion over 0–500K. We report temperature dependent inelastic neutron scattering measurements and ab-initio calculations of the phonon spectrum. The temperature dependence of the experimental phonon spectrum shows strong anharmonic nature of phonon modes of energy around 2.4meV. The ab-initio calculations reveal that the maximum negative Grüneisen parameter, which is a measure of the relevant anharmonicity, occurs for the transverse phonon modes that involve bending motions of the Ag4O tetrahedra. The thermal expansion is evaluated from the ab-initio calculation of the pressure dependence of the phonon modes, and found in good agreement with available experimental data.

RITAC: Reactivity Initiated Transient Analysis Code – An overview

A point reactor kinetics code, coupled with thermal hydraulics of plate type fuel, is developed in order to carry out the reactivity initiated transient analysis of a reactor. Point kinetics equations are solved by matrix algebra, which involves piecewise constant approximation (PCA) i.e. the time dependent functions-reactivity and neutron source terms are assumed to be constant within an interval of time. This code is able to simulate various kinds of reactivity insertion like step, ramp, sinusoidal etc. for transient analysis. Effect of reactivity feedback is included into the code. Solution of kinetics equations gives neutronic power and it is then fed into a thermal hydraulic code where thermal energy conservation equations are solved by finite difference method along with Crank–Nicolson technique to find out the fuel, clad and coolant temperatures during transients. Results of the code corroborate well with the benchmark results.

Modified Block Newton method for the lambda modes problem

AbstractTo study the behaviour of nuclear power reactors it is necessary to solve the time dependent neutron diffusion equation using either a rectangular mesh for PWR and BWR reactors or a hexagonal mesh for VVER reactors. This problem can be solved by means of a modal method, which uses a set of dominant modes to expand the neutron flux. For the transient calculations using the modal method with a moderate number of modes, these modes must be updated each time step to maintain the accuracy of the solution. The updating modes process is also interesting to study perturbed configurations of a reactor. A Modified Block Newton method is studied to update the modes. The performance of the Newton method has been tested for a steady state perturbation analysis of two 2D hexagonal reactors, a perturbed configuration of the IAEA PWR 3D reactor and two configurations associated with a boron dilution transient in a BWR reactor.

Resonance Integral of Neptunium(237Np) from Energy Dependent Differential Neutron Capture Cross-Section by Using the Linac TOF Method and C6D6Scintillation Spectrometer

<TEX>$^{237}Np$</TEX> is very important material in the fission products of nuclear reactors. Resonance integral(RI) tests of this material is necessary to check between the experiments and the evaluated data. Such feedback to the evaluated data is very important to correct data and improve of codes. The RI for the <TEX>$^{237}Np(n,{\gamma})^{238}Np$</TEX> reaction were measured by using the 46-MeV electron linear accelerator (linac) at the Research Reactor Institute, Kyoto University (KURRI). The measurement was performed in the energy region from 0.005 eV and 10 keV. RI obtained as 804.7 barns, compared with those of the evaluated data in JENDL-4.0 and Mughabghab.

First order paramagnetic–ferromagnetic phase transition in Tb 3+ doped La 0.5Ca 0.5MnO 3 manganite

Effect of A-site disorder in half-doped La 0.5Ca 0.5MnO 3 created by the substitution of Tb at La site is studied through temperature dependent neutron diffraction, resistivity, and magnetization on La 0.375Tb 0.125Ca 0.5MnO 3 (LTC) to identify the evolution of phases as a function of temperature and magnetic field. Contrary to the parent system, the paramagnetic (PM) to ferromagnetic (FM) transition is found to be of first order but becomes observable only in fields above 1 T. Absence of this transition in zero and low fields along with the observation of a very intriguing and irreversible M– H curve at low temperature raises the question about the ground state of the system. It is identified from the specially designed measurement protocol that the ground state of the system becomes FM, contrary to the parent LCMO. However, this remains masked because of the hindrance to the transformation kinetics of the PM–FM first order transition in low fields. Significantly, though the thermal hysteresis of the first-order transition decreases with the measurement field, collapse of such a behavior to a critical point is not observed in LTC, unlike earlier observations in some other manganites.

Determination of self shielding factors and gamma attenuation effects for tree ring samples

Determination of tree ring chemistry using Neutron Activation Analysis (NAA) is part of an ongoing research between Penn State University (PSU) and Cornell University, The Malcolm and Carolyn Wiener Laboratory for Aegean and Near Eastern Dendrochronology. Tree-ring chemistry yields valuable data for environmental event signatures. These signatures are a complex function of elemental concentration. To be certain about concentration of signature elements, it is necessary to perform the measurements and corrections with the lowest error and maximum accuracy possible. Accurate and precise values of energy dependent neutron flux at dry irradiation tubes and detector efficiency for tree ring sample are calculated for Penn State Breazeale Reactor (PSBR). For the calculation of energy dependent and self shielding corrected neutron flux, detailed model of the TRIGA Mark III reactor at PSU with updated fuel compositions was prepared using the MCNP utility for reactor evolution (MURE) libraries. Dry irradiation tube, sample holder and sample were also included in the model. The thermal flux self-shielding correction factors due to the sample holder and sample for were calculated and verified with previously published values. The Geant-4 model of the gamma spectroscopy system, developed at Radiation Science and Engineering Center (RSEC), was improved and absolute detector efficiency for tree-ring samples was calculated.

Phase stability and structural temperature dependence in sodium niobate: A high-resolution powder neutron diffraction study

We report investigation of structural phase transitions in technologically important material sodium niobate as a function of temperature on heating over 300-1075 K. Our high resolution powder neutron diffraction data show variety of structural phase transitions ranging from non-polar antiferrodistortive to ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice parameters is found only at 633 K that indicates that the transition of orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm) phase is first order in nature, while other successive phase transitions are of second order. New superlattice reflections appear at 680 K (R phase) and 770 K (S phase) that could be indexed using an intermediate long-period modulated orthorhombic structure whose lattice parameter along <001> direction is 3 and 6 times that of the CaTiO3-like Pbnm structure respectively. The correlation of superlattice reflections with the phonon instability is discussed. The critical exponent ({\beta}) for the second order tetragonal to cubic phase transition at 950 K, corresponds to a value {\beta}$\approx 1/3$, as obtained from the temperature variation of order parameters (tilt angle and intensity of superlattice reflections). It is argued that this exponent is due to a second order phase transition close to a tricritical point. Based on our detailed temperature dependent neutron diffraction studies, the phase diagram of sodium niobate is presented that resolves existing ambiguities in the literature.

Phase Behaviour of CoCl2-MnCl2Mixed Crystals

AbstractThermal properties of CoCl2, MnCl2and their mixed crystals are investigated by means of temperature dependent neutron powder diffraction and time-resolved small angle neutron scattering. The coefficients of thermal expansion of these crystals have been determined as a function of temperature. The temperature-induced structural variations can be explained by distortions of CoCl6/MnCl6octahedra and changes in Co/Mn–Cl bond lengths. CoCl2and MnCl2form homogeneous solid solutions in the entire temperature range between the solidus and 50 K. There is no indication of any miscibility gap as predicted from thermodynamic calculations.

Magnetic structure and magneto-elastic-structural coupling in Cr-modified SrRuO3: A neutron powder diffraction study

Temperature dependent neutron powder diffraction and magnetization studies have been carried out on a Cr-substituted SrRuO3 close to the solubility limit, that is, SrRu0.88Cr0.12O3 (SRC-12). Evidence of ferromagnetic ordering below 200 K was deduced from magnetization studies as well as from neutron powder diffraction. The magnetic lattice coincides with the chemical one (k = 0) and the major component of the ordered magnetic moment has been found along the c-axis. The system exhibits anomalous variation in the cell volume close to the magnetic ordering temperature. The spontaneous magnetostrictive strain below the ordering temperature, estimated by fitting the temperature dependence of the lattice parameters and the cell volume using Debye–Grüneisen function, revealed that the main contribution to the volume magnetostriction comes from the c-axis. The magneto-elastic behavior is further shown to be coupled to the octahedral tilt about the c axis. The analysis of magnetic structure and the magneto-elastic strain suggests c axis to be the easy axis of magnetization for this system.

Neutron diffraction studies on the Heusler alloy Ni50Mn37Sb13

The evolution of martensitic to austenitic transformation in Ni50Mn37Sb13 has been studied usingtemperature dependent neutron diffraction, thermal property, and magnetization studies. Differential scanning calorimetric studies reveal a martensitic transformation TM around 291 K. The magnetization data yield a ferromagnetic ordering temperature of 329 K in the austenitic phase and 230 K in the martensitic phase. The analysis of the powder neutron diffraction data in the temperature range of 325–12 K indicates a structural transition from a high temperature cubic L21 type structure to an orthorhombic structure. At 270 K, both cubic and orthorhombic phases coexist. Anisotropic unit cell changes are observed at the martensitic transformation: The unit cell expands by about 1.5% along the a axis, by about 2.5% along the c axis, and compresses by about 4.28% along the b axis. Both cubic and orthorhombic phases show commensurate collinear ferromagnetic ordering with a magnetic moment of ∼3.67 μB/Mn in Mn (2a and 2f) sites.