PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

The refractive index, n = (1–Nb c 2 /) ½ of Si for thermal neutrons equals unity to within about 1
part in 10 6 . This proposal can thus determine the refractive power, n–1±10 -6 , with a relative
precision of a few parts in 10 6 , and hence the refractive index to a phenomenal precision of a few
parts in 10 12 [156-157].
The refraction corrections of –2.7 * 10 -5 and –1.01x10 -5 fm to b c for {111} and {220} IFM
reflections respectively, due to refraction at air-sample interfaces is of the same order as the Ioffe et
al.’s experimentally determined b c precision of 5 parts in 10 5 . Therefore after refraction
corrections, b c values in Ioffe’s measurement should be modified to 4.15101(21) and 4.15038 (21)
instead of inferred 4.15102(21) and 4.15041(21) values corresponding to IFM reflections {220}
with λ= 1.98 Å and {111} with λ= 2.7 Å, respectively.
5.1.2 Dual nondispersive phase shifter
The dual nondispersive phase shifter comprising two identical and parallel phase shifters placed on
one path, one before and the other after the mirror blade [Fig.55], in the nondispersive
configuration has been recently proposed by Lemmel and Wagh [128]. The effective thickness of
the phase shifter gets doubled which results in doubling of the neutron phase (D) and halving
Ф/. The phase shift I( ) II
( ) , which now can be obtained in a single measurement
with this dual phase shifter placed on a single beam path, is exactly nondispersive. Furthermore,
the temporal as well as spatial displacements of the wave packet effected by the dual sample in the
first gap get cancelled in the second, restoring full interference contrast. A long monolithic sample
with a groove cut in the middle (Fig.56) to accommodate the mirror plate attains the exact
parallelity and the identical thicknesses for its two segments. Both of these segments are
mechanically and thermally coupled. The large nondispersive phase shift can be measured even
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