PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

4.5 Tightening the neutron collimation still further
The tight collimation of neutrons increases their transverse coherence length to greater than 100
μm FWHM and hence forms a nearly plane wave of monochromatic neutrons. The neutron
collimation may be tightened further by reducing the neutron wavelength. For instance, the same
Bragg prism monochromator-analyser pair optimised for 5.26 Å neutrons can super collimate 1.75
Å neutrons using the {333} reflection in the same geometric configuration down further by a factor
of 9. The {333} Si Debye Waller factor is less than that for {111}, since
2
B
with parameters B=0.422Å 2 , ΔB=0.028Å 2 at 293 K [157]. The
DW exp (B B) sin /
corresponding reduction in F H should improve the collimation further. With an optimised 1.75 Å
neutron incidence between 8 and 32 mm from the prism apex, the Bragg prism monochromator is
expected to deliver an angular profile of 0.062 arcsec FWHM (Fig.48). An analyser operating in
opposite asymmetry with S =−51 o and A=16 o with neutron incidence between 34 and 67 mm from
the prism apex would likewise accept a pair of 0.023 arcsec wide neutron peaks separated by 0.22
arcsec (black curve in Fig.49). The rocking curve of the analyser, viz. the convolution of the
monochromator beam profile with analyser acceptance, comprises two 0.065 arcsec wide peaks
separated by 0.25 arcsec (red curve in Fig.49).
One may envisage tightening the neutron collimation further with Bragg prisms operating at still
smaller wavelengths.
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