Applications of neutron diffraction for small samples (<1mm3) or small fiducial areas are limited by the
available neutron flux density. Recent demonstrations of convergent beam electron and x-ray diffraction and focusing of cold (λ>1 Å) neutrons suggest the possibility to use convergent beam neutron diffraction for small sample crystallography. We have carried out a systematic study of diffraction of both monoenergetic and broad bandwidth
neutrons at the NIST Research Reactor and at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory. Combining convergent beams with time-of-flight Laue diffraction is particularly attractive for high efficiency small sample diffraction studies. We have studied single crystal and powder diffraction of neutrons with convergence angles as large as 15° and have observed diffracted peak intensity gains greater than 20. The convergent beam method (CBM) shows promise for crystallography on small samples of small to medium size molecules (potentially even for proteins), ultra-high pressure samples, and for mapping of strain and texture distributions in larger samples.
We have carried out simulations of a time-focused pulsed-source crystal analyzer (inverse geometry) spectrometer using the VITESS Monte Carlo neutron scattering instrument simulation code. The configuration of the instrument is one suggested by the recently reported general theory of this class of instrument. That theory provides the basis for design to accomplish high resolution while allowing other than backscattering geometry and more flexibility in choices of the type of analyzer crystal and the detector location. The VITESS code has all the capabilities needed to treat this type of spectrometer: three-dimensional generality, time-of-flight, off-cut mosaic crystal reflection, and high computational efficiency, all of which we exercised. We analyzed a configuration with a 50.-m incident flight path, 2.-m distance from sample to analyzer, and 1.8-m distance from analyzer to detector, assuming elements 1.-mm thick and considering reflectivity widths up to 0.5°. The Bragg angle at the analyzer was 80.° and the assumed d-spacing was 3.13 Å. We report results concerning the orientations of the moderator (neutron source), the sample, the analyzer crystal, and the detector that prove out the focusing conditions resulting from the theory. Calculations for realistic sizes of elements and 90° scattering angle indicate an elastic-scattering time-of-flight resolution Δt/t approximately 6. x 10-5 (far less than a conventional estimate cot ΘΔΘ) for the instrument geometry alone, absent the contributions from finite moderator emission-time width and finite-width monochromator d-spacing distribution. Simulations of a second analyzer arm at 60° also show the focusing effect, although we have so far been unable to carry this out for the same sample orientation as for 90°, as theory assures should be possible. The calculations also provide indications of the limits of the linear focusing theory.
The present calculations describing the Bonse-Hart Ultra-Small-Angle Neutron Scattering (USANS) Instrument with triple-bounce Si channel-cut crystals show that significant gains in neutron flux and Q-resolution can be achieved using multiple high-order Bragg reflections. These reflections become usable only after combining the Bonse-Hart and Time-of-Flight techniques, thus this variant of the USANS camera needs a pulsed neutron source. We clearly demonstrate that new instruments of that type installed at the SNS water moderator will improve the current state-of-the art USANS camera dramatically increasing the neutron flux and sharpening the Q-resolution by almost one order of magnitude.
The three levels of thermal neutron sources are introduced--University laboratory sources, infrastructure sources and world-class sources--and the needs for each kind and their inter- dependence will be emphasized. A description of the possibilities for University sources based on a (alpha) -Be reactions or spontaneous fission emission is given, and current experience with them is described. A new generation of infrastructure sources is needed to continue the regional programs based on small reactors. Some possibilities for accelerator sources that could not meet this need are considered.
The Small Angle Diffractometer (SAD) at the Intense Pulsed Neutron Source (IPNS) utilizes a 20 X 20 cm2 Borkowski-Kopp type 3He position sensitive detector (PSD) which has reliably performed small-angle neutron scattering experiments for more than a decade. The pulsed-source based SAD employs a small, but fixed, sample-to-detector distance and a pulsed polychromatic neutron beam. The neutron energies are resolved through time-of- flight (TOF) measurements so that a much wider range of momentum transfer is probed in a single measurement compared to the range of spectrometers using monochromatic incident beams. However, the pulsed source requires a short sample-to-detector distance so that the detector covers a large solid angle, but with lower angular resolution, and this situation puts stringent demands on the spatial resolution of the detector. Previously, nonlinearities in the position encoding of detected neutrons required that the outer channels of the detector, representing 40% of the detector area, be discarded. This paper presents a technique to characterize both the position encoding and the position resolution of the entire detector so that the whole detector can be used for SANS measurements.
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