Barring a monumental failure of design execution or of performance estimation, the liquids reflectometer at the SNS will provide unprecedented capabilities for the study of liquid and solid surfaces. Design of the instrument is well underway and procurement of the guide components has begun. Nuetrons from a coupled 20-K supercritical hydrogen moderator will be delivered via a multi-channel supermirror bender and tapered guide onto either a horizontal or tilted surface. Collimating slits select the beam incident angle from a 0-5° vertical intensity distribution provided by optics. With the SNS running at 2 MW, the instrument will be able to accumulate a complete specular reflectivity scan from D2O (R < 10-7, Q > 0.5 Å-1) in less than 10 minutes. We will describe the optical design of the SNS liquids reflectometer, compare it with a conventional instrument, and estimate its time resolution for a model kinetic system.
Neutron specular reflectivity data obtained with a new grazing angle neutron spectrometer (GANS) from a NiC/Ti-multilayer sample were analyzed and modeled for reconstructing the scattering length density profile as a periodic step potential for the layered material. There is some ambiguity in the results due to the uniqueness problem with missing phase information. For more complex layered materials, there is often insufficient knowledge about the layers to use modeling reconstruction without phase information. In the second part, we present a method in which this problem is solved for diffraction data from lipid multilayers: due to changes in chemistry (isomorphous heavy atom method) the phases are determined directly and therefore the density profile of the lipid bilayer can be uniquely determined.
We describe several geometrical configurations of supermirror arrays for polarizing cold and sub-thermal neutrons in transmission with high efficiency, which is particularly important in applications where several polarizers occur in series. The measured polarizing efficiency and reflectivity of Fe-Si supermirror coatings which can be used in these devices are also reported.
The reflectivities of some commercially manufactured supermirror guide coatings are reported and the effects of interfacial roughness and interdiffusion on the specular reflectivity examined. Uniform reflectivity profiles with average reflectivities of .95 or better out to three times the critical angle of ordinary Ni have been measured. Such high quality coatings can dramatically increase conventional cold neutron guide tube conductance and make possible significantly improved thermal guides.
It is in principle possible to recover phase information regarding an unknown, non-magnetic film structure in reflectivity measurements with polarized neutrons if the film is deposited on a substrate with a buried ferromagnetic layer of known thickness. One particular method is described here for inverting plus and minus neutron reflectivity data in the kinematic limit to obtain the chemical density profile of an 'unknown' layer. Application of this method in the dynamical regime is not as straightforward, and will be discussed. Nonetheless, experimental results demonstrate the value of simultaneously fitting plus and minus reflectivity data in the dynamical case in limiting the number of possible ambiguous results.
The importance of modeling in the analysis of neutron and x-ray reflectivity data cannot be overstated. For specular reflectivity, the theory is straightforward and one merely needs a flexible pattern for constructing density profiles. We will describe the parameters used in and the limitations of such models.