Neutron Scattering - JUWEL - Forschungszentrum Jülich

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POWDER DIFFRACTOMETER SPODI 17 6. Setup of the high-resolution neutron powder diffractometer SPODI at FRM II The main components of a constant-wavelength neutron powder diffractometer are: source, monochromator, sample and detector. Between these components collimation systems are installed which have high impact on the instrumental resolution function and the neutron flux. detector primary collimator monochromator angle: 2�M mosaicity: � �1 detector collimator �3 neutron guide secondary collimator �2 sample thermal neutrons Figure 8: illustration of a typical instrumental layout, introducing the parameters used by Caglioti to describe the instrumental resolution function. Instrumental resolution function source As shown by Calgioti, the instrumental resolution function of a constant-wavelength powder diffractometer can be approximated by: FWHM � U tan � �V tan� �W 2 with the Caglioti parameters:
18 POWDER DIFFRACTOMETER SPODI 4 U � tan V � tan � W � 2 2 2 2 2 2 ��1�2��1��2�� 2 2 2 2 �m ��1��2�4�� 2 2 2 � 4� 2 ��1�2�� 2 2 2 �m ��1��2�4�� 2 2 2 2 2 2 2 2 2 1� 2 ��1 �3 �� 2� 3 � 4� ��2��3� 2 2 2 ��4�� 1 2 in this approach it is assumed that all components have Gaussian transmission profiles. The resolution function is determined by the horizontal beam divergences �� the monochromator angle 2�m and the mosaicity of the monochromator �� (Figure 8). As the impact of these parameters on the instrumental resolution function can be estimated, the Caglioti equations help to design an instrument to achieve a designated performance. However, it should be emphasised that in the approximations of Caglioti only the horizontal beam divergences are taken into account, neglecting vertical beam divergences by a vertical focusing monochromator or a vertical divergent neutron guide. Those effects are taken into account by ray-tracing methods, which allow a detailed modelling of the individual components. The powder diffractometer SPODI has been designed to achieve both high resolution and good profile shape. In its standard configuration (highest resolution mode) SPODI uses a unique very high monochromator take-off angle of 155° along with a large monochromatorto-sample distance of 5 meters. An evacuated beam tube of about 4 m in length is located between the monochromator and the sample which also controls both vertical and horizontal neutron beam divergences at the sample position. Thus the natural neutron beam divergence in horizontal plane is 25’ only. It can be reduced down to even 5’ by optional Soller collimators in front of the sample. Monochromator At constant-wavelength diffractometers, the monochromatisation is perfomed using crystals followings Bragg's equation: 2d hkl sin� � � , where the effective transmission band is determined by a derivative �� M cot� M � The width of the wavelength band �� strongly depends on the monochromator angle 2�m and the mosaicity of the monochromator ��m . Thus these parameters have a major impact on the instrumental resolution function and the flux on the sample.
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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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________________________ 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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Neutron Scattering - JUWEL - Forschungszentrum Jülich

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