Wednesday 10 July 2013, Strathblane & Cromdale Halls, 16:30-18:30
Poster session B - Neutron optics
P.162 A minimalistic neutron guide concept
M Bertelsen and Kim Lefmann
University of Copenhagen, Denmark
Monte Carlo ray tracing simulations is an important tool when designing advanced neutron guides as it is possible
to determine the beam properties at the sample quickly. Numerical optimizers are used to vary the parameters
describing the guide to find the optimal guide for given requirements, which is usually flux on sample within some
divergence limits. In almost every case the background is only considered in the initial guide layout, but not within
the figure of merit for the optimization as it is a much larger task to calculate. Thus the resulting guides have a great
signal, but a disproportional amount of background.
This work details how the parameter space can be restricted to only contain guides which limits background to a
minimum, while still maintaining a signal only marginally lower than the global optimum. This is achieved by
matching the phase space requirement at the sample to the geometrical properties of the guide, which yields
constraints that removes free parameters in the optimization. Solutions found in this reduced space will only
transport the neutrons necessary to ensure even illumination of the phase-space corresponding to the requirements,
and thus reduce the background and damage to the super mirrors used in the guide. The performance of guide
geometries found with this method can be described by the brilliance transfer which has an intrinsic maximum of 1
in order to be aware of the maximum performance difference compared to the general solution.
P.163 “m=1” coatings for neutron guides
C Cooper-Jensen and P Bentley
European Spallation Source ESS AB, Sweden
Traditionally, neutron guides have been coated with a single layer of Ni, and the critical angle for the Ni coating is
defined as m=1. To reflect neutrons at angles greater than m=1, Ni58 (m=1.2) or Ni/Ti multilayers have been
widely used. The reason for using Ni based coatings is that Ni has one of the highest neutron scattering densities,
and Ni is relative easy to coat using DC magnetron sputtering. A drawback of using Ni based coatings is the high
absorption cross section, and the high energy of the resulting prompt gamma ray when a neutron is absorbed in the
Ni coating. The prompt gamma ray from the guide dictates the shielding requirements around the guide, and this is
a substantial fraction of total neutron instrument costs. We are investigating Be coatings, which have the same
neutron scattering length density as Ni; and diamond-like carbon (DLC) coatings that have a higher neutron
scattering length density than Ni. Both the Be coatings and DLC coatings have a significantly smaller absorption
cross section compared to Ni. We will present a theoretical study of these neutron mirrors along with preliminary
results.
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P.164 Moving in phase space using time-dependent crystal monochromators
K Habicht1, T Keller2 and C Leo1
1
Helmholtz-Zentrum Berlin für Materialien und Energie, Germany, 2Max Planck Institute for Solid State Research,
Heisenbergstrasse, Germany
Based on the ideas by Friedrich [1] gradient and constant lattice vector phase space transformers have been
investigated in a wavelength range from 2 to 20 Angstroms anticipating that this is the minimum useful range of
incident wavelengths for a crystal spectrometer. While gradient crystals allow keeping the direction of the wave
vector incident on the sample constant and thus allow for a better monochromatisation, rotating crystals with single
lattice vector may offer an acceptable alternative. Gain factors have been calculated analytically. Defining the gain
by the ratio of duration of the Bragg reflected pulse to the ESS pulse length the gain can be derived from the velocity
band accepted by the PST. McStas Monte Carlo simulations confirm intensity gains of factors 5-10 depending on
rotation frequency of the PST. In contrast to optically focussing crystal optics this gain is obtained without an
increase in divergence.
[1]
H. Friedrich in Proc. Workshop on Neutron Scattering Instrumentation for SNQ, Maria Laach, 3-5 Sept.
1984, p 450-455.
P.165 Neutron optics from Helmholtz-Zentrum Berlin
T Krist and J Hoffmann
Helmholtz Zentrum Berlin, Germany
In the last years a variety of neutron optical devices has been developed at HZB, mainly solid state elements where
the neutrons are transported in thin silicon wafers with coated walls. We show results of solid state polarizing
benders, solid state collimators with and without polarizing walls and a solid state radial bender for the polarization
analysis of neutrons over an angular range of 3.8 degrees. Another device consists of a solid state polarizing bender
without absorbing layers used together with a collimator, which allows polarizing or analyzing neutrons without
deflecting them from their original direction. Two-dimensional polarization analysers for an angular range of 5
degrees in both directions are presented. With a focusing solid state lens made from Si wafers coated with m=2 NiTi supermirrors which can transport the full divergence of a neutron guide with the same coating a focus with a
FWHM of 2.4 mm was reached and an intensity increase of 5.6 compared to the intensity there without the lens. A
polarizing cavity in a guide of 9 m length with a cross section of 60 mm x 100 mm was built which polarizes
neutrons with wavelengths above 0.25 nm. In all these polarizing devices polarisations of 95% were realised.
Recently a polarizing S-bender with a cross section of 30 mm x 100 mm was tested, showing at a wavelength of 4.4
Å a polarization above 98% and a transmission above 65%.
P.166 Neutron diffraction studies of dispersive double crystal setting containing fully asymmetric diffraction
geometry (FAD) of a bent perfect crystal (BPC)
P Mikula1, M Vrána1, J Saroun1, J Pilch1, B S Seong2, W C Woo2, V Em3 and D Korytar4
1
Nuclear Physics Institute ASCR, v.v.i., Czech Republic, 2KAERI Daejeon, Korea, 3National Research Centre, 4Institue
of Physics SAS, Slovak Republic
In our contribution first studies of neutron diffraction properties of the fully asymmetric diffraction geometry (FAD) of
a bent perfect crystal (BPC) with the aim of possible applications are presented. The properties of the BPC FAD
Si(311) geometry were studied in the dispersive double-crystal arrangement in combination with the bent Si(220)
crystal slab in the symmetric diffraction geometry. In fact, two possibilities of the FAD diffraction geometry are
possible: with wide beam incident on the largest face and the output beam compression (when passing along the
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longest edge) or with the incident beam passing along the longest edge and the output beam expansion. In this
case we focused our attention to the first geometry. As to a possible practical use, it has been found that thanks to
the mutual dispersive setting between the Si(220) and FAD Si(311) crystals, the FAD geometry of the second
crystal can after a beam condensation provide an intensive highly collimated monochromatic beam of a small width
determined by the thickness of the FAD crystal. Due to the mutual dispersive setting of the BPC slabs, the double
diffracted beam has also a narrow Δλ spread of the order 10-3 or less. Peak reflectivity of the FAD Si(311) and
Si(220) crystals is for the used curvatures larger than 0.6 and 0.9, respectively and the integrated reflectivity of the
DBC-setting corresponds to the overlapping of the individual phase space elements of the BPC Si(220) and FAD
BPC Si(311) crystals.
P.167 Neutron energy analysis and focusing by silicon prisms
F Ott2, J Schulz1, C Hülsen1 and T Krist1
1
Helmholtz Zentrum Berlin, Germany, 2Laboratoire Leon Brillouin, France
Neutron energy analyzing techniques applied to a white beam allow to measure at different neutron wavelengths at
the same time. We tested an energy analyzer consisting of several layers of prism rows, each made of 191 silicon
prisms with a height of 0.25 mm. The refraction of a 0.3 mm wide white beam over a flight distance of 2 m was
measured at the EROS reflectometer at the LLB. At the wavelength of 2.4 Å a wavelength resolution of 34% and at
6.7 Å a wavelength resolution of 5% was measured. Compared to using a chopper with the constant resolution of
34% this results in a gain of 22 at 2.4 Å and of 14 at 6.7 Å.
Due to the low phase space density of neutron beams focusing is most often not an efficient option, however the
imaging of a linear or circular source with a lens promises substantial gains compared to present instruments. A
lens consisting of silicon prism layers was tested. The number of prisms in each layer increases with the distance to
the optical axis, so that the prism system of the lens has the shape of an hourglass. It consisted of 13 layers of
silicon prisms rows. With this lens a 0.2 mm wide slit was imaged over a distance of 1.7 m between the lens and
the detector at the wavelength of 4.9 Å at the V14 reflectometer at the HZB. Within the FWHM of the peak image an
intensity gain of a factor 7.9 was measured compared to the intensity of the direct beam in the same area.
P.168 Quasi- adaptive Neutron Optic
T Panzner, A Bollhalder, T Muehlebach, E Rantsiou and U Filges
Paul Scherrer Institut, Switzerland
One of the strongest requests for neutron experiments in the future is the possibility to measure small samples
(sample size in the range below 2 mm). Different kinds of possible neutron focusing devices have been proposed.
The disadvantage of most of these devices is their optimization to a special phase space constrain. If the experiment
requires another phase space, the optic device cannot be used sufficiently. To get flexibility back, one has to think
about adaptive variants of these devices.
In the last years the neutron optics group at PSI developed a device based on super mirror technology, which allows
easured up to a factor of
12 depending on the incoming beam properties. The construction of this device was done in such a way, that it is
easy to change the parts of the device which determine the focusing properties, such as substrate holders or super
mirrors.
We will present the design of the device, neutron measurements and McStas simulation. The results of the
experiments and simulations match very well and show excellent focusing performance of the device. Also an
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outlook on planned powder diffraction experiments and expected performance enhancement using this device will
be given.
P.169 Measurements of neutron transmission through sapphire crystals
E Rantsiou, T Panzner and U Filges
Paul Scherrer Institute, Switzerland
Sapphire crystals are known to be excellent filters of fast neutrons, while at the same time exhibit moderate to very
little absorption at smaller energies. We have completed an extensive series of measurements in order to quantify
the above effect. Our experiments were performed at the BOA testing beamline at PSI, using a total of 12 crystals
(with a combined maximum thickness of 120mm), acquired by three different commercial providers. We have
measured the dependence of both fast and cold neutron transmission to the crystal’s thickness, and also we have
tested for potential performance differences, due to e.g. inherent impurities (quality check), among the three types
of sapphire crystals from our three suppliers. Furthermore, as an extension of those already performed experiments,
we plan to measure the deterioration of performance of the sapphire filters with usage.
Alongside our experiments, we have performed a series of simulations, in order to reproduce the transmission of
cold neutrons through sapphire crystals. The Monte-Carlo neutron transport code MCNPX and the Monte-Carlo raytracing code McStas were used for that purpose, and overall we have seen an excellent agreement between our
simulations and experiments. The usefulness of reliable simulations becomes apparent, when trying to customize for
example the size of a potential sapphire filter to the needs and characteristics of the beamline/instrument in
question.
We will present results from all the aforementioned measurements and simulations. Our conclusion and belief is
that the specific types of sapphire crystals we have used and analyzed, could successfully be used as fast-neutron
filters in experiments.
P.170 Neutron supermirrors on metallic substrates
M Schneider, C Schanzer and P Böni
SwissNeutronics AG, Switzerland
We have developed a process for polishing metallic substrates leading to an extraordinary low surface roughness
comparable to float glass of high quality. Initially, neutron supermirror coatings on aluminum substrates with R
=91% at the critical angle of reflection of m=2 were produced demonstrating the excellent quality of the aluminum
surface. Recently further developments succeeded to coat m=3.6 supermirror on aluminum and steel substrates
with R= 77%. Supermirror on metallic substrates open new options for advanced neutron optical devices. A
prerequisite for the development of the metallic substrates for neutron mirrors was an adequate method to measure
the surface roughness. With this work we underline the relevance of measurements with atomic force microscopy
(AFM) for the determination of the surface roughness for supermirror substrates. It is shown that AFM provides a
valuable tool for the quality control of the neutron supermirror production.
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