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 22Na has\udbeen investigated in transmission geometry and a fair agreement has been found with previous\udexperimental results where directly comparable. For foils, the dependence on material thickness is\udfound to be similar to the prediction of the Vehanen-M€akinen diffusion model; however, the magnitude\udis 5–10 times lower. A broad consensus is observed between experiment and the results of a\udthree-dimensional model developed in this work. For a given thickness, meshes are found to be\udgenerally better than foils by around a factor of 10 with a maximum efficiency (�10�3) comparable\udto that achieved with thin single crystal foils, in accord with previous measurements and the results\udof the present model.