Efficient generation of low-energy positrons

Abstract

Positrons with a nearly thermal‐energy distribution can be generated by moderating the beta‐decay spectrum of a radioactive β⁺ source. The efficiency ε of this process is improved substantially by using a clean single‐crystal Cu moderator which has been activated by a one‐third monolayer of S. When combined with a low self‐absorption ⁵⁸Co β⁺ source, this moderator has an efficiency (ε=10⁻³) which is within a factor of ∼6 of what one would expect based on what is presently known about positron diffusion, slow‐positron emission, and the stopping of β particles.