PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

yield about an order of magnitude reduction in the D/D contribution to b c /b c . In addition, the
phase also increases by the same factor as D, reducing the Ф/ contribution. Further, one can
maximise d to 3.14 Å by choosing the {111} Bragg reflection for the Si IFM (hence λ = 5.14 Å) to
enhance by about 63% over that obtainable with the {220} Bragg reflection and reduce Ф/. A
thermal enclosure around and vibration isolation of the IFM reduces the phase drift to a fraction of
a degree over a day [1,127]. The effect of this phase drift over a typical measurement duration of a
few hours, is minimised by recording the O and H detector intensities (Fig.50) for the three
positions (I, II and Out) of the sample in succession at each angular setting of the phase flag.
A phase error of about 0.3 deg, thus routinely achieved in interferometric experiments, is included
in Table 3. Good interference contrast can be achieved even for this high interference order due to
the nondispersive configuration [1,126-127]. The contribution from the uncertainty in the refractive
index of air, dependent on variations in the temperature, pressure and relative humidity, can be
larger than that assumed for N a b a /N =9.137(9) x 10 -3
fm in [127]. However, this assumed
uncertainty of 2.2 parts per million in b c precision due to air can be eliminated by performing the
experiment in vacuum.
If the sample happens to be a single crystal, extreme care needs to be exercised to ensure neutron
incidence far off any Bragg reflection of the sample. The sample then just presents an average
refractive index to neutrons. With a crystalline Si sample (Nd =1.57x10 15 cm -2 ), our proposed phase
Ф I-II = -394284.8 o will yield b c with ultrahigh precision of a few parts per million (ppm) as shown
in the last column of Table 3. The exact and approximate phases for δγ=0 in our proposal are
plotted in Fig.53 (bottom curve). The exact phase is greater by about 2.6 o at θ=θ B .
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