Annihilation detection in the APEX electron-positron plasma from volumetric and localized sources

Annihilation gamma rays promise to be a powerful diagnostic for electron-positron pair plasma. The APEX collaboration aims to magnetically confine such pair plasma in a levitated dipole geometry. Gamma rays will be detected and time stamped with FPGA processing of signals from an array of 48 Bismuth-Germanate (BGO) scintillators with photomultiplier tubes. The experiment will generate gamma rays 1) in the bulk plasma from direct annihilation and decay of radiatively recombined positronium and 2) from locally increased annihilation on insertable target probes, injected solid particles, and introduced gas jets. The volumetric signal can be related to the bulk density and the localized signal can be used to diagnose injection efficiencies and loss channels. In order to learn how to differentiate between the volumetric and localized sources, we have conducted measurements with β+ emitters placed inside a circular arrangement of detectors. We compare three methods for identifying localized sources: triangulation from coincident lines of response, single photon detection along collimated views, and distance attenuated single photon detection simultaneously observed with multiple detectors.