Conceptual Design of a Time-of-Flight Ultra-Small-Angle Neutron Scattering (TOF-USANS) Instrument for SNS
We present a conceptual design of a Bonse-Hart-type Time-of-Flight Ultra-Small-Angle Neutron Scattering (TOF-USANS) Double-Crystal Diffractometer, which can be adapted for use at the SNS H2O poisoned decoupled, at H2, or at H2 poisoned decoupled moderator. The experimental set up for this instrument consists of a 15m-long supermirror neutron guide, 2D focused Si(2,2,0) bent pre-monochromator and parallel double-crystal arrangement of the Si(2,2,0) triple-bounce channel-cut crystals located inside a temperature stabilized hutch. The TOF technique allows separating seven Bragg-reflected wavelengths in the range, 0.5 Å < λ < 3.7 Å, arriving at the detector; which gives the opportunity to collect seven sets of USANS data simultaneously. The smallest measurable scattering vector, Qmin, decreases with the increase of the order of Bragg reflection and reaches the value Qmin ˜ 2x10-6 Å-1 for the Si(14,14,0) reflection. This parameter of the TOF-USANS instrument is about one order of magnitude smaller than that for the conventional reactor-based USANS instruments. The combined TOF-USANS scattering curve covers the Q-range 2x10-6 Å-1 < Q < 5x10-3 Å-1. Parallel measurements at different wavelengths seem to be very helpful for direct diagnosing of multiple and inelastic scattering in the USANS experiments, as well as the refraction effects, moreover, time-of-flight operation is likely to reduce the background.
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