Abstract
A description and the performance of the very small angle neutron scattering diffractometer at the National Institute of Standards and Technology are presented. The measurement range of the instrument extends over three decades of momentum transfer q from 2 × 10-4 to 0.7 Å-1. The entire scattering angle range from 8 × 10-5 to π/6 rad (30°) can be measured simultaneously using three separate detector carriages on rails holding nine 2D detector arrays. Versatile choices of collimation options and neutron wavelength selection allow the q resolution and beam intensity to be optimized for the needs of the experiment. High q resolution is achieved using multiple converging-beam collimation with circular pinholes combined with refractive lenses and prisms. Relaxed vertical resolution with much higher beam intensity can be achieved with narrow slit collimation and a broad wavelength range chosen by truncating the moderator source distribution below 4 Å with a Be crystalline filter and above 8 Å with a supermirror deflector. Polarized beam measurements with full polarization analysis are also provided by a high-performance supermirror polarizer and spin flipper, capable of producing flipping ratios of over 100, along with a high-efficiency 3He polarization analyzer.
Highlights
Small-angle neutron scattering (SANS) diffractometers utilizing long flight paths, pinhole collimation, long wavelengths produced by cold neutron sources and 2D detectors to measure scattered intensity at small scattering angles were first developed in the 1970s in Germany at Julich (Schelten, 1972) and are found at most high-flux neutron sources today
USANS instruments of this type can be found at S18 at Institut Laue– Langevin (ILL), Grenoble, France (Hainbuchner et al, 2000), BT5 USANS at NIST, Gaithersburg, MD, USA (Barker et al, 2005), Kookaburra at the Australian Nuclear Science and Technology Organisation (ANSTO) (Rehm et al, 2013) and TOF-USANS at the Spallation Neutron Source (SNS), Oak Ridge, TN, USA (Agamalian et al, 2018)
To protect personnel at the sample area and to reduce further the background on the instrument detectors, a filter containing a length of 20 cm of polycrystalline Be and 20 cm of randomly oriented single crystals of Bi is placed in the beam and cooled to T = 77 K
Summary
Small-angle neutron scattering (SANS) diffractometers utilizing long flight paths, pinhole collimation, long wavelengths produced by cold neutron sources and 2D detectors to measure scattered intensity at small scattering angles were first developed in the 1970s in Germany at Julich (Schelten, 1972) and are found at most high-flux neutron sources today. USANS instruments of this type can be found at S18 at Institut Laue– Langevin (ILL), Grenoble, France (Hainbuchner et al, 2000), BT5 USANS at NIST, Gaithersburg, MD, USA (Barker et al, 2005), Kookaburra at the Australian Nuclear Science and Technology Organisation (ANSTO) (Rehm et al, 2013) and TOF-USANS at the Spallation Neutron Source (SNS), Oak Ridge, TN, USA (Agamalian et al, 2018) Such DCD instruments can measure over a wide q range, they require much longer counting times than traditional pinhole SANS instruments to obtain similar counting statistics, restricting the practical maximum q usually measured to about 0.01 A À1.
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