ASTACUS. A reverse neutron time of flight diffractometer using a Fourier chopper

A neutron diffractometer based on the reverse Fourier time of flight method and intended for the study of powdered samples is described. The Fourier chopper attains a maximum modulation frequency of 120 kHz, allowing a time resolution of about 10 microsec, depending on the frequency window employed. In the geometry, the time focusing principle is used to obtain high resolution. The lattice plane distances are resolved with a precision of Delta d/d = 0.32% at d = 1 A and Delta d/d = 0.16% at 3.0 A. The diffractometer facility, its computer data acquisition hardware as well as the programs developed for controlling measurement and for acquiring and storing data are detailed. The performance of the facility was tested on a nickel powder sample, and the results of the profile refinement procedure for the measured diffraction patterns are given. The accuracy obtained for the lattice parameter determination is to 0.0001, and the room temperature value obtained for the Debye-Waller coefficient of nickel is B sub Ni = 0.425 (+ or - 0.065) A squared.