A 48 BGO detector array to measure volumetric and localized annihilation from a magnetically confined electron-positron pair plasma

The APEX collaboration aims to magnetically confine electron-positron pair plasma in a levitated dipole and diagnose the plasma with FPGA processing of annihilation detections from an array of 48 Bismuth-Germanate (BGO) scintillators. Direct annihilation and decay positronium formed through radiative and three-body recombination produces a volumetric source of gammas. Two-gammas from direct or para-positronium annihilation can be detected in coincidence, allowing for tomographic reconstruction of the volumetric source. Positronium drift and subsequent ionization and pair collisions with neutrals and plasma drive cross-field transport which results in localized sources of two-gamma annihilation at the wall and magnet. The rates of various mechanisms depend on plasma temperature and density and the partial pressures of background gases. Triangulation, distance attenuated single-photon counting, and the ratio between localized and volumetric decays will provide diagnostics for the evolution of the plasma. We are developing techniques to differentiate between volumetric and localized sources and have conducted measurements with β+ emitters placed on rotating turntables to emulate pair plasma distributions.