ORNL DOUBLE-CRYSTAL ULTRA SMALL-ANGLE NEUTRON SCATTERING FACILITY. M. Agamalian, R.Triolo*, G. D. Wignall, Oak Ridge National Laboratory, Oak Ridge, TN 37831

Conventional small-angle neutron scattering instruments (SANS) are widely used for structural studies in the resolution range 10³ - 10Å. The upper resolution limit can be extended by over two orders of magnitude (to ~ 2*10^-5Å) by using the Double Crystal Diffractometer (DCD) with the Bonse-Hart collimation technique. The ultimate resolution of the DCD is determined by the tails of the Bragg reflection curves, which may be minimized by multiple reflections in channel-cut crystals.

A new Double-Crystal Diffractometer for Ultra Small-Angle Neutron Scattering (USANS) has been recently built at High Flux Isotope Reactor, Oak Ridge National Laboratory using the Si(111) triple bounce channel-cut single crystals for both the monochromator and analyzer. The half-width of the rocking curve is about +/- 1.0 arcsec, and the wavelength of the primary neutron beam is 2.59Å.

The DCD has been tested by standard monodisperse polystyrene latex with the radius of 2.5*10⁴Å, as determined by optical microscopy. This radius determined by the DCD is 2.48*10⁴Å, in good agreement with microscopy. The minimum measurable scattering vector is Qmin = 3.5*10^-5Å^-1 which corresponds to the distance in real space 2[[pi]]/Qmin~ 1.8*10⁵Å

The USANS technique has also been used to study a dilute solution of flexible rodlike micelles in D₂O. The radius of gyration obtained in the experiment is Rg = 1.9*10⁴Å, which gives the length of the rodlike micelles L = 6.5*10⁴Å, assuming a rigid rod model.

* University of Palermo, Italy