Moderation and diffusion of positrons in tungsten meshes and foils

摘要

The efficiency of tungsten meshes and thin foils for moderation of fast positrons from ~(22)Na has been investigated in transmission geometry and a fair agreement has been found with previous experimental results where directly comparable. For foils, the dependence on material thickness is found to be similar to the prediction of the Vehanen-Makinen diffusion model; however, the magnitude is 5.1-0 times lower. A broad consensus is observed between experiment and the results of a three-dimensional model developed in this work. For a given thickness, meshes are found to be generally better than foils by around a factor of 10 with a maximum efficiency (～10~(-3)) comparable to that achieved with thin single crystal foils, in accord with previous measurements and the results of the present model.