F. Mezei

The polarizing beam splitter guide at BENSC

Abstract A polarizing beam-splitter guide, theoretically described earlier, was installed in the neutron guide hall at BENSC in the winter of 1993/94. It splits the incoming neutron beam with wavelengths beyond 2.5 A into two oppositely polarized beams. The active elements of the splitter system are Si CoFe supermirrors deposited on Si wafers, which show a good polarization capability between the critical angle of Si of 0.047° per A neutron wavelength and a cut-off angle of 0.21° per A, i.e. twice the critical angle of natural Ni. For single mirrors in the supermirror region an average reflection coefficient of 90% was achieved at a polarization of 96% for the reflected beam while the transmission coefficient amounted to 80% for a polarization of 97%. These values apply for a magnetizing field of 300 G and improve slightly for higher fields. Preliminary results for the whole guide show a polarization above 92% for neutrons with a wavelength of 3 A.

Wide angle NSE: the multidetector spectrometer SPAN at BENSC

Abstract The magnetic field configuration of the spectrometer SPAN allows simultaneous NSE measurements to be performed over a particularly large angular range. The NSE precession field is created by three pairs of coils with diameters 1, 3 and 4.8xa0m, respectively, and the set-up has a symmetry axis, which is perpendicular to the scattering plane and crosses the sample position. The resulting magnetic field integral does not change with the scattering angle and thus fulfils the condition for multidetector NSE. In order to preserve the symmetry of the magnetic field, remanent supermirrors are used as analyzers. These supermirrors perform in reflection, in the stray magnetic field of the precession coils, and are mounted on analyzer frames with an angular opening of 25.7° each. The neutron optical elements (analyzer, polarizing guide) of SPAN have a high transmission over a large wavelength band. As a consequence of this, NSE can be done for incident wavelengths ranging from 2.5 to 9xa0A. At the lowest wavelengths high q values can be reached, which were not previously accessible for NSE experiments (at λ =3xa0A: 0.08xa0A −1 ⩽ q ⩽4xa0A −1 ).

High-performance, short solid state collimators with reflecting walls

Abstract We report about the first experimental test of neutron solid state collimators with reflecting walls. Si wafers are the collimator channel coated on both sides with a reflecting layer covered by an absorbing Gd layer. The collimators produce a beam with quasi-rectangular distribution of angles, which represents an intensity gain at equal resolution compared to the triangular distribution in conventional Soller collimators. These compact collimators can provide for high angular resolution with good transmission in very limited space.

Low dimensional neutron moderators for enhanced source brightness

In a recent numerical optimization study we have found that liquid para-hydrogen coupled cold neutron moderators deliver 3-5 times higher cold neutron brightness at a spallation neutron source if they take the form of a flat, quasi 2-dimensional disc, in contrast to the conventional more voluminous shapes used by now. In the present paper we describe a simple theoretical explanation of this unexpected behavior, which is based on the large difference in para-hydrogen between the values of the scattering mean free path for thermal neutrons (in the range of 1 cm) and its much larger equivalent for cold neutrons. This model leads to the conclusions that the optimal shape for high brightness para-hydrogen neutron moderators is the quasi 1-dimensional tube and these low dimensional moderators can also deliver much enhanced cold neutron brightness in fission reactor neutron sources, compared to the much more voluminous liquid D2 or H2 moderators currently used. Neutronic simulation calculations confirm both of these theoretical conclusions.

NEW COMPACT NEUTRON POLARIZER

Abstract A new type of a neutron polarizing bender was developed in co-operation with BENSC and ANSTO. It is based upon bent thin silicon wafers coated on one side with Siue5f8FeCo polarizing supermirrors and on the other side with Gd. Initial tests at BENSC in a 300 Oe magnetic field yielded a transmission of spin-up neutrons of about 55% over an angle range of 0.75° and flipping ratios > 30. Subsequent tests at ANSTO at 1200 Oe yielded a transmission of 48% with a flipping ratio > 45.

A new time-of-flight small-angle scattering instrument at the Helmholtz-Zentrum Berlin: V16/VSANS

Small-angle scattering methods have become routine techniques for the structural characterization of macromolecules and macromolecular assemblies like polymers, (block) copolymers or micelles in the spatial range from a few to hundreds of nanometres. Neutrons are valuable scattering probes, because they offer freedom with respect to scattering length density contrast and isotopic labelling of samples. In order to gain maximum benefit from the allotted experiment time, the instrumental setup must be optimized in terms of statistics of scattered intensity, resolution and accessible range in momentum transfer Q. The new small-angle neutron scattering instrument V16/VSANS at the Helmholtz-Zentrum in Berlin, Germany, augments neutron guide collimation and pinhole optics with time-of-flight data recording and flexible chopper configuration. Thus, the available Q range and the respective instrumental resolution in the intermediate and high momentum transfer regions can be adjusted and balanced to the individual experimental requirements. This renders V16/VSANS a flexible and versatile instrument for soft-matter research.

Structure, roughness and magnetization behaviour of triode sputtered Co11Fe89-Si multilayers

A study of monochromator-type neutron mirrors, produced under different sputtering conditions by a triode sputtering system is presented, and the effect on the magnetic behaviour and the film structure is revealed. Special attention was paid to the dependence of the interface roughness in respect to the deposition parameters. A structural characterization of the specimen was performed by x-ray reflectometry and diffraction, supported by transmission electron microscopy and electron diffraction studies, made on ultramicrotom cuts oriented perpendicular to the surface. SQUID magnetometry and polarized neutron reflectometry in combination with a simulation of the reflection data on the Co11Fe89-Si system was used to examine the magnetization profile and its evanescence near the chemical interface. Mosbauer spectroscopy clarified the model of interpretation.

Compact neutron optical elements

Abstract We report about the performance of neutron polarising benders and collimators in which the neutrons transmit thin coated Si wafers. A compact polarising bender was tested with neutrons at wavelengths of 4.7 and 2.4xa0A using either the neutrons reflected or transmitted at a supermirror coating. Transmissions of up to 68% of the spin-up component and a polarisation of more than 95% were reached. Using flat Si wafers coated with Gd compact collimators could be realised.

Polarizing Ti1-uXu/FexCoyVz supermirrors

Abstract Polarizing multilayers and supermirrors composed of Ti1−uXu and FexCoyVz have been produced using DC-magnetron sputtering. The structure of the samples has been characterized by neutron and X-ray diffraction. The results show that the smoothness of the layers can be improved by adding controlled amounts of alloying materials, an effect that has already been observed for Ni/Ti supermirrors. Flipping ratios of more than 40 and reflectivities of more than 85% at twice the critical angle of Ni are obtained, depending on the concentrations of the materials, u, x, y, z. The sputtering process can be optimized in such a way that the magnetization within the plane is anisotropic, thus allowing a saturation of the samples in low fields.

Non-specular reflectivity of spin flipped neutrons

Abstract For the first time we observed large intensity non-specular reflection peaks of neutrons incident at small grazing angles on not fully magnetized ferromagnetic samples. In addition to the normal specular reflection spin flipped neutrons are reflected at distinctly higher (lower) angles if the incoming spin down (spin up) neutrons hit the surface at angles near the respective critical angle. The same effect occurs in transmission. The angles at which the non-specular reflection occurs can be calculated by a simple model, which assumes a spin flip without a change of direction and takes into account the different refraction angles of the original and the flipped spin state when the neutrons leave the sample.