No surface with scattering properties superior to the tungsten single crystal  was found but the observed angular scattering was within acceptable limits. Generally, the angular scattering increases with sample complexity, i.e. layered samples displayed a larger scattering than bulk materials. This is due to surface roughness - a bulk material can be very well polished whereas the layered samples depend on the flatness of each of the layers grown on the substrate. The more specular reflection at lower energies might be due to the smoother surface potential seen from a slower particle compared to a fast one. Slower particles would be reflected further away from the surface and thus be less influenced by the surface corrugation. Furthermore sputtering is less likely at lower energies and the angular distribution of sputtered particles is expected to be wide.
From the investigated surfaces the CVD diamond seems to be best suitable for use as start surface in the NPD sensor. The relative detection efficiency surpasses clearly all other samples (Table 4.2). The angular scattering of the CVD diamond is comparable to the tungsten surface measured by . The high secondary electron yield for oxygen on this surface contributes to the high relative detection efficiency. Even at 193eV per atom, the lower secondary electron yield (Figure 3.6) is approximately compensated by the more specular scattering (Figures 4.3 and 4.2).
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland