Gamma-Ray Imaging by Silicon Detectors in Space: the AREM Method

We present the Agile REconstruction Method (AREM) for gamma-ray (30 MeV - 50 GeV) direction reconstruction applicable to high-resolution Silicon Tracker detectors in space. It can be used in a "fast mode", independently of Kalman filters techniques, or in an "optimized mode", including Kalman filter algorithms for track identification. AREM correctly addresses three points of the analysis which become relevant for off-axis incidence angles: (1) intrinsic ambiguity in the identification of the 3-dimensional $e^+/e^-$ tracks; (2) proper identification of the 3-dimensional pair production plane and reconstructed direction; (3) careful choice of an energy weighting scheme for the 3-dimensional tracks. We apply our method to simulated gamma-rays in the AGILE detector. The excellent spatial resolution obtained by the AGILE Silicon Tracker, providing crucial analog information, makes it possible to improve the spatial resolution of previous detectors (e.g. EGRET) by a factor of ~ 2 in containment radius at gamma energy ~ 400 MeV. In this paper we present the results of our 3D-method for a selected sample of photon incidence angles and energies. A more comprehensive and complete discussion including the use of Kalman filter algorithms will be the subject of forthcoming papers.