EFFICIENT GENERATION OF LOW-ENERGY POSITRONS

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.