PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

are incident at angles θ and θ+δθ B on the sample. Neutron refraction at the airsample
interfaces introduces a small correction to the phase due to the sample. 93
Fig.52 Phase dispersion in a 26.5 mm thick Si sample placed on path I of an IFM as a
function of neutron incidence angle θ at central wavelength λ 0 (=5.14Å). The
phase becomes nondispersive only for incidence at the Bragg angle θ B and
hence mandates sample alignment to arcsec precision. 94
Fig.53 Variation in the difference Ф I -Ф II between phases arising with the sample on paths
I and II with θ near θ B for three wavelengths (top). The approximate phases for
λ 0 are also shown (bottom). Ф I -Ф II remains nondispersive over ~ arcminute,
relaxing the required sample alignment precision. 97
Fig.54 Variation of the allowed sample thickness with Bragg angle of the IFM. 99
Fig.55 The experimental setup (schematic) employing a large symmetric LLL IFM with
dual sample placed in subbeam II. 103
Fig.56 Dual Si-sample mounted on a plate. 104
Fig.57 A photograph of the experimental setup at S18 ILL, depicting a pair of amorphous
Si prisms each of 120 o apex angles placed before the symmetric {220} LLL IFM
to remove the λ/2 from monochromatic neutron beam. Dual sample mounted on a
plate, is hanging from the top. 106
Fig.58 Optimisation of the sample rotation and tilt. 109
Fig.59 Typical interferograms for path I and II as a function of phase flag. 110
Fig.60 Variation in phase with the interferogram oscillation frequency normalised to that
expected theoretically. 110
Fig.61 Oscillation frequency and phase with scan number. 111