PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

The experiment was performed with =2.36 Å corresponding to θ B = 38 o and an empty IFM
rocking curve with 2.52 arcsec FWHM was recorded with respect to monochromator to align IFM.
The empty IFM contrast, defined by Eq.(42), dropped from 80% at θ B =30 o (=1.92Å) to about
40% at θ B = 38 o since now neutrons sampled the less perfect outer regions of the mirror. By
translating the IFM horizontally and vertically, the contrast was optimised to about 65%.
Next, the O+H intensity recorded as a function of the sample translation transverse to Bragg planes
in the IFM, exhibited a plateau each in paths I and II. The sample position for each path was set at
the centre of the respective plateau. With the sample placed in either path I or II, O and H
interferograms were recorded. The contrast dropped to ~ 60% on placing the sample at either of
these positions. For the Si sample, the attenuation cross-section (σ attn = 0.175 barn) does not
significantly influence the real part of the refractive index and hence a large neutron path length of
58.5 mm within the sample in nondispersive configuration still yielded a good interference
contrast.
5.2.2 Data analysis
For each interferogram, intensity oscillations O*/(O+H) were fitted to sinusoidal variation
I I
O(1 A cos(B )), yielding the average intensity I O , contrast A, normalized frequency B
and phase . The phases extracted from interferograms acquired at several rotations ε and tilts γ of
the sample displayed the expected parabolic variations (Fig.58). The sample was then set to the
correct orientation (ε 0 , γ 0 ) where the phase magnitude exhibited a minimum. A large number of
successive sample-in and sample-out interferometric scan pairs were then made alternately for path
I and II (Fig.59), punctuated with realignments of the IFM whenever the contrast dropped below
about 50%. Phases inferred as a function of oscillation frequency are plotted in Fig.60. The
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