Development and use of a thin-film transmission positron moderator

Abstract

A narrow energy beam of slow positrons can be generated by moderating the beta‐decay spectrum of a radioactive β⁺ source. We report here the development and use of the first moderator in which the low‐energy positrons are not extracted from the surface facing the β⁺ emitter but from the opposite surface, i.e., the transmission moderator. An advantage is that such a configuration eliminates the problem of moderated positrons passing by the radioactive source. This is an essential problem when the source is physically large such as the present commerically available Na²² sources. This moderator is a 5000‐Å self‐supporting W (100) film. The growth and treatment of the film were found to provide a high‐quality positron moderator. The slow to fast positron conversion efficiency, using a Na²² source, was found to be 4×10⁻⁴.