David G. Narehood

Preparation and synthesis of Ag2Se nanowires produced by template directed synthesis

We have prepared silver selenide nanowires by a novel synthesis technique. The nanowires were produced through template directed synthesis, in which we used a porous alumina membrane as the template. Silver was deposited on one surface and in the pores close to that surface of the template, followed by electro-deposition of selenium into the pores. Silver selenide nanowires of high quality were produced inside the pores.

Location of frame overlap choppers on pulsed source instruments

A detailed study has been performed to investigate the effect of frame overlap in a cold neutron chopper spectrometer. The basic spectrometer is defined by two high-speed choppers, one near the moderator to shape the pulse from the moderator, and one near the sample to define energy resolution. Using ray-tracing timing diagrams, we have observed that there are regions along the guide where the trajectories of neutrons with different velocities converge temporally at characteristic points along the spectrometer. At these points of convergence, a frame overlap chopper would be totally ineffective, allowing neutrons of all velocities to pass through. Conversely, at points where trajectories of different velocity neutrons are divergent, a frame overlap chopper is most effective. An analytical model to describe this behaviour has been developed, and leads us to the counterintuitive conclusion that the optimum position for a frame overlap chopper is as close to the initial chopper as possible. We further demonstrate that detailed Monte Carlo simulations produce results which are consistent with this model.

Inelastic neutron scattering of H2 in xerogel

The properties of molecular hydrogen adsorbed in Britesorb were studied through inelastic neutron scattering. We have measured both the rotational energy levels and the momentum distribution at bilayer and nearly full pore fillings. Splitting of the J=1 rotational energy levels is observed for molecular hydrogen adsorbed on the surface, while the rotational properties of the hydrogen adsorbed after monolayer completion is consistent with behavior in the bulk. Additionally, the measurement of the momentum distribution showed that the mean kinetic energy of the molecules in the bilayer is 88 K±7 K. The kinetic energy measured in the nearly full pore was 81 K±6 K, which is consistent with a simple model in which the behavior of the monolayer is dominated by the interaction with the surface of the pore wall but H2 adsorbed after monolayer completion is bulk like.

Monte Carlo simulation of instrument response for direct geometry time-of-flight spectrometers

A full Monte Carlo simulation of sample scattering and the final flight path for direct geometry time-of-flight spectrometers has been developed. This allows the scattering from systems with both realistic and complex scattering geometries as well as realistic scattering functions to be modeled. This simulation, PULSCAT, interfaces with commonly available ray tracing programs, such as VITESS, that simulate the incident beam. Spectra with elastic and inelastic features resulting from scattering from isotropic scattering systems in addition to multiple scattering for amorphous scattering systems can be modeled with PULSCAT. The sample geometry used in the simulation is entered through a GUI interface. Due to the large flexibility in the input parameters for the sample, sample environment equipment can be included in the simulations allowing for scattering from ancillary equipment (such as from a standard orange cryostat) to be modeled. This makes PULSCAT a powerful tool for simulating systems and investigating spurious effects present in collected spectra.