Methods and Instruments for Neutron Scattering
DNS - Diffuse Neutron Scattering Spectrometer: Full Vector Polarization Analysis
Equipped with polarizing supermirrors, the DNS instrument is capable of polarizing neutrons and analysing the polarization using a multi-detector system. Common applications are (i) the separation of coherent and incoherent scattering, particularly for hydrogeneous material typical of soft matter, and (ii) the separation of magnetic scattering from directional dependence. In addition to the standard longitudinal polarization analysis, the so-called xyz-method, a new, unique application method is possible using the DNS, based on a precessing incident polarization that enables full vector polarization with arbitrary rotation of the polarization. In this way, and without the need for additional apparatus, polarization spins for all types of impulse and energy variations can be measured simultaneously, making this an important tool for exploring complex magnetic phenomena.
DNS - Diffuse Neutron Scattering Spectrometer: Single Crystal Time-of-flight Spectroscopy
Currently we are working on renewing the time-of-flight option with a 2-disc chopper system operating at higher speeds of 300 Hz; this offers better resolution with repetition rates of up to 900 Hz. The set-up with two phase controlled choppers alternatively eliminates the lambda/2 background or selects lambda/2 instead of lambda, using a higher take-off angle for better resolution on a shorter wavelength. Furthermore, the previous detector system with its 50 small detectors will shortly be replaced by 128 position-sensitive He-3 tubes of 1 m in height, increasing the covered solid angle by more than one order of magnitude to 1.8 sr. In addition to the improved Q-resolution, this adds a new dimension: the vertical Q component. The DNS will then present greatly improved opportunities to conduct single crystal TOF spectroscopy facilitating efficient measurements in all 4 dimensions of S(Q,E).
IN12 Relocation and Upgrade
The IN12 instrument is operated by Forschungszentrum Jülich in cooperation with the CEA Grenoble as a CRG-B instrument at the Institut Laue Langevin in Grenoble, France. As a three axis spectrometer for cold neutrons, IN12 is dedicated to high resolution-studies of low energy excitations. In the past, the instrument has demonstrated excellent performance as one of the best spectrometers available for neutron scattering research.
Within the framework of ILL’s Millennium Program, the IN12 will move to an end position on its own new guide.
This permits further improvements to be carried out which will secure its position as a powerful and unique instrument. Key components are:
- a new m=2 guide with an optimized ballistic shape and the virtual source concept together with a new double focusing PG monochromator
- an extended wavelength range of 1.5 - 6.3 A (now 2.4 - 6 A)
The new design promises an increase in the overall count rate of an order of magnitude.
Additional components are planned to further improve the performance and efficiency of the spectrometer:
- a new monochromator shielding adapted to the requirements of the end position; the higher flux and the larger angular range of the extended wavelength band will be constructed in-house
- a velocity selector to eliminate higher order contributions and guarantee a low background
- a guide changer with cavities and a transmission polarizer to ensure an efficient and easy-to-use polarization of the incident beam
- a guide changer in front of the monochromator to permit a collimated beam option (high and tunable Q-resolution) instead of the focusing (high divergence) option will be installed at a later date - a rarity on many spectrometers but indispensable for many experiments such as high pressure studies or UFO (see below)
- in the future, a perfectly bent silicon monochromator to focus the beam on small sample volumes will be made optional.
As a further option, IN12 will be equipped with a two-dimensional position sensitive detector and an array of fifteen analysers which can be rotated and positioned individually in order to map the scattered beam on a user-chosen path in Q-space. Depending on the requirements, the user can, for example, define scans with a constant energy transfer or scans that map a path along a certain Q-direction (linear Q-scan). We refer to this concept as IN12-UFO (Universal Focusing Option). With this set-up, IN12 will be capable of meeting future challenges and fulfilling its role as a fully-operating multiplex instrument.
KWS-3 Focusing Small Angle Camera
This instrument occupies a unique position as the only small angle neutron scattering instrument in the world with focusing mirror optics. It is known to improve resolution by one order of magnitude
MARIA, a Reflectometer Dedicated to the Investigation of Nano Structures and Thin Magnetic Layers.
The new reflectometer MARIA is optimised for investigations of thin magnetic layers down to the sub mono range with small sample sizes of 1cm2. Therefore MARIA offers the opportunity to analyse magnetic nano structures, which are the critical building blocks of important magnetoelectronical devices, such as the magnetic random access memory or patterned recording media. Due to the necessary miniaturization of such devices and the proximity of the super paramagnetic limit, the magnetic interaction between neighbouring cells is becoming an increasingly important parameter, not just to monitor but also to be fully understood.
To analyse these small structures along with a tiny amount of sample material, a dynamic range of 9 orders of magnitude is needed. In order to achieve the necessary basic intensity, the wavelength resolution can be relaxed down to 10% in a band of 4.5A to 11A. Additionally the neutron guide is elliptic, focused in the vertical direction to increase the intensity on the sample position. For the investigation of the magnetic structures, polarization analysis is taken to be a standard, with an Fe/Si neutron guide acting as a polarizer and a He3-cell as an analyser. Furthermore, the fast switching between reflectometer and GISANS mode enables structures on a length scale range from nm to µm to be investigated.
POWTEX - High-Intensity Time-of-Flight Neutron Diffractometer
In order to provide the large chemistry, geo and materials science communities with a powerful tool for rapid neutron-data acquisition, the novel time-of-flight diffractometer POWTEX will be installed at the new MLZ reactor in Munich. POWTEX - a combination of the words POWder and TEXture - was approved by the BMBF and was built by RWTH Aachen University and Forschungszentrum Jülich within the framework of the JARA project in association with Göttingen University.
We expect to outperform comparable monochromator instruments by one order of magnitude in intensity (> 1×107 neutrons/cm2s) for samples of less than a cubic centimeter. This extraordinary performance will be achieved by combining several new concepts: the ballistic beam compressor with double focusing super-mirrors, the four-unit disk-chopper system, including the double-disk pulse chopper, the use of position-sensitive detectors to access the full Debye-Scherrer cones by a huge solid angle (> 9 steradian) and significantly,, the multiple measurements of all reflections using the time-of-flight strategy. The high neutron current at the sample position will allow us, for example, to investigate in-situ chemical reactions and to characterize phase transitions as a function of temperature, pressure, magnetic fields (or other sample environments) in comparatively short measurement times. Applications in the field of geo and materials sciences for this instrument are mainly related to texture measurements on natural samples and, additionally, to measurements during in-situ deformation and re-crystallization/annealing experiments with special sample environments. As POWTEX is under construction at present, we expect to conduct our first measurements in the year 2012.
TOPAS, a Novel Time-of-flight Spectrometer
TOPAS is involved mainly with the investigation of elementary excitations in novel materials. The energy transfer between sample and neutrons is determined by time-of-flight analysis, leaving three angular coordinates free for the determination of the momentum transfer and hence the full dispersion relation. The combination of polarization analysis and a large position-sensitive detector provides a unique insight into the dynamics of single crystalline materials. For magnetic materials in particular, this opens up new opportunities for measuring dynamics.
Due to the small inelastic scattering cross section, a high neutron flux is crucial for the instrument. Using an elliptical neutron guide, the neutrons are focused both vertically and horizontally onto the sample. A system of Fermi choppers selects the incoming neutron energy and pulses the beam. This approach allows the measurement of excitations of up to 140 meV with good energy resolution and a high neutron flux.
3He Neutron Spin Filters
Spin-polarized neutrons provide a very powerful tool for the study of matter. Nuclear spin polarized 3He is an especially versatile filter for both the production of the incident-polarized neutrons and analysis of the polarization of neutrons scattered from the sample. We are now building advanced apparatus for generating highly polarized 3He using two different optical methods:
Metastable Exchange Optical Pumping (MEOP)
the direct polarization of the 3He gas at low pressure followed by compression to the required pressure (typically a few bars)
Spin Exchange Optical Pumping (SEOP)
indirect (spin-exchange) polarization for which an alkali metal such as rubidium or potassium is optically polarized followed by the transfer of the polarization to 3He cores by means of collisions.
Polarized Neutron Techniques
The largest Joint Research Activity of the Integrated Infrastructure Initiative for Neutron Scattering and Muon Spectroscopy, NMI3, within the EU 6th Framework Program, PNT, is coordinated by the Jülich scientist Dr. Alexander Ioffe. It includes 11 partners from 7 countries and observers from USA, Japan and Australia. The goal is to develop:
- devices for the three-dimensional analysis of polarization in inelastic/quasielastic neutron scattering, diffraction and reflectometry
- improved neutron spin echo techniques for an increase in the resolution of inelastic scattering instruments
- thin film-based Larmor precession devices
- novel Larmor precession-based instrumentation for reflectometry, SANS and diffraction.