Small-angle neutron scattering at pulsed spallation sources

Abstract

The importance of small‐angle neutron scattering (SANS) in biological, chemical, physical and engineering research mandates that all intense neutron sources be equipped with SANS instruments. Four existing instruments at pulsed sources are described and the general differences between pulsed‐source and reactor‐based instrument designs are discussed. The basic geometries are identical, but dynamic range is generally achieved by using a broad band of wavelengths (with time‐of‐flight analysis) rather than by moving the detector. This allows optimization for maximum beam intensity at a given beam size over the full dynamic range with fixed collimation. Data‐acquisition requirements at a pulsed source are more severe, requiring large fast histograming memories. Data reduction is also more complex, as all wavelength‐dependent and angle‐dependent backgrounds and nonlinearities must be accounted for before data can be transformed to intensity versus momentum transfer (Q). A comparison is shown between the Los Alamos pulsed instrument and DII (Institut Laue–Langevin) and examples from the four major topics of the conference are shown. The general conclusion is that reactor‐based instruments remain superior at very low Q or if only a narrow range of Q is required, but that the current generation of pulsed‐source instruments is competitive at moderate Q and may be faster when a wide range of Q is required.

Phase stability of an HT-9 duct irradiated in FFTF

“…The accessible length scale range of LQD is 1-100 nm. All SANS data were reduced to differential scattering cross section per unit volume, I(Q) (cm À1 ), as a function of the magnitude of the scattering vector, Q (Å À1 ) using standard data reduction methods for time-of-flight low-Q data [20]. Data analysis and scattered intensity corrections for background scattering and detector sensitivity were done using standards.…”

SANS at Pulsed Neutron Sources: Present and Future Prospects

“…Wavelength-dependent corrections are required for the incident spectrum shape, detector efficiency and sample transmission. The process has been described in some detail by Seeger & Hjelm (1991), so will only be outlined here, in a form slightly modified to suit LOQ at ISIS. The number of neutrons of wavelength ~.…”

Long ferromagnetic correlation length in amorphousTbFe2

“…Data from SANS-J is mapped directly into intensity ͑after averaging over the appropriate range of azimuthal angles͒ as a function of momentum transfer, Q ϭ4/ sin , where is the incident neutron wavelength and 2 is the scattering angle. LQD uses time-of-flight techniques; the data from this instrument are reduced using standard techniques 42,43 to give the intensity I in absolute units of differential cross section per unit area dP/d⍀ ͑cm 2 /cm 2 ͒ as a function of magnitude and direction of scattering vector Q. Dividing the intensity by the sample thickness in cm gives the differential cross section per unit volume, usually noted as d⌺/d⍀ ͑cm Ϫ1 ͒.…”

“…Measurements were made either in a turbo-pumped vacuum furnace or in a He cryocooler to permit sample temperatures from 20 to 473 K. Background, transmission, and calibration standards were measured to permit absolute measurements of scattering. 42 Absorption by the sample was measured and found to be negligible.…”