PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

we were able to achieve empty IFM contrasts up to about 75 %. On placing the sample in path I or
II, the intensity dropped only by about 5% (Fig.65) and the interference contrast remained
essentially unaltered over a majority of scans (Fig.66). We note that the actual neutron path length
of 58.5 mm within the Si dual sample though very large, still yielded a very good contrast
underscoring the importance of measurements in nondispersive sample configuration. Further,
small intensity decrease in measurements made with the sample IN positions compared to sample
OUT scans is due to the incoherent scattering and absorption of neutrons within sample. The
phases extracted from interferograms acquired at several rotations ε and tilts γ of the sample in path
I and II were fitted to parabolic curves (Fig.67). 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 -out interferometric scan pairs were then made alternately for path I and II. A typical
interferogram for this set of measurements is shown in Fig.68. The regulated air flow within the
temperature-controlled aluminium enclosure yielded good interference contrasts, but also reduced
the relative air humidity to between 5 and 10% which, in turn, led to a large variation in the
measured phases. The frequency of interference oscillations remained fairly constant throughout
the measurements.
To arrive at the correct phase, we carefully analysed the temperature and humidity data. We
observed that the measured phases on either path remained closely constant when thermal gradients
in the IFM vicinity were low and the relative humidity inside the IFM cabin ranged between 35 and
40%. From 95 such interferograms for paths I and II and using the previous measurements of b c for
Si [127], we deduced phases of – 455 X 360 – 54.811 0.154 deg and + 455 X 360 – 256.49
0.117 deg for paths I and II respectively so that Φ I-II = – 455 X 720 + 201.679 0.193 deg. After
considering a 1μm uncertainty in the sample thickness of 35.96 mm (Fig.69), thermal expansion of
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