Surface-induced parasitic scattering in Bonse-Hart double-crystal diffractometers

Bonse-Hart double-crystal diffractometers (DCD) with multibounce channel-cut crystals show rocking curves which depart dramatically in their wings from dynamical diffraction theory. This intrinsic background is many orders of magnitude higher than the predictions of dynamical diffraction theory. This effect has been studied at the ultra-small-angle neutron scattering (USANS) facility at Oak Ridge National Laboratory by the measurement of rocking curves from different volume elements of a thick single-bounce Si(111) perfect crystal and from triple-bounce channel-cut Si(111) crystals. Analysis of these data, together with the rocking curves from an X-ray Bonse-Hart DCD, allows it to be established that the rocking curve of the multibounce channel-cut crystals contains parasitic scattering generated at the surface of the crystals. The intensity of this component can be reduced by very deep etching of the crystal surface. Using this technique, the signal-to-noise ratio of the Bonse-Hart DCD at the ORNL USANS facility has been improved by one order of magnitude to similar to 5 X 10(5) [I(0)/I(theta = 10 ")].