Ultra-small-angle neutron scattering with azimuthal asymmetry

Small-angle neutron scattering (SANS) measurements from thin sections of rock samples such as shales demand as great a scattering vector range as possible because the pores cover a wide range of sizes. The limitation of the scattering vector range for pinhole SANS requires slit-smeared ultra-SANS (USANS) measurements that need to be converted to pinhole geometry. The desmearing algorithm is only successful for azimuthally symmetric data. Scattering from samples cut parallel to the plane of bedding is symmetric, exhibiting circular contours on a two-dimensional detector. Samples cut perpendicular to the bedding show elliptically dependent contours with the long axis corresponding to the normal to the bedding plane. A method is given for converting such asymmetric data collected on a double-crystal diffractometer for concatenation with the usual pinhole-geometry SANS data. The aspect ratio from the SANS data is used to modify the slit-smeared USANS data to produce quasi-symmetric contours. Rotation of the sample about the incident beam may result in symmetric data but cannot extract the same information as obtained from pinhole geometry.

Design and performance of a thermal-neutron double-crystal diffractometer for USANS at NIST

An ultra-high-resolution small-angle neutron scattering (USANS) double-crystal diffractometer (DCD) is now in operation at the NIST Center for Neutron Research (NCNR). The instrument uses multiple reflections from large silicon (220) perfect single crystals, before and after the sample, to produce both high beam intensity and a low instrument background suitable for small-angle scattering measurements. The minimum detector background to beam intensity ratio (noise-to-signal, N/S) forq≥ 5 × 10−4 Å−1is 4 × 10−7. The instrument uses 2.38 Å wavelength neutrons on a dedicated thermal neutron beam port, producing a peak flux on the sample of 17300 cm−2 s−1. The typical measurement range of the instrument extends from 3 × 10−5 Å−1to 5 × 10−3 Å−1in scattering wavevector (q), providing information on material structure over the size range from 0.1 µm to 20 µm. This paper describes the design and characteristics of the instrument, the mode of operation, and presents data that demonstrate the instrument's performance.

A `beam-selection' high-resolution X-ray diffractometer

A new diffractometer that can be described as a high-intensity low-background high-resolution diffractometer for analysing perfect, nearly perfect and highly imperfect materials on a routine basis is presented. The instrumentation is very simple and uncomplicated, yet the way in which it works is less obvious. The sample requires minimal sample alignment, the resolution can be adjusted to optimize the experiment and the wavelength dispersion can be controlled. This diffractometer can produce near perfect profiles from bent and imperfect samples. The illuminated area can easily be varied from greater than 3 mm down to 50 µm diameter, offering great opportunities in microdiffraction with high resolution. The instrument appears similar to a double-crystal diffractometer in reverse,i.e.the sample and collimating crystal of a conventional double-crystal diffractometer are reversed; however, the concept is quite different.

Fine-rotation attachment to standard goniometers

A new type of fine-rotation stage has been constructed and tested. It can be attached to standard goniometers used in X-ray and neutron crystallography. The device consists of a shaft and a bar that is fitted tightly to a hole traversing the shaft. The diameter of the shaft is 5 to 10 times larger than the diameter of the bar and the length of the bar is about 5 times larger than the height of the shaft. The bottom of the shaft is attached to the top plate of the goniometer and a goniometer head can be fitted to the other end of the shaft. The free end of the bar is pushed tangentially by a linear actuator to produce a torsion moment at the shaft. The dimensions and materials of the prototype were chosen such that a 1 mm bend of the bar corresponded to a torsion angle of the shaft of about 20 µrad. The rotation angle was measured using a double-crystal diffractometer in the non-dispersive setting, with MoKα1radiation from a fine-focus X-ray tube. Accurately known angular deviations were produced by refraction in a prism and the shifts in the rocking-curve position were measured. The measured torsion angle agreed within 4% with the value calculated from the elastic constants and dimensions of the device. The repeatability of the angle was ±20 nrad (0.004 arcsec).