Positron re-emission from tungsten surfaces

Positron re-emission properties of 50 mm thick polycrystalline tungsten foil and single crystalline tungsten specimen with 110.surface were measured as a function of successive heat treatment with and without oxygen by a pulsed monoenergetic positron beam. The tungsten specimens after in situ annealing with oxygen have significantly higher positron re-emission fraction and lower positronium fraction than those after in-situ annealing without oxygen. Furthermore, re-emission fraction at high temperatures of 800–10008C with an oxygen partial pressure of 2=10y7 Torr is significantly higher than that without oxygen, suggesting that surface oxygen inhibits positronium formation. This feature is good for electron-linac based slow positron beamlines. q1999 Elsevier Science B.V. All rights reserved.