Design concept and modeling of a new Positron Identification by Coincident Annihilation Photons (PICAP) system

The Positron Identification by Coincident Annihilation Photons (PICAP) system is a new design concept to measure moderate energy (∼few MeV) positrons and negatrons in space. Positron measurements in this energy range would open a window on cosmic ray propagation in the Galaxy, Solar modulation of cosmic rays (particularly charge-sign effects) and Solar energetic particles. Electrons of both charge signs are first identified by the conventional dE/dx versus residual energy technique. Coincident 511 keV annihilation photons are used to identify stopping positrons. This technique is far simpler than magnet spectrometers, which require elaborate tracking systems and associated electronics. We present a baseline design and Monte-Carlo modeling results using EGS4. We estimate the baseline instrument would mass ∼4 kg and use ∼3 W of power making such an instrument highly attractive for resource-constrained space missions.