THE GEOMETRIC ORIGIN OF ASYMMETRIC SMALL-ANGLE NEUTRON-SCATTERING FROM AN ELASTICALLY BENT DOUBLE-CRYSTAL DIFFRACTOMETER

We have investigated the Small Angle Neutron Scattering (SANS) from a Double Crystal Diffractometer (DCD) in the (1, -1) setting using a foc using Si monochromator crystal in the symmetric (111) case and a defoc using analyser Si crystal in a completely asymmetric (111) case. We ha ve found that SANS data from porous glassy carbon and Silica spheres a re asymmetric about the Bragg Peak centre. We attribute this observati on to a combination of factors, particularly geometrical effects, rela ted to the angle of acceptance chi into the analyser crystal when a SA NS sample is positioned between the crystal pair. A simple procedure f or SANS data correction is outlined and shown to be effective in this case.