High-resolution neutron Fourier diffractometer at the IBR-2 pulsed reactor: A new concept

Abstract

The High Resolution Fourier Diffractometer (HRFD) is a neutron time-of-flight spectrometer with a fast Fourier chopper and a correlation mode of data acquisition designed to study atomic and magnetic structures of crystalline materials with high resolution in reciprocal space. The HRFD was constructed in the early 1990s by a collaboration of JINR (Dubna), PNPI (Gatchina) and VTT (Espoo) and began operating routinely in 1994 at the IBR-2 pulsed reactor in Dubna. If correlation analysis is used, the HRFD Δd/d resolution is determined by the maximum rotational speed of the Fourier chopper. For Vmax = 6000 rpm, “high-resolution” patterns are measured by backscattering detectors with Δd/d ≈ 0.0015 despite a very short flight path (L ≈ 20 m) between chopper and sample. Switching to a regular TOF-mode (without Fourier chopper) makes HRFD one of the world’s best high-intensity diffractometers, capable of measuring diffraction patterns in the range of ∼1 min and less. In this paper, the experience of using HRFD at IBR-2 is considered and a possible solution for some technical and methodical problems of Fourier diffractometry at a long-pulse neutron source recognized during its operation is discussed. New possibilities appearing after the recent substantial HRFD upgrade and further perspectives on the method are also presented.