Small Angle Neutron Scattering Facility

The beam from the guide passes through a liquid-nitrogen cooled filter consisting of 20 cm of polycrystalline beryllium and 15 cm of single-crystal bismuth to remove fast neutrons and core gamma rays, respectively, from the beam. Following the filter, a variable speed, helical vane, mechanical velocity selector is used to tune the mean wavelength from 5 to 20 A with a fixed wavelength resolution, delta lambda/lambda (FWHM) = 0.25. The beam is collimated along a 4.5 m evacuated pre-sample flight tube with either a pair of circular apertures at its entrance and exit or, for higher resolution measurements, a 7-beam, converging pinhole collimation system that can be inserted in the flight path.

The instrument has an evacuable sample chamber with a computer-controlled translation stage that accommodates multi-specimen sample changers with up to 5 sample positions. The sample chamber is removable, providing access to a 60 cm diameter rotatable sample table for large pieces of apparatus such as electromagnets and cryostats.

The post-sample evacuated flight path consists of three shielded cylindrical sections which are mounted on a frame that can be rotated about the sample position by about 15 to reach larger scattering angles. The section which houses the detector rolls along rails in the frame to allow the sample-to-detector distance to be changed from 3.6 m to 2.0 m. Built into the detector shield is an externally adjustable positioning device for locating a beam stop in front of the detector.

The detector is a 64 cm x 64 cm position-sensitive proportional counter that utilizes rise-time position encoding and has a spatial resolution of about 10 mm. Counts from the detector accumulate in histogramming memory and are periodically transferred to disk on the instrument's dedicated data acquisition computer as well as to a color monitor which displays a current image of the data being collected. The instrument is operated through a user-friendly menu-driven interface. Completed data sets can be reduced on-line and transferred electronically to a remote site, or to PC/Mac diskettes in ASCII format.