Cryogenic Positron Beams for Atomic Physics Experiments

Trapped positron plasmas are routinely used to generate positron beams that can be used for a wide variety of experiments. For example, positron attachment to molecules occurs via the excitation of vibrational Feshbach resonances, yielding large peaks in the annihilation rate. These rates are measured as a function of positron energy by passing a beam through a molecular gas. While current beam generation techniques are sufficient for the measurement of positron-molecule binding energies,\footnote{G. F. Gribakin, et al., {\it Rev. Mod. Phys.} {\bf 82}, 2557 (2010).} more detailed studies are limited by beam energy resolution. Described here is a new method of positron beam formation using a buffer gas cryogenically cooled to 50 K. Simulations of the beam formation process are discussed and used to predict an energy resolution of $\approx$ 9 meV FWHM; a factor of 5 improvement over current techniques. Various possible physical measurements using this technique are discussed, including the ability to resolve individual multimode features in the resonant spectra,\footnote{A. C. L. Jones et al., {\it Phys. Rev. Let.} {\bf 108}, 093201 (2010).} and more detailed studies of annihilation involving intramolecular vibrational energy redistribution (IVR).