Tasks & Partners

One of the main challenges in neutron scattering is to increase the luminosity of spectrometers. Here we propose several new concepts for neutron reflectometers. The aim is to maximize the use of the neutrons available in the guide by performing reflectivity measurements in parallel for all wavelengths. Since there is no monochromatization, or chopping of the beam, the expected gains in flux are of the order of 10 to 100. These ideas, especially the energy analysis, may eventually have a broader scope of applications on other types of spectrometers.

Doc: J. Stahn SELENE concept (PDF)
F. Ott – High Flux Reflectometry (PDF)

Sub-Task 17.2.1: Compact Energy Analyzer

We propose the design of an energy analyzing device based on reflective optics. The device combines multi-layer monochromator mirrors and would achieve an energy resolution of the order of 7% which is sufficient for most reflectometry measurements.

Sub-Task 17.2.2: Refraction-encoded reflectometry

We propose to use neutron prisms with a very good flatness, using the contrast in scattering length density between air and the prism material to build an energy analysis device.

Sub-Task 17.2.3: Wavelength-encoding by Bragg diffraction

We propose to place many diffracting crystals in the reflected beam, diffracting the different wavelengths in different directions onto a position-sensitive detector, such that the wavelength is encoded in the position on the detector. The method can be implemented either by placing an array of single crystals one behind the other and each tilted with respect to the other, or by producing single-crystal fibres in bundles.

Sub-Task 17.3.1: Multichannel focusing guide

The use of focusing guides is a well-known technique to significantly increase the neutron flux for small samples (< 1cm2). A multi-channel guide can provide much greater focusing than a conventional single-channel guide. The performance optimisation of such a guide is made difficult by the large number of parameters and thus needs to be coupled to a very efficient optimisation algorithm. We propose to use the Monte-Carlo simulation tools McStas and/or VITESS, which currently do not support multi-channel tapered focusing guides. Such a software component will need to be written as part of this task.

Sub-Task 17.3.2: Adaptive Optics for extreme Environments

During recent years it was shown that very effective focusing devices using non-linearly tapered guides can be fabricated. It is possible to achieve flux gains of the order of 30 – 40 on spot sizes of the order of 1 mm2. However, during the practical use of elliptic focusing guides at regular neutron beam lines at FRM-II and SINQ, the following problems became apparent: difficulty in aligning the focal point on tiny samples, adaptation of beam size to the sample size, optimization of the divergence of the neutron beam with respect to the sample. We suggest resolving these problems by means of focusing guides with adaptive optics. An adaptive focusing guide will be implemented at a triple axis spectrometer for the investigation of elastic and inelastic scattering from samples at high pressure and in extremely high magnetic fields.
R. Valicu – Adaptive Optics (PDF, 1.527 MB)

Sub-Task 17.3.3: High resolution imaging using reflective optics

As a first task we propose to demonstrate by means of Monte-Carlo calculations that elliptic neutron guides only lead to minor distortions of the phase space and can thus be efficiently implemented for neutron radiography and tomography. As a second task we would like to develop focusing devices using a nested Wolter-type mirror system and advanced supermirror technology to produce focal spots with a diameter of the order of 0.1 mm. The emerging beam can be used as a bright neutron source for a cone beam geometry allowing us to increase the resolution and at the same time magnify the object, as is done with X-rays from micro-focus tubes. This way the limited spatial resolution of present day neutron detectors may no longer limit radiography.

Sub-Task 17.3.4: Focusing SANS

The aim of this task is to propose flexible optics to optimize SANS spectrometers by using focusing optics. For SANS instruments, the possibility of focusing the beam on the detector has the potential to increase the useful part of the neutron beam by about one order of magnitude. Using reflective optics, we propose a new concept of neutron focusing on SANS spectrometers using a combination of curved super mirrors. The device, which combines advanced neutron optical elements such as parabolic and elliptic super-mirrors, is achromatic and has no absorption. Proper optical elements such as neutron lenses would substantially increase the performance of SANS in two directions: (i) an order of magnitude increase of intensity in particular at small momentum transfer Q and (ii) an extension of Q to the order of 10-4 Å-1. Spherical biconcave lenses of MgF2 with a diameter of 3cm are currently used at several SANS instruments. Watching recent developments in the creation of lenses for SANS, we believe that there is the potential to go one step further and to create aspherical lenses which avoid the imaging faults of spherical aberration and would allow focusing of larger beam areas.

J. Fuzi – Multibeam SANS (PDF,4.218 MB)