Performance of a multi-wavelength mono­chromator system for neutron single-crystal and powder diffraction

In order to improve significantly data collection efficiency, the advantages of using multi-wavelengths in single-crystal neutron diffraction as well as in powder neutron diffraction were established by experiments using a stacked monochromator assembly of two elastically bent perfect Si(111) and Si(220) crystals. The TAS-2 spectrometer on JRR-3M at JAERI (Japan Atomic Energy Research Institute) was employed for these experiments. Each crystal was elastically bent in order to increase the flux density at the sample position. Two wavelengths, λ₁ = 1.80 Å for Si(220) and λ₂ = 2.94 Å for Si(111), were selected. The alignment of the stacked monochromator was successful and a well defined rocking curve of approximately 0.3° full width at half-maximum (FWHM) was obtained. Diffraction patterns of Si powder were collected using the TAS-2 point detector as well as a neutron imaging plate (NIP) detector to compare the relative performance of the two detector systems. With the NIP detector, it was possible to acquire multiple reflections simultaneously with better intensity, better statistics and higher spatial resolution with an open geometry. The NIP detector was used to obtain the diffraction pattern of a single crystal of the organic compound piperidine cobaloxime. For the systems investigated in this study, it was possible to assign unambiguously rings or spots to diffraction by λ₁ or by λ₂ in the case of powder and single-crystal diffraction.