PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

analytic expressions for intensity fraction I H and exit angle θ H of neutrons diffracted from a
Bragg prism, are derived. With a judicious choice of the Bragg reflection, its asymmetry and the
apex angle, a Bragg prism can collimate a neutron beam to sub-arcsec widths (Section 4.2). In
conjunction with an analyser in the opposite asymmetry likewise tailored to accept a pair of even
narrower peaks, it would yield a rocking curve comprising a pair of sub-arcsec peaks separated
by up to a few arcsec. Experimental achievement of the first ever neutron beam of sub-arcsec
widths is presented in Section 4.3. This novel setup has facilitated SUSANS (Super Ultra-Small-
Angle Neutron Scattering) experiments probing wave vector transfers Q ~ 10 -6 Å -1 (Section 4.4)
and hence characterisation of up to 150 μm-size agglomerates in samples. The transverse
coherence length of 175 μm of the monochromated beam is the highest achieved to date,
allowing us to record the first neutron diffraction pattern from a macroscopic grating of 200 μm
period [8-16].
Chapter 5 describes high-precision interferometric determination of the coherent scattering
length b C
by optimising various parameters of the experiment. A finely surfaced thick dual
sample in the nondispersive configuration and a large interferometer (IFM) with spacious
splitter-mirror and mirror-analyser gaps operating at a large Bragg angle reduce imprecision in
previous b C
measurements down to a few ppm. It is then imperative to correct the b inferred
C
from the observed phase for neutron refraction effects at the sample-ambient interfaces. The
refractive index for neutrons can thus be determined to a phenomenal precision of a few parts in
10 12 . Section 5.2 describes the interferometric experiment which thus determined b C
of silicon to
within 27 parts in 10 6 [17-22].
Chapter 6 concludes this thesis and provides a summary of the main results and future directions
for the use of these novel techniques and devices in research. It enumerates several firsts scored
III