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The detection of photons with energies greater than a few tenths of an MeV, interacting via Compton scattering and/or pair production, faces a number of difficulties. The reconstruction of single-scatter Compton events can only determine the direction of the incoming photon to a cone, or an arc thereof and the angular resolution of pair-conversion telescopes is badly degraded at low energies. Both of these difficulties are partially overcome if the density of the interaction medium is low. Also no precise polarization measurement on a cosmic source has been obtained in that energy range to date. We present the potential of low-density high-precision homogeneous active targets, such as time-projection chambers (TPC) to provide an unambiguous photon direction measurement for Compton events, an angular resolution down to the kinematic limit for pair events, and the polarimetry of linearly polarized radiation., Comment: Invited chapter for the Handbook of X-ray and Gamma-ray Astrophysics

The Advanced Energetic Pair Telescope gamma-ray polarimeter uses a time projection chamber for measuring pair production events and is expected to generate a raw instrument data rate four orders of magnitude greater than is transmittable with typical satellite data communications. GammaNet, a convolutional neural network, proposes to solve this problem by performing event classification on-board for pair production and background events, reducing the data rate to a level that can be accommodated by typical satellite communication systems. In order to train GammaNet, a set of 1.1x10^6 pair production events and 10^6 background events were simulated for the Advanced Energetic Pair Telescope using the Geant4 Monte Carlo code. An additional set of 10^3 pair production and 10^5 background events were simulated to test GammaNet's capability for background discrimination. With optimization, GammaNet has achieved the proposed background rejection requirements for Galactic Cosmic Ray proton events. Given the best case assumption for downlink speeds, signal sensitivity for pair production ranged between 1.1 +/- 0.5% to 69 +/- 2% for 5 and 250 MeV incident gamma rays. This range became 0.1 +/- 0.1% to 17 +/- 2% for the worst case scenario of downlink speeds. The application of a feature visualization algorithm to GammaNet demonstrated decreased response to electronic noise and events exiting or entering the frame and increased response to parallel tracks that are close in proximity. GammaNet has been successfully implemented and shows promising results., Comment: Citation: Richard L. Garnett, Soo Hyun Byun, Andrei R. Hanu, Stanley D. Hunter, Event selection and background rejection in time projection chambers using convolutional neural networks and a specific application to the AdEPT gamma-ray polarimeter mission, NIM A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2020, 164860, ISSN 0168-9002, 10.1016/j.nima.2020.164860

The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Though they play a key role in cultivating the cosmological environment and/or enabling our studies of it, there is still much we do not know about AGNs and GRBs, particularly the avenue in which and through which they supply radiation and energetic particles, namely their jets. This White Paper is the second of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy will bring to understanding their energetic particle phenomena. The focus of this white paper is active galactic nuclei and gamma-ray bursts., Comment: 11 pages (including references), 2 figures; Submitted to the Astro2020 call for science white papers

The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Energetic particles in our own galaxy, galactic cosmic rays (GCRs), engage in a complex interplay with the interstellar medium and magnetic fields in the galaxy, giving rise to many of its key characteristics. This White Paper is the first of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy will bring to understanding their energetic particle phenomena. The focus of this white paper is galactic cosmic rays, supernova remnants, protostellar jets and superbubbles, and colliding wind binaries., Comment: 9 pages (including references), 2 figures; Submitted to the Astro2020 call for science white papers

We describe the science motivation and development of a pair production telescope for medium-energy gamma-ray polarimetry. Our instrument concept, the Advanced Energetic Pair Telescope (AdEPT), takes advantage of the Three-Dimensional Track Imager, a low-density gaseous time projection chamber, to achieve angular resolution within a factor of two of the pair production kinematics limit (~0.6 deg at 70 MeV), continuum sensitivity comparable with the Fermi-LAT front detector (<3x10-6 MeV cm-2 s-1 at 70 MeV), and minimum detectable polarization less than 10% for a 10 millicrab source in 106 seconds., Comment: Version to be published in Astroparticle Physics, 32 pages, 11 figures