| We have constructed and studied a nozzle source helium beam and a detector for use in a dynamic helium atom scattering (DHAS) apparatus. DHAS will someday be a useful tool for surface study, having both the surface sensitivity of traditional helium scattering experiments and the high-energy resolution of dynamic light scattering techniques. Coherence requirements, engineering considerations, and the physics of the helium gas expansion place an upper limit on the signal that can be obtained in a DHAS setup. This thesis summarizes work on the optimization of detection and beam brightness in order to maximize DHAS signal. Helium beams from micron size glass nozzle sources are investigated over a wide range of speed ratio, or ratio of terminal velocity spread to terminal velocity. The brightest of these sources were found to be smaller nozzles operated at pressure and temperature just below values for which quantum effects on the helium scattering cross section arise. A field ionization tip was chosen for the detector due to its high ionization efficiency over a sub-micron region. Field ionization tips were studied in a diffuse gas and in a well-collimated beam. The dependence of detection efficiency on voltage, on gas temperature and on orientation to the beam has been determined. |
PDF Full Text Request