Gamma diagnostics for the APEX collaboration

Annihilation gamma rays will be a primary means of diagnosing the low-temperature, e+e- pair plasmas that the APEX collaboration is working toward creating and studying -- both in a compact levitated dipole trap (APEX-LD) and in an optimized stellarator (EPOS). Even when positrons are below plasma densities (i.e., they have no collective interactions among themselves), they represent a sensitive probe for particle dynamics & transport -- whether in vacuum magnetic fields or through interactions with electrons, ions, or neutrals. An array of BGO detectors has been assembled and commissioned. A subset of the array was used to study the injection, confinement (including toroidal dynamics – due to spatial and velocity spreads), and loss mechanisms of posi­tron bunches in a supported dipole trap (based on a permanent magnet). Measurements with the full array have been benchmarked with radioactive sources. Modeling of full-orbit e+ trajectory simulations and gamma ray scattering yields very good agreement with the experiments.