Thin films for slow positron generation

We report on calculated slow-positron yields from m thick W(110) and Cu(111) single crystals in a transmission geometry to be used in conjuction with a²²Na ⁺ source. The fast to slow positron moderation efficiencies were found to be comparable ( 10^–3 to the more conventional single-crystal moderators used in the backscattering geometry with a⁵⁸Co source. This allows the construction of more simplified slow-positron beams. We also calculate the brightness enhancement and efficiency of ultrathin single-crystal metal foils used for remoderation of the slow-positron beam. The results are consistent with recent experiments, while they are smaller than previous theoretical estimates.