Monochromatic Neutron Research Articles

Bent perfect crystal monochromator at the monochromatic focusing condition

Abstract. In conventional diffractometry with a mosaic monochromator due to a nonnegligi-ble λ−θ correlation of the monochromatic beam impinging the sample, the resolution consid-erably differs in the parallel (+,-) and anti-parallel (+,+) setting. However, a bent perfect crystal (BPC) monochromator at monochromatic focusing condition can provide equal reso-lution properties at both parallel and anti-parallel setting and gives a chance to use both sides of the scattering plane. Three diffraction geometries with the BPC monochromators were tested and the received data of FWHM of the line profiles and the corresponding ∆ d/d prove the possibility of simultaneous measurements in both settings. Introduction Conventional neutron diffractometers employing mosaic monochromators are used only in the parallel (+,-) measurement setting, because the resolution of the anti-parallel (+,+) setting is much worse [1]. However, if we could use bot h settings with the equivalent resolution, the measurement efficiency could be twice of that related to the traditional mode. A strong dif-ference in the resolution properties in the parallel and antiparallel settings is brought about by a nonnegligible λ−θ correlation of the monochromatized beam impinging the sample in the scattering plane. In order to overcome this problem the momentum distribution of the monochromatized neutrons should be symmetric with respect to the axis of the beam imping-ing the sample. Three monochromator arrangements which could provide a rather highly collimated monochromatic neutron beam with a negligible λ−θ correlation and a reasonable ∆λ (keeping the luminosity and the instrument resolution on a feasible level) have been al-ready tested. These are: 1.

A new polarized neutron reflectometer at Dhruva for specular and off-specular neutron reflectivity studies

A polarized neutron reflectometer for vertical sample geometry has been installed in the guide tube laboratory of Dhruva reactor at Trombay, India. The instrument uses a monochromatic neutron beam of 2.5 Å in a step scan mode with a linear position s

Determination and validation of prompt k0-factors with a monochromatic neutron beam at the Dhruva reactor

Prompt Gamma-ray Neutron Activation Analysis (PGNAA) was carried out using a reflected neutron beam of 0.018 eV energy at the Dhruva research reactor, Bhabha Atomic Research Centre, Mumbai, India. The neutron beam characteristics, such as dimension, homogeneity and thermal equivalent flux were evaluated. The prompt k 0-factors of about 15 elements were determined versus the 1951.1 keV gamma-ray of the 35Cl(n,γ) reaction. These prompt k 0-factors are compared with the recommended k 0-values for thermal neutrons and were found to be in good agreement, except for Gd, Cd and Hg. The internal mono-standard method was applied to analyze a meteorite and a stainless steel alloy (SS 316 M) using the recommended k 0-values from the literature. As to the alloy, the measured concentrations were in good agreement with the nominal composition. For the meteorite sample, the concentrations of the major elements were in good agreement with the values determined using conventional neutron activation analysis.

Suppression of Surface Effect by Using Bent-Perfect-Crystal Monochromator in Residual Strain Scanning

Using our long experience in Bragg diffraction optics we successfully used focusing principles for substantial increasing both luminosity and resolution of the strain scanners with respect to the conventional devices. Monochromatic neutrons are selected by the cylindrically bent monochromator from the reactor spectrum. There is a strong correlation between divergences of incoming and outgoing beams with respect to the polycrystalline sample, which can be easily manipulated by changing the monochromator bending radius. By setting a proper value of the radius, a narrow, quasiparallel and highly luminous output beam can by adjusted. The strong correlation between wavelength and direction of incoming and outgoing beams depending on the monochromator bending radius can be as well used for suppression of surface effect in residual strain scanning. In scanning near a sample surface aberration peak shifts arise due to the fact that the gauge volume defined by input and output slits is partially out of the sample and its value can be of the same order as the residual strain effects. In this work we demonstrate that by changing of the bending radius of monochromator, this surface effect can be suppressed to values smaller then experimental errors in residual strain scanning.

Conceptual monochromatic digital neutron radiography using continuous cold neutron beam

Conceptual monochromatic digital neutron radiography using continuous cold neutron beam has been discussed. Neutron filters to obtain monochromatic cold neutron are a series of combinations of mosaic single crystal monochrometer, Bragg cut-off polycrystalline and critical angle-based super-mirror. An experimental test is going to be conducted using the new cold neutron beam of the modified CNRF, JRR-3M, JAERI.

First phase-contrast tomography with thermal neutrons

Attenuation-contrast tomography with monochro-matic thermal neutrons becomes a standard tool at many places in the world. A new tomographic neutron technique-the inter-ferometric imaging or neutron phase-contrast tomography (nPCT)-is introduced. nPCT is similar to x-ray phase-contrast tomography (xPCT) and offers a complete three-dimensional (3-D) investigation of the attenuation, the small angle scattering, and the phase-shifting properties of isotope distributions in the sample. As a first demonstration of the feasibility of nPCT, an aluminum screw has been imaged in an aluminum block of slightly different composition. A spatial resolution of 100 /spl mu/m has been achieved and the high sensitivity of phase-contrast measurements with thermal neutrons was verified.

Low efficiency 2-dimensional position-sensitive neutron detector for beam profile measurement

A gas-filled position-sensitive wire detector has been developed for monitoring thermal neutron beams with flux up to 10 7 n/cm 2/s over an area ≈80 cm 2. It is filled with CF 4(4 atm) and a small quantity of 3He, has 1.2 mm spatial resolution, 0.1% efficiency (at 0.18 nm) and can achieve a count rate of 350 kHz. Experiments at the 30 MW HANARO reactor showed that the fabricated PSD is a very useful and convenient tool for adjusting neutron monochromators, especially focusing monochromators, and for monitoring monochromatic beams. It can be used for various neutron optics experiments.

Characterization and modelling of germanium assembled crystals for the diffraction of thermal neutrons

Two assembled germanium crystals, produced for use as neutron monochromators, are characterized by means of neutron and X-ray diffraction. The neutron and synchrotron experimental data (reflectivity profiles and topographs) are compared with calculations based on analytical models and Monte Carlo simulations. Our results show the limits of the standard theories and indicate that full Monte Carlo simulations can be a valid tool for interpreting real crystal reflectivities.

Neutron diffraction studies on La2−x Dy x Ca2xBa2Cu4+2x Oz superconductors

Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength lambda = 1.249 Angstroms. A series of samples with La2-xDyxCa2xBa2Cu4+2xOz stoichiometric composition, for x = 0.1 - 0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, exhibiting no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties are discussed in detail.

Neutron protein crystallography in JAERI

Neutron diffraction provides an experimental method of directly locating hydrogen atoms in proteins. After developing an original neutron detector (neutron imaging plate) and a novel practical neutron monochromator (elastically bent perfect Si monochromator), BIX-type diffractometers which were equipped with these tools were efficiently constructed at JRR-3 in Japan Atomic Energy Research Institute (JAERI), Japan and they have finished many protein crystallographic measurements and interesting results have come one after another. At the same time a method of growing large protein single crystals and a database of hydrogen and hydration have also been developed. In the near future, a pulsed neutron diffractometer for biological macromolecules has been proposed at J-PARC in JAERI.

Production of copper and Heusler alloy Cu 2MnAl mosaic single crystals for neutron monochromators

The growth of high-quality, large single crystals of copper allows the production of very efficient double focusing copper monochromators at ILL. The main difficulty of adapting the crystal mosaic to the instrument requirements has been overcome through the development of specific techniques such as the “onion peel” method and plastic deformations at high temperature. Several instruments have been equipped with new copper monochromators allowing a significant gain in the neutron flux onto the sample. ILL also produces Heusler single crystals with a controlled mosaic for polarized neutrons. Recently, an order of magnitude increase in polarized neutron flux has been reached on the triple axis spectrometer IN20 using a new Heusler monochromator. In addition, the Neutron Optics Laboratory carries out developments in the field of new materials for neutron monochromators. Mixed gradient crystals have been successfully grown with a variation in the d spacing Δ d/ d≈10 −2. Studies on new polarizing crystal alloys are also in progress and NiMnSb Heusler alloy having partially enriched Ni with 60Ni may be interesting for polarized neutron applications.

Bragg diffraction optics in neutron diffractometry

A review of recent practical applications of Bragg diffraction optics in neutron diffractometry is presented. Besides examples of a direct comparison of the flat Ge-mosaic and a bent perfect Si crystals for neutron monochromatization, the results of new tests of dispersive monochromator, multiple reflection monochromator and curved TOF analyzer for high- and ultra-high resolution studies as received within a wide collaboration are introduced.

Development of NiC/Ti multilayer neutron monochromators using ion beam sputtering instrument at KURRI

We have succeeded in fabricating high-reflectivity and small d-spacing which are 5.7 and 7.0 nm NiC/Ti multilayer neutron monochromators using ion beam sputtering technique. The reflectivities of these monochromators with d=5.7 and 7.0 nm are 0.81 and 0.91, respectively.

Cold pressing of copper single crystals for a large-area doubly focusing monochromator

The design for a large-area doubly focusing neutron monochromator consists of a large number of small square copper single crystals mounted onto thin aluminium blades that both buckle and rotate independently. This avoids the need for large individual alignment mechanisms for each crystal that introduce unacceptably high levels of background. However, it does require that the crystals be oriented such that the diffracting crystallographic planes are parallel to the crystal face. Cold pressing broadens the natural narrow mosaic of the virgin crystal discs to increase the diffracted intensity. This introduces anisotropy into the crystal that determines its orientation in the final monochromator. The alignment procedure used for each crystal before cutting out the square tile in the correct orientation is described. A few crystals are characterized in detail by neutron diffraction at various stages of the operation, revealing the variation in the mosaic width and the angular position of the reciprocal-lattice vector as a function of the azimuthal angle by rotating the crystal about the normal to its face. The twofold symmetry of the mosaic width of the pressed crystal is modulated by the 2π periodicity introduced by the precession of the reciprocal-lattice vector around the crystal face normal. Satisfactorily aligned crystals have a variation in the angular position for diffraction within the allowed tolerance.

Development of NiC/Ti multilayer neutron monochromators using ion beam sputtering instrument at KURRI

We have succeeded in fabricating high-reflectivity and small d-spacing which are 5.7 and 7.0nm NiC/Ti multilayer neutron monochromators using ion beam sputtering technique. The reflectivities of these monochromators with d=5.7 and 7.0nm are 0.81 and 0.91, respectively.

Bragg diffraction optics in neutron diffractometry

A review of recent practical applications of Bragg diffraction optics in neutron diffractometry is presented. Besides examples of a direct comparison of the flat Ge-mosaic and a bent perfect Si crystals for neutron monochromatization, the results of new tests of dispersive monochromator, multiple reflection monochromator and curved TOF analyzer for high- and ultra-high resolution studies as received within a wide collaboration are introduced.

Direct comparison of plastically deformed Ge mosaic crystal and bent perfect Si crystal for neutron monochromatization

The results of direct comparison of some monochromator properties of plastically deformed (mosaic) Ge single crystal with cylindrically bent perfect crystal (BPC) of Si are presented. Using a double-crystal diffractometer and α o=20′ collimation of the incident polychromatic beam we compared monochromatic beam current at the sample position for the take off angles of 40°, 50° and 60°. The comparison was carried out on the diffraction planes (3 1 1) M when the monochromators operated at nearly the same asymmetric diffraction geometry. For the sake of comparison of their resolution properties, Fe(1 1 0) and Fe(2 1 1) diffraction profiles of the beam scattered by a solid and well annealed α-Fe polycrystalline sample are also presented. It has been found that under these specific experimental conditions often used for residual strain scanning, the focusing bent Si crystal is a good cheap alternative to the flat mosaic Ge monochromator.

On a possible use of multiple Bragg reflections for high-resolution monochromatization of neutrons

Multiple diffraction effects resulting in Umweganregung have been studied in cylindrically bent perfect silicon single crystals. The aim of this experiment was to find strong umweg-effects providing a highly monochromatic and collimated beam for a possible practical use. Using three different cuts of the Si crystal slabs, several very intensive umweg-effects in the thermal region from 0.1 to 0.25 nm which could be used for high-resolution monochromatization namely at high-flux neutron sources have been found. The advantage of such umweg-monochromators would be their tunability through the variation of the curvature and thus the variability of the bandwidth Δ λ/ λ in the range from 10 −4 to 10 −3 and collimation of the orders of minutes of arc without any employment of Soller collimators.

Response of SIMPLE SDDs to monochromatic neutron irradiations

We report measurements of the response of large-volume superheated droplet detectors for the SIMPLE project to filtered monochromatic neutron beams of energies 54 and 149 keV. The results continue to indicate a significantly higher than anticipated response at low temperatures, but provide good support for the theoretical device calibration in temperature. A significant moderation of the incident beams by the aqueous gel of the detector and the temperature-stabilizing water bath is observed, such that the detector response is primarily governed by the superheating factor of the detector refrigerant. Translated into background neutron sensitivity of WIMP searches, these features indicate that a good choice of operating conditions is 9°C and 2 atm, with the temperature implying a mass sensitivity >5 GeV given that sufficient background reduction below this temperature can be achieved.

Developmental study of a plasma source of 14-MeV neutrons. I. General concept

The concept of a plasma source of monochromatic 14-MeV neutrons is proposed. It is shown that the fast neutrons can be produced in a moderately heated D 3He plasma irradiated by a flux of thermal neutrons. The efficiency of this n(thermal) → n(14 MeV) transformation in the compact device with a size of 10– 20 cm can be sufficient for providing an appreciable yield of the fast neutrons. Temperature of the D 3He plasma required for such a scenario is found to be only 100 eV for ions, and few keV for electrons. Key problems related to a further development of the present concept are discussed.