Bragg prism monochromator and analyser for super ultra-small angle neutron scattering studies

We have designed, fabricated and operated a novel Bragg prism monochromator-analyser combination. With a judicious choice of the Bragg reflection, its asymmetry and the apex angle of the silicon single crystal prism, the monochromator has produced a neutron beam with sub-arcsec collimation. A Bragg prism analyser with the opposite asymmetry has been tailored to accept a still sharper angular profile. With this optimized monochromator-analyser pair, we have attained the narrowest and sharpest neutron angular profile to date. At this facility, we have recorded the first SUSANS spectra spanning wave vector transfers Q ～ 10^?6 Å^?1 to characterize samples containing agglomerates up to tens of micrometres in size.     

Neutron Transmission of Single-crystal Sapphire Filters

An additive formula is given that permits the calculation of the nuclear capture, thermal diffuse and Bragg scattering cross-sections as a function of sapphire temperature and crystal parameters. We have developed a computer program that allows calculations of the thermal neutron transmission for the sapphire rhombohedral structure and its equivalent trigonal structure. The calculated total cross-section values and effective attenuation coefficient for single-crystalline sapphire at different temperatures are compared with measured values. Overall agreement is indicated between the formula and experimental data. We discuss the use of sapphire single crystal as a thermal neutron filter in terms of the optimum cystal thickness, mosaic spread, temperature, cutting plane and tuning for efficient transmission of thermal-reactor neutrons.     

In the wonderland of ultra-parallel neutron beams

Bragg reflections from single crystals yield angular widths of a few arcsec for thermal neutron beams. The Bonse-Hart proposal to attain a sharp, nearly rectangular profile by Bragg reflecting neutrons multiply from a channel-cut single crystal, was realized in its totality three and a half decades later by achieving the corresponding Darwin reflection curves for 5.23 Å neutrons. This facilitated SUSANS (Super USANS) measurements in the Q ～ 10^?5 Å^?1 range. The polarized neutron option was introduced into the SUSANS set-up by separating the up- and down-spin neutron beams by ～10 arcsec with a magnetic (air) prism. The neutron angular width has recently been reduced further by an order of magnitude to ～0.6 arcsec by diffracting 5.3 Å neutrons from a judiciously optimized Bragg prism. This constitutes the most parallel monochromatic neutron beam produced to date. I present the first SUSANS spectra probing the Q ～ 10^?6 Å^?1 domain, recorded with this beam.     

Single crystal diffraction with X-rays and neutrons: High quality at high pressure?

Abstract

Single crystal X-ray and neutron diffraction is essential for determining occupancies, positional as well as static and dynamic displacement parameters in crystalline matter by measuring Bragg, satellite or diffuse reflections. In our contribution a new low-temperature high pressure cell for neutron single crystal diffraction will be presented. It is designed to operate from a few K to ambient temperature in “orange” cryostats at pressures up to at least 3 GPa. We will present first neutron diffraction results obtained at E4/HMI Berlin and discuss the quality of the data and the significance of the results. A software package has been written (as a part of the PROMETHEUS system), which deals with the data reduction for both X-ray and neutron high pressure cell single crystal data. Likewise a data collection program has been developed for single crystal data collection on four-circle diffractometers using Merrill-Bassett cells. A series of single crystal experiments on H₂O and D₂O ice VI and KDP (KH₂PO₄) show that results of very high quality can be obtained routinely including even higher order terms in the atomic displacement parameters.     

Geometric formula for prism deflection

While studying neutron deflections produced by a magnetic prism, we have stumbled upon a simple ‘geometric’ formula. For a prism of refractive indexn close to unity, the deflection simply equals the product of the refractive powern − 1 and the base-to-height ratio of the prism, regardless of the apex angle. The base and height of the prism are measured respectively along and perpendicular to the direction of beam propagation within the prism. The geometric formula greatly simplifies the optimisation of prism parameters to suit any specific experiment.     

Neutron diffraction study of quasi-one-dimensional spin-chain compounds Ca<Subscript>3</Subscript>Co<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>6</Subscript>

We report the results of the DC magnetization, neutron powder diffraction and neutron depolarization studies on the spin-chain compounds Ca₃Co_2–x Fe _x O₆ (x = 0, 0.1, 0.2 and 0.4). Rietveld refinement of neutron powder diffraction patterns at room temperature confirms the single-phase formation for all the compounds in rhombohedral structure with space group Rc. Rietveld refinement also confirms that Fe was doped at the trigonal prism site, 6a (0, 0, 1/4) of Co. The high temperature magnetic susceptibility obeys the Curie-Weiss law; the value of the paramagnetic Curie temperature (θ _p) decreases as the concentration of iron increases and it becomes negative for x = 0.4. No extra Bragg peak as well as no observable enhancement in the intensity of the fundamental (nuclear) Bragg peaks has been observed in the neutron diffraction patterns down to 30 K. No depolarization of neutron beam has been observed down to 3 K confirming the absence of ferro- or ferrimagnetic-like correlation.      

X-ray Bragg diffraction in asymmetric backscattering geometry

We observe three effects in the Bragg diffraction of x rays in backscattering geometry from asymmetrically cut crystals. First, exact Bragg backscattering takes place not at normal incidence to the reflecting atomic planes. Second, a well-collimated (approximately 1 microrad) beam is transformed after the Bragg reflection into a strongly divergent beam (230 microrad) with reflection angle dependent on x-ray wavelength--an effect of angular dispersion. The asymmetrically cut crystal thus behaves like an optical prism, dispersing an incident collimated polychromatic beam. The dispersion rate is approximately 8.5 mrad/eV. Third, parasitic Bragg reflections accompanying Bragg backreflection are suppressed. These effects offer a radically new means for monochromatization of x rays not limited by the intrinsic width of the Bragg reflection.     

Characterisation of novel magnetic materials using the USANSPOL technique

USANSPOL is a novel ultra-small-angle scattering technique with polarised neutrons for investigation of magnetic materials. It represents a polarised neutron extension to traditional USANS which works with unpolarised neutrons. The high angular resolution of this technique relies on the narrow reflection width of perfect crystal reflections and is employed in a double-crystal diffractometer. Corresponding to the μrad resolution of the set-up, micro-structures of the order of a few tenths of a micrometre up to a few tens of micrometres may be investigated. Neutron polarisation is achieved by insertion of birefringent magnetic prisms between the monochromator crystal and the sample. Rocking the analyser crystal produces a scattering pattern for both neutron spin states in a single measurement but well separated in reciprocal space. By this technique, we have recently studied various amorphous Galfenol soft-magnetic ribbons which were produced by spinning from melt at different manufacturing conditions. USANSPOL allows for a determination of domain sizes of the non-magnetised samples and a study of the growing of magnetically homogeneous regions with increasing externally applied magnetic field. The manufacturing process of the ribbons is reflected in the magnetic micro-structure of the different specimens.     

On the Use of Depleted Uranium Dioxide as a Neutron Filter

A simple additive formula is given which, for neutron energies in the range 10^-4 < E<10 eV, permits calculation of the nuclear capture, thermal diffuse and Bragg scattering cross-sections of a crystalline material as a function of its crystal constants and temperature. Computer codes PUO and SUO were developed to calculate the total attenuation of reactor neutrons through poly- and mono-crystalline samples, respectively. The codes were applied to calculate the total cross-section of polycrystalline uranium and thorium dioxides in the energy range from 4 meV to 1 eV. The obtained agreement between the calculated values and available experimental values justifies the applicability of the used formula and the computer codes. A feasibility study on using depleted polycrystalline uranium dioxide as a cold neutron filter and the single crystal as a thermal neutron filter is given. The optimum crystal parameters and thickness for efficiently transmitting the thermal reactor neutrons while strongly attenuating both fast neutrons and -rays accompanying the thermal ones is also given.     

Carbide precipitates in solution-quenched PH13-8 Mo stainless steel: A small-angle neutron scattering investigation

This paper deals with the small-angle neutron scattering (SANS) investigation on solution-quenched PH13-8 Mo stainless steel. From the nature of the variation of the functionality of the profiles for varying specimen thickness and also from the transmission electron microscopy (TEM), it has been established that the small-angle scattering signal predominantly originates from the block-like metallic carbide precipitates in the specimen. The contribution due to double Bragg reflection is not significant in the present case. The single scattering profile has been extracted from the experimental profiles corresponding to different values of specimen thickness. In order to avoid complexity and non-uniqueness of the multi-parameter minimization for randomly oriented polydisperse block-like precipitate model, the data have been analyzed assuming randomly oriented polydisperse cylindrical particle model with a locked aspect ratio.     

On measuring the neutron coherent scattering length with ultrahigh precision

We propose an order of magnitude improvement in the present five parts in 10⁵ precision of a nondispersive interferometric measurement of the neutron coherent scattering lengthb _c. For this purpose we make a judicious selection of the Bragg angle for the interferometer and the sample thickness. The precision is further improved by an optimal choice of the Bragg reflection (and a consequent neutron wavelength). By performing the experiment in vacuum, errors arising from possible variations in the pressure, composition or humidity of the ambient air can be eliminated. On attaining such precision, we ought to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correctb _c from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimateb _c. The refractive index for neutrons can thus be determined to a phenomenal precision of a few parts in 10¹².     

A nano-polycrystalline diamond anvil cell with bulk metallic glass cylinder for single-crystal neutron diffraction

ABSTRACT

A high-pressure cell for in-situ single-crystal neutron diffraction was developed. The cell uses nano-polycrystalline diamond anvils in a tubular load frame made of bulk metallic glass which is highly transparent to neutrons and does not produce Bragg reflections. Diffraction peaks from a sample can be measured from almost any direction and the simple geometry of the cell allows accurate attenuation corrections. We demonstrate the operation of the cell by ambient-pressure experiment using a single-crystal of NaCl on the D9 diffractometer at the Institute-Laue-Langevin. A high-pressure experiment was also carried out on a single crystal of ice VII at 2.35?GPa showing the potential to detect weak diffraction spots. The correct integration of weak reflections together with the simple attenuation correction will help to carry out precise structure analysis and address new scientific problems using neutron diffraction.     

Interaction of neutrons with layered magnetic media in oscillating magnetic field

New experimental possibilities of investigating layered magnetic structures in oscillating magnetic fields are discussed. Spin-flip and nonspin-flip neutron reflection and transmission probabilities show a frequency dependency near the magnetic neutron resonance condition. This allows to increase the precision of the static magnetic depth profile measurements of the magnetized matter. Moreover, this opens new possibilities of measuring the induction of the oscillating field inside the matter and determining the magnetic susceptibility of the oscillating magnetic field. Refraction of neutrons as they pass through a magnetic prism in the presence of an oscillating magnetic field is also investigated. A non-polarized neutron beam splits into eight spatially separated neutron beams, whose intensity and polarization depend on the strength and frequency of the oscillating field. Also, it is shown that the oscillating magnetic permeability of an angstrom-thick layer can be measured with a neutron wave resonator.     

A POSSIBLE WAY TO DETECT THE INTERFERENCE BETWEEN THE NEUTRON SPINOR COMPONENTS

The interference effect between the neutron spinor components i s investigated. It is shown that the diffusion of the neutron wave-packet under an unhomogeneous magnetic field is related to the phase shift angle between the two spinor components. We propose that this effect could be used to detect the phase -shifk of a coherent neutron beam having passed through some medium. In the interference experiment based upon this.principle one will not need to use Bragg reflection from crystal planes for splitting and recombining the beam, and the dimension of the phase shift region will be much longer than that of the usual BonsAe-Hart type neutron interferometer.     

Soft X-ray resonant magnetic diffraction

We have conducted the first soft x-ray diffraction experiments from a bulk single crystal, studying the bilayer manganite La2-2xSr1+2xMn2O7 with x=0.475 in which we were able to access the (002) Bragg reflection using soft x rays. The Bragg reflection displays a strong resonant enhancement at the L(III) and L(II) manganese absorption edges. We demonstrate that the resonant enhancement of the magnetic diffraction of the (001) is extremely large, indeed so large that it exceeds that of the nonresonant Bragg diffraction. Resonant soft x-ray scattering of 3d transition metal oxides is the only technique for the atomic selective measurement of spin, charge, and orbital correlations in materials, such as high temperature superconductors, colossal magnetoresistance manganites, and charge stripe nickelates.     

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.      

Multilayer neutron monochromator-polarizer based on iron

In this paper, a multilayer neutron monochromator-polarizer, consisting of 490 alternating layers of iron and niobium, is proposed. Each layer of iron or niobium has a thickness of 25 Å. The use of thin layers in the structure period improves the quality of the monochromator-polarizer and yields a neutron beam reflected from this structure with high wavelength resolution and a small fraction of minor nonmonochromatic contributions to the intensity of the reflected beam. The fact that the layer thicknesses are the same in the period suppresses even-order Bragg peaks. The parameters of the first-order Bragg peak for eight model neutron monochromators-polarizers based on iron and cobalt are given. It follows from the calculated curve of the coefficient of neutron reflection from the Fe/Nb structure that the first-order Bragg peak has a half width of 1.1%, a relative reflection coefficient of 0.997, and a maximum polarizing efficiency of unity. We also present the first experimental curves of the coefficient of neutron reflection from this structure, depending on the momentum transfer for both spin components of the beam.     

Anomalous behavior of the dispersion of a neutron transmitting through a crystal at nearly Bragg energies

The features of the propagation of a neutron through a crystal at nearly Bragg energies has been studied within the framework of the preparation of an experiment on the search for the electric dipole moment of a neutron by the crystal diffraction method. The time of passage of the neutron through the crystal has been studied as a function of the deviation from the Bragg condition. The anomalous behavior of the dispersion of the neutron, i.e., the energy dependence of its average velocity, has been observed. It has been shown that the derivative dv/dE for the diffracting neutron at nearly Bragg energies can be three or four orders of magnitude larger than this derivative for a free neutron. This opens new possibilities in precision neutron spectroscopy.     

A stopping layer concept to improve the spatial resolution of gas-electron-multiplier neutron detector

In recent years, gas electron multiplier (GEM) neutron detectors have been developing towards high spatial resolution and high dynamic counting range. We propose a novel concept of an Al stopping layer to enable the detector to achieve sub-millimeter (sub-mm) spatial resolution. The neutron conversion layer is coated with the Al stopping layer to limit the emission angle of ions into the drift region. The short track projection of ions is obtained on the signal readout board, and the detector would get good spatial resolution. The spatial resolutions of the GEM neutron detector with the Al stopping layer are simulated and optimized based on Geant4GarfieldInterface. The spatial resolution of the detector is 0.76 mm and the thermal neutron detection efficiency is about 0.01% when the Al stopping layer is 3.0 μ m thick, the drift region is 2 mm thick, the strip pitch is 600 μ m, and the digital readout is employed. Thus, the GEM neutron detector with a simple detector structure and a fast readout mode is developed to obtain a high spatial resolution and high dynamic counting range. It could be used for the direct measurement of a high-flux neutron beam, such as Bragg transmission imaging, very small-angle scattering neutron detection and neutron beam diagnostic.     

Bragg reflection and isochromat spectroscopy of electrons from a magnetic single crystal surface: Novel polarization detectors?

Two effects, Bragg reflection and isochromat spectroscopy of electrons from a magnetic single crystal surface are proposed as candidates for a low energy spin polarization detector. A qualitative estimate shows that besides Fe or Ni a Gd single crystal surface might be a good choice for testing both effects.