PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

CHAPTER 4
First sub-arcsec collimation of a monochromatic neutron beam
4.0 Introduction
I present in this chapter, a novel device producing a tail-free plane-wave-like neutron beam of subarcsec
angular width. Neutrons incident on a single crystal prism in an asymmetric Bragg
configuration undergo, in addition to the Bragg reflection, diffraction at the exit face [138] of the
prism. Two small diffraction peaks I H , one on either side of the Bragg reflection, appear outside the
Fig.26 Asymmetric Bragg prism diffraction (schematic). Outside the total reflectivity domain,
unreflected neutrons traverse the prism at an angle Δ to the incidence surface and exit the side face
in diffracted (I H ) and forward diffracted (I O ) beams. At large enough angles Δ, neutrons reach the
part of the side face cut along the diffracted beam direction and exit wholly into the I O beam, thus
truncating I H on the larger-|y| side as well to yield a tail-free, sharp angular profile.
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