A very-low-Q diffractometer for an advanced spallation source [electronic resource].
- Washington, D.C. : United States. Dept. of Energy, 1993.
Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
- Physical Description:
- Pages: (14 pages) : digital, PDF file
- Additional Creators:
- Los Alamos National Laboratory
United States. Department of Energy
United States. Department of Energy. Office of Scientific and Technical Information
- Proposals to build new, more powerful spallation sources and the introduction of advanced moderator concepts will result in neutron sources that are 20 times more luminous than the brightest available today. These developments provide opportunity and challenge to expand the capabilities of present low-Q instruments using new designs. A particularly interesting case is the design of an instrument capable of measurements to very low'' momentum transfer, say Q [approx] 0.0007 [Angstrom][sup [minus]1]. We consider an instrument to be built on a 20 Hz, 330 kW target and viewing a coupled liquid-hydrogen moderator. The instrument would use a frame-definition chopper to select a wavelength band suitable for the required Q-domain. Monte Carlo optimization of the geometry was performed by choosing the minimum observable Q always to be 0.0007 [Angstrom][sup [minus]1] and then maximizing intensity/variance at Q = 0.0020 [Angstrom][sup [minus]1] while maintaining reasonable constraints. The resulting design is 48 m long, with a maximum wavelength band 16.9 [Angstrom] [le] [lambda] [le] 20.5 [Angstrom]. The Monte Carlo simulations of instrument performance include wavelength-dependent effects from aluminum and fused silica windows, air, chopper opening and closing times and phase jitter, measured spectrum and detector efficiencies, sample transmission and multiple scattering, and gravity. The results were normalized by the measured flux from the present LANSCE moderator, and scaled by the expected performance of a coupled hydrogen moderator. The results compare well with the 76-m configuration of the D11 instrument at ILL in both count rate and Q-precision, due in part to the ability to correct for gravitation effects using the time of flight of the detected neutrons. The validity of these comparisons was demonstrated by comparison of measurements and Monte Carlo simulations made on the present LANSCE Low-Q instrument (LQD) and on D11.
- Published through SciTech Connect.
12. International collaboration of advanced neutron sources meeting (ICANS), Abingdon (United Kingdom), 24-28 May 1993.
Hjelm, R.P.; Seeger, P.A.
- Funding Information:
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