We review the methods of simulating the neutron optics of three-axis spectrometers to optimize their resolution and luminosity and to interpret experimental data collected with them. The program package RESTRAX includes both a high-speed analytical (Gaussian) convolution algorithm and a Monte Carlo ray-tracing code providing enhanced accuracy in description of most of the spectrometer components. The program is designed for a fixed spectrometer layout, with emphasis on the choice of a complete set of usual neutron optical devices and on their realistic representation. The fixed layout permits to generate a highly optimized Fortran code, producing sets of 1000 - 10000 successful events per second. This speed on the turn allows for a truly interactive work when designing a new instrument or refining a model describing the scattering behavior of a sample under investigation. As an illustration, we give several examples of TAS configurations, including high-luminosity focusing arrangements and multianalyzer setups as well a demonstration of the data fitting part of RESTRAX in the case of magnetic excitations.
The three-axis spectrometer IN20 has been upgraded to enhance significantly the data collection rate in experiments using polarized neutrons to study magnetic excitations in the (higher) thermal energy range. To increase the monochromatic polarized neutron flux, a new geometry of the primary spectrometer, optimized by detailed ray-tracing simulations, has been adopted. The main ingredients are a neutron source of a diameter increased from 100 mm to 170 mm and a large double focusing monochromator, illuminated through a heavy input slit (virtual source) of adjustable width. This geometry permits to keep the background at a possibly low level while maximizing the solid angle available for monochromatic focusing. The real challenge of the project has been the new Heusler monochromator. With its active surface of 230 x 150 mm2, consisting of 75 crystal plates mounted in 15 columns, it is the largest polarizing crystal assembly ever built. In combination with the horizontally focusing analyzer of a similar design, implemented in spring 2000, the data collection rate in the polarization analysis mode has increased by a factor 30 - 50 in April 2001 as compared to the original IN20, which up to now has provided world's highest polarized neutron flux in the thermal energy range.
Using Bragg diffraction optics, an unconventional DBC diffractometer was tested for medium resolution small-angle neutron scattering experiments. The diffraction geometry of the analyzer enables to transform the angular beam distribution into the positional distribution and, consequently, to analyze it by means of a one-dimensional position sensitive detector. First experimental results obtained with a sample of PE+graphite proves a compatibility and a higher speed of data collection compared to a standard DBC diffractometer.
Experimental tests of elastically bent Si crystals as monochromator and analyzer in a triple- axis setup for investigation of stress fields in polycrystalline materials are presented. It is also demonstrated that if certain focusing conditions for a bent monochromator are met the beam diffracted by a polycrystalline sample becomes quasi-parallel which enables high resolution measurements directly with a PSD without the use of a collimator or a crystal-analyzer. In the three axis setup maximum sensitivity in determination of (Delta) d/d <EQ 10-4 can be achieved permitting profile-broadening analysis for reasonable sample volumes and counting times.