Investigation of positron work function and moderation efficiency of Ni, Ta, Pt and W(1 0 0)

Intense slow positron beams are essential for depth dependent measurements using positron annihilation technique. For this purpose efficient positron moderators are searched for. In this study, the moderation efficiency and positron work function of polycrystalline Ni, Ta, Pt, and of W(1 0 0) were investigated. The slow positron beam facility at Technical University of Munich is operated with a 6 mCi 22Na source and a W(1 0 0) foil (1 μm thick, in situ annealed at 2600 K) as moderator in transmission geometry. For the material dependent measurements, the tungsten foil was replaced by 1 μm thick foils of Ni, Ta and Pt. The energy distribution of transmitted positrons could be measured by variation of the acceleration voltage. The transmitted positrons were magnetically guided by a curved beam tube to a beam catcher. There, the yield of slow positrons was determined by detection of the 511 keV annihilation quanta. In order to characterise surface contamination and adsorbate concentration, X-ray induced photoelectron spectra (XPS) were recorded for each sample.