Positronium Emission and Low-Energy Positron Diffraction from Metal Surfaces.
This dissertation reports on two different experiments. In both, high energy positrons from a ('58)Co source are converted into slow positrons and directed through a transport system onto the surface of a metal sample mounted inside of an ultrahigh vacuum bell jar. The first experiment measured the fraction of incident positrons being emitted as positronium as functions of surface contamination, incident beam energy, and sample temperature. The slow positrons were guided to the target through a magnetic solenoid and then their annihilation products were detected and analyzed under computer control using a NaI(T1) scintillator and single and multichannel analyzers. For Ag, Al, and Ni, the cleaner the surface was, the more positronium was emitted; for Cu, the maximum emission occurred with (TURN)15% contamination of both C and O. In the incident energy studies, the higher the energy of the beam, the less positronium was emitted; the deeper penetrating positrons being less likely to diffuse back to the surface where they could form positronium. The study of the positronium emission fraction versus temperature suggested that both thermally and nonthermally activated emission occurs. For a 30 eV incident beam the fraction of nonthermally activated positronium ranged from 0.43 to 0.47. The thermal emission is analyzed using a surface state model based on a one step thermal activation process which yields, coupled with the positronium binding energy and the electron work function, both an activation energy and a positron binding energy; these are, respectively, 0.44 (+OR-) 0.04 and 2.81 (+OR-) 0.06 eV for Al(100), 0.64 (+OR-) 0.04 and 2.87 (+OR-) 0.06 eV for polycrystalline Ag, and 0.75 (+OR-) 0.05 and 2.60 (+OR-) 0.07 eV for Ni(100). The second experiment, the discovery and analysis of low energy. positron diffraction, was done using an electrostatic transport system. capable of delivering monochromatic positrons to the target with a. reasonably constant flux, 5.5 (+OR-) 1.0 mm diameter spot over a 40-400 eV. incident energy range. The incident beam energy and the data. collection was controlled by computer; particle detection was effected. using a goniometer mounted retarding field energy analyzer and channel. electron multiplier. The intensity of the elastically scattered positrons was. measured as a function of both angle and energy for a Cu(111) sample. The angular data shows peaks at the predicted locations for (00), (01),(' ). and (02) beams. Curves of diffracted intensity versus incident energy(' ). for both zero and first order scans exhibit maxima corresponding to the primary Bragg peaks. These intensity versus energy curves are also compared with theoretical calculations and shown to agree well within the present precision of the data.
- Pub Date:
- Physics: Condensed Matter