Composite germanium monochromators for high resolution neutron powder diffraction applications

Abstract

To perform optimally, reactor-based high resolution neutron powder diffractometers need to have monochromating crystals that satisfy an unusually stringent set of conditions: (i) The crystals should have reflecting planes with small plane spacings. This is necessary to provide a narrow wavelength distribution (required for good resolution) centered at a wavelength short enough to give the instrument an adequate scattering vector (Q) range. (ii) The scattering-amplitude-weighted coherent structure factor in the crystal reflecting plane of choice must be large. This limits beam penetration into the crystal and minimizes flux loss from absorption and broadening of the reflected beam. (iii) A high Debye temperature is desirable to reduce the flux loss due to temperature diffuse scattering. (iv) Absorption and incoherent scattering cross sections should be small to keep absorption losses and beam-related background to a minimum. (v) A diamond structure is preferred to eliminate second order contamination of the reflected beam. (vi) The horizontal mosaic distributions of the crystals should be laterally uniform and well-matched to the angular acceptance of the collimation system. This is necessary to provide both optimal intensity and a smooth, symmetric, structure- free beam profile at the sample. (vii) The vertical mosaic distributions should be small and laterally uniform to provide optimal vertical focusing (needed for reasons of intensity) at the sample position. (viii) Large single crystals of good quality should be readily available at reasonable prices.