PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

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4.3.2 Experiment The experiment was carried out at the V12b Double Crystal Diffractometer setup of BER II 10 MW Reactor in HZB, Germany. The analyser rotation could be adjusted in two stages; first in 1 arcsec steps with a geared step motor and then with a piezocrystal driven stage, with the smallest step size of 0.156 arcsec. The analyser tilt could be set to within 0.9 arcsec. The monochromator (Fig.35), covered strategically with Cd sheets, could be translated along the incident beam direction to direct either its Bragg reflection or prism diffraction towards the analyser. A Cd sheet could be translated in front of the He 3 detector to cut off the Bragg reflection from the analyser. 4.3.3 1 st ever sub-arcsec collimation An asymmetric monochromator (θ S =53.5 o ) was placed in the 5.26 Å neutron beam from the PG pre-monochromator and its rotation and tilt angles adjusted to maximise its Bragg reflection. The analyser was illuminated with this Bragg reflection and aligned using its prism diffraction. The analyser tilt had to be set to within 9 arcsec. The analyser rocking curve comprised a well resolved pair of ~1.7 arcsec wide peaks (circles) (Fig.37). The theoretical convolution (smooth curve) of monochromator Bragg reflection and analyser acceptance is also plotted for comparison. The experimental curve reproduces the theoretical peak widths [150-151] fairly well vindicating the analyser Bragg prism performance. The flat tops of the theoretical peaks however have got rounded off in the observed spectrum. The relative intensities of the two peaks do vary statistically from spectrum to spectrum, but more often than not, the higher-angle peak is weaker. The reason for this left-right asymmetry in Figs.37, 40-42 is not clear. A Bragg prism monochromator with θ S =50.1 o was next tested. Its direct Bragg reflection, being much stronger and wider than prism diffraction, was first used, facilitating a quick and easy 74
Fig.37 Bragg reflection from the θ S =53.5 o monochromator observed with prism diffraction (cf. inset) of the optimal analyser. alignment of the analyser whose Bragg reflection as well as prism diffraction were monitored (Fig.38). The analyser rocking curve yielded the expected ~ 3.1 arcsec wide peak (circles) with a peak intensity of about 468 neutrons/s. The monochromator was then translated to illuminate the analyser with its prism diffraction alone. The corresponding analyser rocking curve of ~ 2.5 arcsec FWHM (cf. Fig.39) agrees well with theory (smooth blue curve), displaying a peak intensity of 68.5 neutrons/s, After optimising the analyser alignment, the Cd sheet before the detector was translated to stop Bragg reflected neutrons from the analyser. With these Bragg prism diffractions, the analyser tilt adjustment became even more critical and had to be made in 0.9 arcsec steps, since prism diffraction peaks are narrower and sharper than the Bragg reflections. The rocking curve (Fig.40) consisting of a pair of 0.62 arcsec wide peaks separated by 2.2 arcsec (squares), is in fair 75
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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 ,
Page 46 and 47: exploiting multiple successive iden
Page 48 and 49: We shall confine our discussion hen
Page 50 and 51: The depth Z is measured from the in
Page 52 and 53: wings (large-y regions). The freque
Page 54 and 55: expressed as T(y, t )R(y, t )R(
Page 56 and 57: studies. With such beams, the and
Page 58 and 59: CHAPTER 3 Neutron forward diffracti
Page 60 and 61: eing the separation of points M and
Page 62 and 63: 3.1 Neutron forward diffraction Con
Page 64 and 65: excited by the incident wave (Fig.1
Page 66 and 67: Therefore, the forward diffracted i
Page 68 and 69: Fig.15 Calculated cr and I O for a
Page 70 and 71: (1 I ) D tan b . ID cot( B S)
Page 72 and 73: Fig.18 Experimental layout to measu
Page 74 and 75: Fig.20 Bragg reflection intensity f
Page 76 and 77: fraction I O through the Bragg pris
Page 78 and 79: Fig.22 Experimental (points) and th
Page 80 and 81: Table 2: Observed deflection sensit
Page 82 and 83: total reflectivity domain. These ar
Page 84 and 85: Here the term C() accounts for the
Page 86 and 87: Fig.28 Theoretical curves for Si {1
Page 88 and 89: I H (θ H ) peaks of 0.18 height an
Page 90 and 91: other with a compression factor lim
Page 92 and 93: an overall compression factor of 14
Page 94 and 95: 4.3 Experimental 4.3.1 Bragg prism
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 142 and 143: selected asymmetric Bragg prism may
Page 144 and 145: III. High precision determination o
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REFERENCES [1] H. Rauch and S. Wern
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[36] J. S. Nico and W. M. Snow, Ann
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[67] V. F. Sears, Can. J. Phys., 56
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[97] A. Cabello, S. Filipp, H. Rauc
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[127] A. Ioffe, D. L. Jacobson, M.
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2009, (Grenoble, France, 2010) 3-15
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PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

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