High-pressure neutron diffraction studies at LANSCE

The development of neutron diffraction under extreme pressure (P) and temperature (T) conditions is highly valuable to condensed matter physics, crystal chemistry, materials science, and earth and planetary sciences. We have incorporated a 500-ton press TAP-98 into the HiPPO diffractometer at the Los Alamos Neutron Science Center (LANSCE) to conduct in situ high-P–T neutron diffraction experiments. We have developed a large gem-crystal anvil cell, ZAP, to conduct neutron diffraction experiments at high P. The ZAP cell can be used to integrate multiple experimental techniques such as neutron diffraction, laser spectroscopy, and ultrasonic interferometery. More recently, we have developed high-P low-T gas/liquid cells in conjunction with neutron diffraction. These techniques enable in situ and real-time examination of gas uptake/release processes and allow accurate, time-dependent determination of changes in crystal structure and related reaction kinetics. We have successfully used these techniques to study the equations of state, structural phase transitions, and thermo-mechanical properties of metals, ceramics, and minerals. We have conducted researches on the formation/decomposition kinetics of methane, CO₂ and hydrogen hydrate clathrates, and hydrogen/CO₂ adsorption of inclusion compounds such as metal–organic frameworks (MOFs). The aim of our research is to accurately map out phase relations and determine structural parameters (lattice constants, atomic positions, atomic thermal parameters, bond lengths, bond angles, etc.) in the P–T–X space. We are developing further high-P–T technology with a new 2000-ton press, TAPLUS-2000, and a ZIA (Deformation-DIA type) cubic anvil package to routinely achieve pressures up to 20 GPa and temperatures up to 2000 K. The design of a dedicated high-P neutron beamline, LAPTRON, is also underway for simultaneous high-P–T neutron diffraction, ultrasonic, calorimetry, radiography, and tomography studies. Studies based on high-pressure neutron diffraction are important for multidisciplinary sciences, particularly for theoretical/computational modeling/simulations.     

Plasma enhanced CVD of SiOxCyHz thin film on different textile fabrics: Influence of exposure time on the abrasion resistance and mechanical properties

In order to improve textile fabric abrasion resistance, in this work a SiO_xC_yH_z thin film was realized by low pressure plasma chemical vapour deposition (PCVD) at room temperature, using hexamethyldisiloxane (HMDSO) as precursor compound. To test changes in the performance properties of the surface finished samples as a function of the type of the substrate, the deposition was carried out on different textile fabrics. The polymerization processes were followed by weight measurements of textile fabrics. It was found that, after PCVD, a significantly lower fabric weight loss was observed on treated samples after rubbing than on the untreated samples. The morphology, elemental composition and type of chemical bonding present in the film applied on textile fabrics were also investigated using electron scanning microscopy (SEM), energy dispersive X-ray (EDX) and infrared spectroscopy techniques (FT-IR (ATR)). The results showed a substantial enhancement of wear resistance for the surfaces modified with the presented process, while tensile and tearing strength were adversely affected.     

Neutron inelastic scattering spectrum and valence force field for neopentane

The neutron inelastic scattering spectrum of neopentane has been measured between 10 and 2000 cm^?1. A partial reassignment of the normal modes of vibration is presented. A vibrational analysis based on the Snyder and Schachtschneider valence force field has been carried out using the frequencies of neopentane and neopentane-d₁₂. The calculated neutron spectrum, using our new force field, is found to be in good agreement with the observed neutron data.     

A comparison between neutron-fluence measurements using metal-activation monitors and standard glasses calibrated via thin uranium-fission monitors and via ηq method

The two fundamental approaches to fission-track dating involve either an explicit determination of the thermal neutron fluence (φ-method) or a calibration against age standards (ζ-method). The neutron fluence measurements are carried out with metal-activation monitors or with uranium-fission monitors, co-irradiated with the samples. Uranium-fission monitors consist of either a thin “mono-atomic”) film, or a thick fission source (standard uranium glass) irradiated against a muscovite external track detector. In this work, different techniques for performing neutron-fluence measurements were compared: based on thin-film calibration, based on thick-source calibration, and based on gamma spectrometry of co-irradiated metal monitors (Au, Co). The results suggest that more experiments are needed to make all calibrations consistent, including new measurements of the length of etched induced tracks in mica. Also the standard glass calibration carried out with thin films should be confirmed with a greater number of calibrating irradiations.     

Iron speciation in ancient Attic pottery pigments: a non‐destructive SR‐XAS investigation

The present work reports a detailed investigation on the speciation of iron in the pigments of decorated pottery fragments of cultural heritage relevance. The fragments come from the Gioiosa Guardia archaeological site in the area of the `Strait of Messina' (Sicily, Southern Italy), and date back to VI–V century BC. The purpose of this study is to characterize the main pigmenting agents responsible for the dark‐red coloration of the specimens using non‐destructive analytical techniques such as synchrotron radiation X‐ray absorption spectroscopy (SR‐XAS), a well established technique for cultural heritage and environmental subjects. Absorption spectra were collected at the Fe K‐edge on the Italian beamline for absorption and diffraction (BM8‐GILDA) at the European Synchrotron Radiation Facility in Grenoble (France). In order to determine the speciation of Fe in the samples, principal component analysis and least‐squares fitting procedures were applied to the near‐edge part of the absorption spectra (XANES). Details on the local structure around the Fe sites were obtained by analyzing the extended part of the spectra (EXAFS). Furthermore, an accurate determination of the average Fe oxidation state was carried out through analysis of the pre‐edge peaks of the absorption spectra. Samples resulted composed of an admixture of Fe₂O₃ (hematite or maghemite) and magnetite (Fe₃O₄), occurring in different relative abundance in the dark‐ and light‐colored areas of the specimens. The results obtained are complementary to information previously obtained by means of instrumental neutron activation analysis, Fourier transform infrared absorbance and time‐of‐flight neutron diffraction.     

Investigation of adsorption behaviour of fission products in soils using multitracer technique

The adsorption mechanism of fission products in soils has been investigated by using multitracer technique. The multitracer method used in this work contained fission products as carrier free isotopes which were prepared by neutron irradiation of ²³⁵U at Kyoto University Research Reactor. After the soil sample was mixed into the multitracer solution for two weeks, the solution was separated from soil sample. The gamma-ray spectrometry was carried out for the separated solution and the original multitracer solution which has not been mixed with soils. Distribution coefficients of fission products for soil samples were estimated by measured photoelectric peaks. In this work, distribution coefficients of ten elements for thirty typical Japanese soils were observed.     

A study of 192Ir production conditions at an electron accelerator

This communication deals with the conditions of ¹⁹²Ir isotope production under a nonreactor technology via the ¹⁹³Ir(γ, n)¹⁹²Ir reaction. It can be carried out by irradiation of a target from natural iridium with the high-energy X-ray of an electron accelerator. The possibility of increasing the photonuclear yield of the target isotope by addition of the ¹⁹¹Ir(n, γ)¹⁹²Ir reaction induced by moderated photoneutrons has been shown. For this, an X-ray converter and a target were placed inside a neutron moderator. Data on the ¹⁹²Ir and admixture yields for the techniques using the moderator and without it have been obtained by computer simulation and experimentally.     

Characterisation of the high-energy neutron beam at iThemba LABS

Monoenergetic and white neutron beams with a maximum energy of 200 MeV are produced at the neutron beam facility of the iThemba Laboratory for Accelerator-Based Sciences (iTL) by (p, n) reactions on Li, Be and C targets. The iTL separated-sector cyclotron can accelerate protons from 25 MeV to 200 MeV. Time of flight (TOF) measurements can be carried out by increasing the time separation between proton bunches up to 500 ns with a beam pulse selector. In the present work, the methods of beam characterisation used at this facility are discussed. Comparisons with metrological standards are presented. Specific issues related to monitoring the neutron fields at different accelerator settings are analysed.     

Neutron polarizations from the9Be(d,n)10B and11B(d,n)12C reactions

Neutron polarization angular distributions were measured for the⁹Be(d,n)¹⁰B and¹¹B(d,n)¹²C reactions for deuteron energies of 5.47 and 5.34 MeV, respectively. Using neutron time-of-flight techniques, polarizations to eight states in¹⁰B and to six states in¹²C have been measured. Neutron polarization energy excitation functions for both reactions were measured for deuteron energies from 3.0 to 5.5 MeV in 0.25 MeV steps at 30° (lab). Distorted wave method calculations carried out to compare theoretical calculations against experimental results were not in good agreement with the data.     

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho_2−xY_xTi₂O₇ (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1 −1 0) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6 K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30 mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi₂O₇ and compare our results with pure spin ice Ho₂Ti₂O₇. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.     

Corona-charged polypropylene electrets analyzed by XPS

In the present study, we use X-ray photoelectron spectroscopy (XPS) analysis to clarify how different polarities of corona initiate various changes in the surfaces of polypropylene (PP) electrets subject to corona discharge. The samples were charged in three-electrode corona discharge system using positive and negative corona polarities at both −20 and 75 °C temperatures. The tests were divided into four groups. The surface potentials of the electret samples were measured using the vibrating electrode method with compensation. XPS studies were carried out by means of a VG ESCALAB Mk II electronic spectrometer using an Al Kα excitation source (hν=1486.6 eV). The spectra of C1s, O1s and N1s lines for all groups and for untreated samples were recorded and analyzed. The investigations that we carried out show that for negative-corona-charged samples, the oxygen content is approximately 2.4 times higher than that in positive-corona-charged samples. Based on the results we have obtained, we may assume that the changes in oxygen content in samples charged by different polarity coronas lead to the formation of different surface local levels. This assumption agrees well with the experimental measurement made on the electrets.     

Negative thermal expansion in framework compounds

We have studied negative thermal expansion (NTE) compounds with chemical compositions of NX₂O₈ and NX₂O₇ (N=Zr, Hf and X=W, Mo, V) and M₂O (M=Cu, Ag) using the techniques of inelastic neutron scattering and lattice dynamics. There is a large variation in the negative thermal expansion coefficients of these compounds. The inelastic neutron scattering experiments have been carried out using polycrystalline and single crystal samples at ambient pressure as well as at high pressures. Experimental data are useful to confirm the predictions made from our lattice dynamical calculations as well as to check the quality of the interatomic potentials developed by us. We have been able to successfully model the NTE behaviour of these compounds. Our studies show that unusual phonon softening of low energy modes is able to account for NTE in these compounds.      

Ground-state properties of neutron magic nuclei

A systematic study of the ground-state properties of the entire chains of even–even neutron magic nuclei represented by isotones of traditional neutron magic numbers N = 8, 20, 40, 50, 82, and 126 has been carried out using relativistic mean-field plus Bardeen–Cooper–Schrieffer approach. Our present investigation includes deformation, binding energy, two-proton separation energy, single-particle energy, rms radii along with proton and neutron density profiles, etc. Several of these results are compared with the results calculated using nonrelativistic approach (Skyrme–Hartree–Fock method) along with available experimental data and indeed they are found with excellent agreement. In addition, the possible locations of the proton and neutron drip-lines, the (Z, N) values for the new shell closures, disappearance of traditional shell closures as suggested by the detailed analyzes of results are also discussed in detail.     

HgO-added YB<Subscript>a2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>−δ superconductors

The HgO-added YB_a2Cu₃O_7−δ (YBCO) superconductor has been studied for its structural and superconducting properties. Polycrystalline YBCO samples were synthesized through solid-state reaction method by adding HgO in different concentrations without using oxygen annealing. All the samples showed a sharp superconducting transition temperature around 90 K. The X-ray diffraction patterns of all the samples revealed monophasic Y-123 nature. The structural studies were carried out by neutron scattering and Rietveld analysis. The neutron scattering revealed that Hg is not incorporated in the Y-123 system and has shown optimum oxygen concentration. The significant role played by the HgO is to provide oxygen ambient through its decomposition, thus changing the oxygen balance in favour of high Cu-valence state.     

Determination of the emission rate of an Am–Be neutron source with a Bonner sphere spectrometer

The Neutron Measurements Laboratory of the Technical University of Madrid (LMN) has an automated panoramic irradiator with a 111 GBq ²⁴¹Am–Be neutron source installed in a bunker-type large room. This facility is going to be used for calibration purposes. Recently, a spectrometry campaign involving four research groups working with different Bonner Sphere Spectrometers (BSS) and using different spectral unfolding codes was carried out. As part of these measurements the emission rate, B(t), was estimated. The application of the generalized fitting method to account for the scattering contribution is difficult due to specific characteristics of the neutron installation. A reduced fitting method, which includes room-return and in-scatter, has instead been used to overcome this problem.Detailed Monte Carlo simulations (with MCNPX code) were also performed to estimate the fluence rate using the measured source strength value. This was performed at different points. Results were then compared with measurements.Finally, the ambient dose equivalent rate measured with a neutron monitor (LB6411) was compared with results using the BSS.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

Fission-track dating of Macusanite glasses with plateau and size correction methods

Two Macusanite pebbles (MB1 and MB2) were dated with the fission-track method. Six irradiations were carried out in different nuclear reactors: Pavia (Italy), IPEN-CNEN (Brazil) and IPEN-Lima (Peru). Measurements of the thorium and uranium induced-fission per target nucleus using natural thorium thin films and natural U-doped glasses calibrated against natural uranium thin films, together with a λ_F of 8.46×10⁻¹⁷ a⁻¹ were used to determine the ages. The apparent ages were corrected using the plateau and size correction methods. Track measurements were performed by different analysts, using different counting criteria. In addition, tracks were measured on samples which had been submitted to thermal treatment as well as on samples which had not been heated. Thermal treatments were carried out to erase the fossil tracks before neutron irradiation. No significant differences have been found in individual results, using the two Macusanite pebbles and the different nuclear reactors, age correction techniques, analysts, track-counting criteria, and thermal treatments before neutron irradiation. The great majority of the results (14/17) is compatible with the Ar–Ar ages of 5.12±0.11 and 5.10±0.11 Ma, Macusanite MB1 and MB2, respectively. However, the fission-track ages are systematically less (8%) than the Ar–Ar ages of the two Macusanite samples studied.     

Measurements of effective delayed neutron fraction in a fast neutron reactor using the perturbation method

A perturbation method is proposed to obtain the effective delayed neutron fraction β_(eff) of a cylindrical highly enriched uranium reactor.Based on reactivity measurements with and without a sample at a specified position using the positive period technique,the reactor reactivity perturbation △ρ of the sample in β_(eff) units is measured.Simulations of the perturbation experiments are performed using the MCNP program.The PERT card is used to provide the difference dκ of effective neutron multiplication factors with and without the sample inside the reactor.Based on the relationship between the effective multiplication factor and the reactivity,the equation β~(eff)=dκ/△ρ is derived.In this paper,the reactivity perturbations of 13 metal samples at the designable position of the reactor are measured and calculated.The average β_(eff) value of the reactor is given as 0.00645,and the standard uncertainty is 3.0%.Additionally,the perturbation experiments for β_(eff) can be used to evaluate the reliabilities of the delayed neutron parameters.This work shows that the delayed neutron data of ~(235)U and ~(238)U from G.R.Keepin's publication are more reliable than those from ENDF-B6.0.ENDF-B7.0,JENDL3.3 and CENDL2.2.