High-Resolution Analysis Using Bent Perfect Crystal in Powder Diffraction: Part II
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igh-Resolution Analysis Using Bent Perfect Crystal in Powder Diffraction: Part II P. Mikulaa, * , J. Šarouna, J. Stammersa, and V. Emb a
Nuclear Physics Institute, Czech Academy of Sciences, Řež, 250 68 Czech Republic b National Research Centre “Kurchatov Institute,” Моscow, 123182 Russia *e-mail: [email protected] Received July 5, 2019; revised August 17, 2019; accepted August 19, 2019
Abstract—As a continuation of our recent studies of α-Fe(211) samples at a scattering angle of 2θS = 88° (Part I), a three-axis setup using bent perfect crystal monochromator and several analyzers was tested in the diffraction study of polycrystalline α-Fe(110) pins with a diameter of 8 and 2 mm at a scattering angle of 2θS = 47.1°. After realizing the focusing conditions in real and momentum space at a neutron wavelength of 0.162 nm, a high angular resolution was achieved up to FWHM(Δd/d) = 1.4 × 10–3 and FWHM(Δd/d) = 2.5 × 10–3 for α-Fe samples with a diameter of 2 and 8 mm, respectively. This resolution was obtained by using open beams, i.e., without Soller collimators within the used range of the curvatures of the analyzers. Such settings can be used in powder diffraction, namely, in studies of high-resolution residual stresses and/or analysis of diffraction profiles with a small difference in the lattice parameters or a small change due to the application of an external load. Keywords: powder diffraction, focusing, bent crystal analyzer DOI: 10.1134/S1027451020070332
INTRODUCTION The need for structural studies of complex samples, as well as small structural changes by neutron diffraction, requires constant attention to the luminosity of the device and its resolution. Usually, in the case of using mosaic monochromators, this is achieved, for example, by reducing the collimation and Δλ spread of the monochromatic beam, due to the use, possibly, of a large take-off angle of the monochromator 2θM and/or by utilizing small volume samples. However, in the case of using elastically bent perfect crystals (BPC), open beams without Soller collimators can be used, and in addition to real space focusing, one can also benefit from focusing in momentum space. In our case of the three-axis setup (bent perfect crystal monochromator + polycrystalline sample + bent perfect crystal analyzer), the resolution properties for a specific scattering angle of the sample 2θS depend on the curvature of the selected monochromator and analyzer, as well as their reflections. Therefore, with constant curvatures of the monochromator and analyzer, high resolution can be maintained in a limited range of the scattering angle and practically used in experiments [1–12]. Moreover, as was already shown in the first part of the work [13], using focusing in real and momentum space, high resolution can be obtained on samples of rather large sizes. After studying the luminosity and resolution properties with
α-Fe(211) samples at a scattering angle 2θS = 88°, we decided to carry out a similar extensive investigation of the three-axis setup with the polycrystalline samp
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