Development and Testing of the Positron Identification by Coincident Annihilation Photons (PICAP) System

Moderate energy positrons (~few to ~20 MeV) have seldom been observed in the Heliosphere. Their detection would have implications in the study of Galactic cosmic rays, Solar particle events, Solar wind transport and the modulation of energetic particles in the Heliosphere. Their lack of observation is due primarily to there not having been dedicated instruments for such measurements; positron measurements have, to date, been aimed at significantly higher energies (~few GeV). The Positron Identification by Coincident Annihilation Photons (PICAP) system is designed specifically to measure these moderate energy positrons by simultaneously detecting the two 511-keV gamma rays that result from the a positron stopping in the instrument and the subsequent electron-positron annihilation. It is also expected by this method to effectively discriminate positrons from other charged particle species, particularly protons and negatrons. The instrument is comprised of (Si) solid-state detectors, plastic scintillation detectors, as well as high-Z BGO scintillator suitable for detecting the 511-keV gamma rays. PICAP offers a low mass, low power option for measuring positrons, electrons and ions in space. Following extensive computer modeling, we have designed and built a PICAP laboratory prototype that is easily adaptable to a space-flight design. This prototype will be taken to particle accelerator facilities and exposed to energetic positrons, electrons, and protons with the goal of determining performance and validating modeling. We plan to present the model calculations and preliminary results. This work was 95% supported by NASA Grant NNX10AC10G.; One-quarter cutaway showing the interior of the PICAP prototype instrument.