Neutron Scattering - JUWEL - Forschungszentrum Jülich

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PUMA 7 4. Triple Axis Spectrometer PUMA The three-axis instrument is the most versatile instrument for use in inelastic scattering because it allows one to probe nearly any coordinates in energy and momentum space in a precisely controlled manner. The three axes correspond to the axes of rotation of the monochromator (axis1), the sample (axis2), and the analyzer (axis3). The monochromator crystal selects neutrons with a certain energy from the white neutron beam emanating from the reactor. The monochromatic beam is then scattered off from the sample (second axis). The neutrons scattered by the sample can have a different energy from those incident on the sample. The energy of these scattered neutrons is then determined by the analyzer crystal (third axis). All three angles (�M, �S, �A) can vary during an experiment, the sample table and analyzer are equipped with air pads, so that they can glide over the “Tanzboden” (dancing floor). Below, we describe in detail each component of a triple-axis spectrometer. Monochromator A crystal monochromator is used to select neutrons with a specific wavelength. <strong>Neutron</strong>s with this wavelength interact with the sample and are scattered off at a similar (elastic) or different wavelength (inelastic). The energy of the neutrons both incident on and scattered from the sample is determined by Bragg reflection from the monochromator and analyzer crystals, respectively. For a specific Bragg plane (hkl) characterized by an interplanar spacing dhkl, the crystal is rotated about a vertical axis. A pyrolytic graphite with d002 = 3.35 Å (PG(002)) and a copper with d220 = 1.28 Å (Cu(220)) monochromators are available at PUMA. The angular range of the monochromator 2�M is of 15 o - 115°. The PG(002) is usually used for energies below 50meV (�>1.3Å). For higher incident energies the Cu(220) can be used. Sample table The sample table from the company Huber provides a possibility to vary independently both 2�s and �S. It is equipped with a goniometer moving the sample in the three translation axes x, y and z and tilting. The tilt angle is ±15°. Single crystal experiments can be performed with an Euler cradle at PUMA. The sample environment includes magnets, pressure cells, cryostats and high temperature furnace. Analyzer Like the monochromator, the PG(002) analyzer consist of 20x5 separate analyzer crystal plates are mounted in an aluminum frame. There is an option to measure with the flat or horisontaly and verticaly focused analyser. The angular range of the analysator 2�M is of -130 o - 130°. Detector and monitor The detector consists of five counter tubes which are filled with a 3 He pressure of 5 bar. To be able to monitor the neutron flux incident on the sample, a low-efficiency neutron counter monitor is usually placed before the sample. Such a monitor is required so that flux variation caused by, for example, the reactor power fluctuations and the change in reflectivity of the monochromator with neutron wavelength can be automatically corrected for.
8 O. Sobolev, A. Teichert, N. Jünke Fig.4 PUMA spectrometer. Slits, Collimators, Filter Additional components like slits or collimators are used to define the beam cross section. Collimators (�1- �4) are used for the improvement of the resolution and to specify the beam divergence. They consist of multiple parallel arranged Gd2O3 coated foils with a defined angle to the beam. The angular divergence of the collimator in the horizontal plane � is defined by the distance between foils �d and the length of the collimator l (tan � = �d / l). Different collimators with a horizontal divergence between 10’ and 60’ are available at the instrument. One of the problems of the TAS method is the possible presence of higher harmonics in the neutron beam. Higher harmonics arise from higher order (hkl) in Bragg’s law (2). This means that if the monochromator (analyzer) crystal is set to reflect neutrons with a wavelength of � from a given (hkl) plane, it will also reflect neutrons with wavelength �/n. This leads to the appearance of several types of spurious peaks in the observed signal. Different filters are used to eliminate the high-order neutrons and to reduce the background. There are a sapphire filter (Al2O3) and an erbium filter (Er) at PUMA. They are installed in front of the monochromator. Sapphire filter is used wavelengths ��> 1 Å and reduce the background inducing by the epithermal neutrons. Erbium filter is suitable as �/2 filter for � between 0.5 and 1Å as well as �/3 filter for � between 0.7 and 1.6Å.
Page 1 and 2: Member of the Helmholtz Association
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20 M. Meven For a given spectrum
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22 M. Meven References [1] Th. Hahn
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SPHERES Backscattering spectrometer
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SPHERES 3 1 Introduction Neutron ba
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SPHERES 5 Fig. 2: The monochromator
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SPHERES 7 While the primary spectro
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SPHERES 9 neutron is scattered, it
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SPHERES 11 The stationary equilibri
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SPHERES 13 4. Summarize the tempera
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________________________ DNS Neutro
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DNS 3 1 Introduction Polarized neut
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DNS 5 Monochromator horizontal- and
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DNS 7 3 Preparatory Exercises The p
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DNS 9 important and always necessar
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DNS 11 Contact �� Phone:
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2 O. Holderer, M. Zamponi and M. Mo
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10 O. Holderer, M. Zamponi and M. M
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2 H. Frielinghaus, M.-S. Appavou Co
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4 H. Frielinghaus, M.-S. Appavou Th
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6 H. Frielinghaus, M.-S. Appavou Fi
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8 H. Frielinghaus, M.-S. Appavou mo
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________________________ KWS-3 Very
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KWS-3 3 1 Introduction Ultra small
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KWS-3 5 2. The standard Q-range of
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KWS-3 7 Table 2. Samples for practi
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KWS-3 9 References [1] Eskilden, M.
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________________________ RESEDA Res
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RESEDA 3 1 Introduction Neutrons ar
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RESEDA 5 various length values l ac
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RESEDA 7 In this expression, a norm
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RESEDA 9 spin flip from up to down
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RESEDA 11 The neutrons are counted
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RESEDA 13 Contact ��
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2 S. Mattauch and U. Rücker Conten
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4 S. Mattauch and U. Rücker Fig. 2
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6 S. Mattauch and U. Rücker 4 Expe
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8 S. Mattauch and U. Rücker Contac
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Schriften des Forschungszentrums J
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