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.
Some results are presented of strain measurements carried out using a novel, modified version of the energy- dispersive neutron transmission diffraction method which has been implemented on a conventional diffractometer at a steady-state reactor. This method, which has so far been used only on time-of-flight instruments, can also be easily and successfully used at steady-state sources. The EDNTD method presented is based on the precise measurement of the Bragg diffraction edge by using focusing crystals and is complementary to the method of strain measurements by Bragg diffraction angle analysis. It allows changes (Delta) d/d of lattice spacings with sensitivity down to some 10-5 to be investigated for reasonable, irradiated sample volumes and counting times, even at a medium-power research reactor.
Novel modification of the neutron diffraction method - method of Bragg diffraction angle analysis, which is routinely used for strain/stress investigations in polycrystalline materials in NPI Rez is presented. It provides sufficiently high resolution permitting investigations of both the macro- and microstains of the order 10-4. Basic feature of the modification is the exploitation of Bragg diffraction optics - focusing in real and momentum space - using curved perfect crystal slabs. Properties of this modification are documented by results of experimental measurements.
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.