PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

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selected asymmetric Bragg prism may be experimentally verified and employed in applications. The use of Bragg prisms can also be explored in neutron interferometric Laue phase measurements [164-165]. Within the channel-cut monochromator as well as analyser silicon crystals, each performing 7 {111} Ewald reflections, an amorphous Si wedge each was inserted after 3 reflections to deflect neutrons of 5.24 Å by about 4 arcsec. Treimer et al. [112] thus attained an analyser rocking curve width ~ 2 arcsec, reduced from Darwin width ~ 6 arcsec. The angular width achievable thus can be trimmed down to a fraction of Darwin width by placing a Si-Bragg prism instead of an amorphous prism albeit with a concomitant loss of intensity. One may envisage further, the tuning of the angular width of such rocking curves by Bragg prism rotation. The observations presented in Chapter 3 have led to the design and operation of a novel supercollimator Bragg prism which produced a neutron beam with a sub-arcsec angular width. II. First sub-arcsec collimation of monochromatic neutron beam Chapter 4 reported attainment of the first sub-arcsec collimation of a monochromatic neutron beam with a Bragg prism, viz. a single crystal prism operating in the vicinity of Bragg incidence. Analytical as well numerical computations based on the dynamical diffraction theory, led to the optimised collimator configuration of a Si {111} Bragg prism for 5.26Å neutrons. This optimal collimator produced a nearly plane wave neutron beam with a 0.58 arcsec angular width. With a similarly optimised Bragg prism analyser of opposite asymmetry, we recorded a 0.62 arcsec wide virgin rocking curve. The availability of such a sharp rocking curve has enabled several experimental firsts. We have recorded the first ever SUSANS spectrum in Q ~ 10 -6 Å -1 range with a hydroxyapatite casein protein sample and demonstrated the instrument capability of characterising agglomerates up to 150 m in size. The super-collimated monochromatic beam has also enabled us 120
to record the first neutron diffraction pattern from a macroscopic grating of 200 m period. The transverse coherence length of 175 m (FWHM) extracted from the analysis of this pattern is the greatest achieved to date for Å wavelength neutrons. A magnetic air prism between the monochromator and analyser Bragg prisms can separate the upand down-spin components of the neutron beam, thus providing a polarised SUSANS facility [166- 168]. As indicated in Chapter 4, one can produce even tighter neutron collimation by employing other Bragg reflections, asymmetry and apex angles. The monochromator and analyser Bragg prisms characterised in Chapter 4 can be used to produce a sharper neutron beam by employing higher order reflections. This Bragg prism pair operating near {333} Bragg incidence of 1.75 Å neutrons can achieve a 0.065 arcsec wide rocking curve. The transverse coherence length of 525 μm (FWHM) of this beam will facilitate SUSANS studies down to 3x10 -7 Å -1 and characterise agglomerates up to 450 μm in size. A magnetic air prism between the monochromator and sample would then yield a Polarised submicro-Å -1 SUSANS facility. One may envisage tightening the neutron collimation further with Bragg prisms operating at still smaller wavelengths. However, there are practical limits such as the mechanical stability of the apparatus, the minimum rotation step achievable for the goniometer and the inherent weakness of neutron sources in terms of flux. Only when the next generation neutron sources like Inertial Confinement Fusion based or advanced pulsed sources, becomes operational, may Bragg prisms be able to deliver sufficiently strong neutron beam with extremely narrow angular profiles. However, with X-rays, these novel Bragg prisms can produce sharper angular profiles than neutron beams as photon flux available currently is many orders of magnitude greater compared to neutrons. 121
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Studying neutron dynamical diffract
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DECLARATION I, hereby declare that
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ACKNOWLEDGEMENTS I sincerely, thank
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Fig.10 Boundary conditions on wave
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Fig.28 Theoretical curves for Si {1
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sample. Least-squares fit to the sa
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Fig.62 Interference contrast and av
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3.4 Experimental 49 3.5 Results and
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SYNOPSIS The present thesis aims to
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analytic expressions for intensity
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Research A, 634 (2011) S41. [9] S.
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spallation sources yield peak neutr
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SU(2) phase, and separation of geom
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[33-34,48]. The bi-refracting natur
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application of the dynamical theory
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I. One of the major disadvantages o
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nuclear physics very important as i
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potential for most nuclei in spite
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Due to the rapid strides made latel
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for a positive value of b c i.e. fo
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k b k O H . ni sin( B S) . (18)
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factor exp(-W). The centre, θ C ,
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exploiting multiple successive iden
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We shall confine our discussion hen
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The depth Z is measured from the in
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wings (large-y regions). The freque
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expressed as T(y, t )R(y, t )R(
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studies. With such beams, the and
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CHAPTER 3 Neutron forward diffracti
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eing the separation of points M and
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3.1 Neutron forward diffraction Con
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excited by the incident wave (Fig.1
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Therefore, the forward diffracted i
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Fig.15 Calculated cr and I O for a
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(1 I ) D tan b . ID cot( B S)
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Fig.18 Experimental layout to measu
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Fig.20 Bragg reflection intensity f
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fraction I O through the Bragg pris
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Fig.22 Experimental (points) and th
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Table 2: Observed deflection sensit
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total reflectivity domain. These ar
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Here the term C() accounts for the
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Fig.28 Theoretical curves for Si {1
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I H (θ H ) peaks of 0.18 height an
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other with a compression factor lim
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Page 94 and 95: 4.3 Experimental 4.3.1 Bragg prism
Page 96 and 97: 4.3.2 Experiment The experiment was
Page 98 and 99: Fig.38 Bragg reflection rocking cur
Page 100 and 101: 4.4 Applications of novel SUSANS se
Page 102 and 103: Fig.42 First Q ~ 10 -6 Å -1 SUSANS
Page 104 and 105: 2 (1) -(δk i .Δ i ) /2 Γ (Δ) =
Page 106 and 107: 2 I( ) A( ) . (76) The least squa
Page 108 and 109: Fig.46 Analyser rocking curve for a
Page 110 and 111: This instrument was also employed t
Page 112 and 113: Fig.48 Theoretical flat-topped angu
Page 114 and 115: difference between the two neutron
Page 116 and 117: Fig.52 Phase dispersion in a 26.5 m
Page 118 and 119: the beam and remains visible up to
Page 120 and 121: cos bc 4NDd1 1 2cot 2 III 2 2
Page 122 and 123: yield about an order of magnitude r
Page 124 and 125: The refractive index, n = (1-Nb c
Page 126 and 127: with a white, and hence high-intens
Page 128 and 129: proposed dual sample. The dual samp
Page 130 and 131: The experiment was performed with =
Page 132 and 133: Fig.59 Typical interferograms for p
Page 134 and 135: Fig.63 Path I, II and I-II phases d
Page 136 and 137: Fig.65 Average intensity with the s
Page 138 and 139: Fig.69 New metrological mapping of
Page 140 and 141: Si at 26.2 o C, applying a correcti
Page 144 and 145: III. High precision determination o
Page 146 and 147: REFERENCES [1] H. Rauch and S. Wern
Page 148 and 149: [36] J. S. Nico and W. M. Snow, Ann
Page 150 and 151: [67] V. F. Sears, Can. J. Phys., 56
Page 152 and 153: [97] A. Cabello, S. Filipp, H. Rauc
Page 154 and 155: [127] A. Ioffe, D. L. Jacobson, M.
Page 156: 2009, (Grenoble, France, 2010) 3-15
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PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

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