UC San Diego

The focus of this dissertation is the development of a positron beam with significantly improved energy resolution over any beam resolution previously available. While positron interactions with matter are important in a variety of contexts, the range of experimental data available regarding fundamental positron-matter interactions is severely limited as compared to analogous electron-matter processes. This difference is due largely to the difficulties encountered in creating positron beams with narrow energy spreads. Described here is a detailed investigation into the physical processes operative during positron cooling and beam formation in state-of-the-art, trap-based beam systems. These beams rely on buffer gas traps (BGTs), in which positrons are trapped and cooled to the ambient temperature (300~K) through interactions with a molecular gas, and subsequently ejected as a high resolution pulsed beam.

Experimental measurements, analytic models, and simulation results are used to understand the creation and characterization of these beams, with a focus on the mechanisms responsible for setting beam energy resolution. The information gained from these experimental and theoretical studies was then used to design, construct, and operate a next-generation high-energy-resolution beam system. In this new system, the pulsed beam from the BGT is magnetically guided into a new apparatus which re-traps the positrons, cools them to 50~K, and re-emits them as a pulsed beam with superior beam characteristics.

Using these techniques, positron beams with total energy spreads as low as 6.9~meV FWHM are produced. This represents a factor of $\sim 5$ improvement over the previous state-of-the-art, making it the largest increase in positron beam energy resolution since the development of advanced moderator techniques in the early 1980's. These beams also have temporal spreads of $0.9~\mu$s FWHM and radial spreads of 1~mm FWHM. This represents improvements by factors of $\sim 2$ and $10$, respectively, over those of the previous beam resolutions. Future experimental applications of this new technology are also discussed.