PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

I. One of the major disadvantages of neutrons, also shared by X-rays, is their very small magnitude
of refractive power |n – l| ~ 10 -6 –10 -5 [4-5]. Therefore, thermal neutrons traversing an amorphous
prism undergo a deflection in the arcsec regime and a few-degree variation in the incidence angle
brings about only ~ arcsec change in the neutron deflection. Further, contrary to light, since
refractive index is usually smaller than unity for neutrons, the deflection will be towards the apex
of prism. Schneider and Shull [108] have carried out precision measurements of prism deflection
angles from which refractive indices, and hence scattering lengths, were determined. Wagh and
Rakhecha [109] showed that for neutron propagation parallel to the prism base, the neutron
deflection just equals the product of the refractive power and base-height ratio of the prism. Over
the past five decades, variations of the forward diffracted (I O ) and diffracted (I H ) intensities from
single crystal prisms with the incidence angle have been observed for X-rays [38,110] and neutrons
[79]. Due to the nonavailability of arcsec wide neutron beams at that time however, the neutron
I H (θ) variation could only be mapped by scanning a narrow slit across the diffracted beam.
However, deflection sensitivities of the amorphous prisms remained abysmally low. In Chapter 3,
we overcome this problem by presenting a novel Bragg prism, viz., a single crystal prism operating
in the vicinity of a Bragg reflection, offering deflection sensitivity of more than three orders of
magnitude greater than an amorphous prism achieved with a 2 arcsec wide neutron beam [111-
112].
II. Experimenters’ insatiating thirst for more and more parallel monochromatic neutron beam, has
led to development of numerous types of monochromator–collimator setup using single crystal
[4,24-27]. Ultra-small angle neutron (or X-ray) scattering (USANS/USAXS) studies [113] which
probe wavevector transfers Q ~ 10 -5 Å -1 , require an incident beam collimation down to a few
arcsec, to characterize samples [26-27,114] containing agglomerates of μm dimensions. A Bragg
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