Neutron Scattering - JUWEL - Forschungszentrum Jülich

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SPHERES 7 While the primary spectrometer (everything before the sample) is mainly in vacuum, the secondary spectrometer is not. To minimize neutron losses in the secondary spectrometer, the entire instrument housing can be flooded with argon. For the labcourse, we preferentially remove the argon so that participants can accede the housing. However, since refilling takes at least one full day, time constraints may prevent us from doing so. In this case, a video will be shown to present the interior of the spectrometer. 2.3 Measuring Spectra So far we have introduced a static arrangement with fixed energies Ei = Ef. Such an arrangement is actually used to measure the fraction of elastic versus total scattering, called the Debye- Waller factor for coherent scattering and the Lamb-Mössbauer factor for incoherent scattering. More often, however, one wants to measure full spectra S(Q, ω). Therefore, one must find a way to modify the energy transfer �ω = Ei − Ef. (3) This can be done using the Doppler effect: The monochromator is mounted on a linear drive that performs a cyclic motion. In the monochromator’s rest frame, the backscattered energy is always the value E0 =2.08 meV given by the lattice constant of Si(111). Depending on the monochromator’s velocity v, the value in the laboratory frame is Ei(v) = mn 2 (v0 + v) 2 where v0 = 631 m/s is the neutron velocity at E0 = mn/2 v 2 0. The Doppler drive of SPHERES has a linear amplitude of ±75 mm and achieves a velocity amplitude of ±4.7 m/s, resulting in an energy range −30.7 μeV < �ω
8 J. Wuttke S (q,ω) (μeV -1 ) 1 0.1 0.01 0.001 SNR > 1700:1 FWHM = 0.66 μeV SPHERES September 2010 |E|max = 31μeV -20 0 20 hω (μeV) Fig. 6: Resolution function of SPHERES, measured on a user provided sample at a low temperature where the scattering is purely elastic. not the full story: the quality of an instrument also depends on the shape of the resolution functions, especially of the deep wings. The resolution of SPHERES is slightly asymmetric. This is related to the (δΘ) 2 term in the wavelength spread of a backscattering analyzer: all deviations from the perfect Θ=90 ◦ geometry lead to the transmission of longer wavelengths, never of shorter ones. Another important figure of merit is the signal-to-noise ratio (SNR). It depends strongly on the ratio of scattering to absorption cross sections and on the thickness and geometry of the sample. With argon filling, the best value obtained in user experiments has been 1700:1; without argon, 1200:1. On the other hand, for strongly absorbing samples it is sometimes less than 100:1. 3 Applications In the following, two different applications of neutron backscattering are explained: hyperfine splitting in a magnetic material, and methyl group tunneling. 3.1 Hyperfine Splitting The measurement of hyperfine splitting has been historically the first application of neutron backscattering, 4 and to this day, it is the conceptually simplest one. Since the neutron has spin S =1/2, its magnetic quantum number can take the values Sz = ±1/2. In a scattering event, this quantum number can change. In more pictorial words: when a 4 A. Heidemann, Z. Phys. 238, 208 (1970).
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Forschungszentrum Jülich GmbH Jül
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Page 78 and 79: SPHERES Backscattering spectrometer
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Page 92 and 93: ________________________ DNS Neutro
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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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