Positron-diffraction experiments with a brightness-enhanced slow positron beam
The construction of a high brightness slow positron beam has enabled us to extend Low Energy Positron Diffraction (LEPD) measurements well beyond the previously accessible range of energies. Details of the design and construction of this beam, which utilizes two stages of reflection remoderation for a 500-fold increase in brightness-per-volt, are presented, with emphasis on the positron optics. The positron beam is maintained in an ultra-high vacuum environment. This, together with the inclusion of facilities for display LEED and Auger Electron Spectroscopy, make the apparatus well suited for surface studies. Two sets of experiments performed with this new apparatus are described. Measurements of the specularly diffracted intensity from Cu(001) and Cu(111) single crystals show large peaks in the previously inaccessible 10-30 volt range. These peaks are well displaced from the kinematic Bragg positions and may be influenced by the emergence of non-specular diffraction beams, but the sharp series of features characteristic of beam threshold effects has not been observed. In the second set of experiments neutral positronium atom production is measured as a function of incident positron energy. The observed positronium production is correlated to the intensity vs. energy spectrum for diffracted positrons, suggesting that positronium formation is an important channel for elastically scattered positrons. The angular distribution of positronium formed changes with incident beam energy and may contain information on the details of the potential in the surface region.
- Research Organization:
- Braddock, Dunn and McDonald, Inc., El Paso, TX (USA)
- OSTI ID:
- 5150232
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
Molecular & Chemical Physics-- Beams & their Reactions
656002 -- Condensed Matter Physics-- General Techniques in Condensed Matter-- (1987-)
656003 -- Condensed Matter Physics-- Interactions between Beams & Condensed Matter-- (1987-)
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
AUGER ELECTRON SPECTROSCOPY
BEAMS
COHERENT SCATTERING
COLLISIONS
CONSTRUCTION
COPPER
CRYSTALS
DESIGN
DIFFRACTION
ELASTIC SCATTERING
ELECTRON SPECTROSCOPY
ELEMENTS
ENERGY SPECTRA
LEPTON BEAMS
METALS
MONOCRYSTALS
PARTICLE BEAMS
POSITRON BEAMS
POSITRON COLLISIONS
POSITRONIUM
SCATTERING
SPECTRA
SPECTROSCOPY
SURFACES
TRANSITION ELEMENTS
USES
VACUUM SYSTEMS