PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

other with a compression factor limit of 47.62 (Fig.31) adding up to a single large peak. Thus by
increasing A, we achieve stronger and narrower diffraction peaks due to better compression and
hence superior collimation.
By a judicious optimisation of the Bragg reflection, the asymmetry of the Bragg configuration and
apex angle A, the angular profile of the diffracted beam emerging from the side face can be
narrowed down to the sub-arcsec domain. For 5.26 Å neutrons and Si {111} reflection, the optimal
monochromator with A=172 o and θ S =50.1 o yields vide Eq.(65) a pair of diffraction peaks each of
9.9 arcsec FWHM in incidence angle which get compressed to 0.67 arcsec FWHM in exit angle
(Fig.32). Thus the variation in the compression factor from 0 at |y|=1 to 47.62 at |y|>>1 results in
Fig.32 Exit angular profiles of prism diffraction (theory) for parameters as in Fig.28. Long tails of
the large curves obtainable with the Bragg prism get clipped (small curves) on cutting the prism
side face along the diffracted beam direction beyond a depth of 5 mm.
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