Reemission Intensity and Energy Spectrum Measurements of Slow Positron Beams for Various Moderators

Abstract

Various characteristics of moderators for slow positron beam experiments using a 22Na source in non-ultra-high vacuum (UHV) were studied. Relative reemission intensities of slow positrons for various moderators: tungsten (W) annealed at various temperatures, rubbed by the polishing material SiC powder, etched chemically, covered by evaporation films of Cu metals, iridium (Ir) films and synthesized diamond films were relatively measured. Measurements of the beam energy spectrum of reemitted positrons just after annealing and after a long time in use are presented. By measurement of the energy spectrum, the value of the positron work function for W was found to be 2.1±0.3 eV. Intensity and spectrum studies for the ribbon, plate and mesh type W moderators were also carried out. An Ir moderator shows 20% stronger intensity than a W-ribbon moderator, but is not convenient for practical use. A W-mesh moderator is the best choice for gas scattering experiments in non-UHV. The scanning transmission microscope (STM) and atomic force microscope apparatus (AFM) were used to observe the surface state of the W moderator for slow positrons. The relationship between the surface state and the reemission intensity was analyzed in the atomic and sub-micron scales.