Neutron Sciences

The DEMAND Diffractometer goniometer has a full χ circle with a 4–800 K closed-cycle helium refrigerator. The detector is a scintillator-based 2D Anger Camera. The upper 2Θ limit is 160°. A multilayer-[110]-wafer silicon monochromator with the reflection from planes of the <011> zone ensures sharp diffraction peaks in specified ranges of detector angles by control of the horizontal radius of curvature. Any plane from the <011> zone can be set in Bragg position, but only the (331), (220) with (440), and (111) with (333) reflections are of practical interest. For the fixed monochromator angle of 47.5°, these reflections provide principal incident wavelengths of 1.005 Å, 1.546 Å, and 2.541 Å, respectively. A PC-based LabView system provides user-friendly diffractometer control and data acquisition. The beam size is 6.3 mm in diameter, and the minimum measured crystal size is 0.02 mm³. The maximum crystal dimension is usually limited to 5 mm. The flux on the sample can be up to 2.2 × 10⁷ n/cm²/s. The horizontal bending of the monochromator can be changed to optimize the Q-resolution or flux depending upon the needs of the measurement. The longer wavelength of 2.541 Å has ~5% λ/3 contamination and is mainly used for polarized neutron diffraction (S-bender will filter out the high-order contamination). The 1.546 Å-wavelength has the highest flux but with ~1.4% λ/2 contamination (PG filter is available to reduce the contamination below 10^-4), is mainly used for determining magnetic structures. The 1.005 Å wavelength is monochromatic and is good for precisely determining both nuclear and magnetic structures although the flux is 8 times lower than the highest flux at 1.546 Å.