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Annihilation of low energy antiprotons in silicon sensors

The aim of the AEg̅IS experiment is to measure the gravitational acceleration for anti-hydrogen in the Earth's gravitational field, thus testing the Weak Equivalence Principle, which states that all bodies fall with the same acceleration independent of their mass and composition. AEg̅IS will make use of a gravity module which includes a silicon detector, in order to measure the deflection of anti-hydrogen from a straight path due to the Earth's gravitational field, by detecting the annihilation position on its surface. A position resolution better than 10 μm is required to determine the gravitational acceleration with a precision better than 10%. The work presented here is part of a study of different silicon sensor technologies to realise a silicon anti-hydrogen detector for the AEg̅IS experiment at CERN. We here focus on the study of a 3D pixel sensor with FE-I4 readout, originally designed for the ATLAS detector at the LHC, and compare it to a previous monolithic planar detector studied, the MIMOTERA. The direct annihilation of low energy anti-protons (~ 100 keV) takes place in the first layers and we show that the charged annihilation products (pions and nuclear fragments) can be detected by such a sensor. The present study aims at understanding the signature of an annihilation event in a 3D silicon sensor, in order to assess the accuracy that can be achieved by such a sensor in the reconstruction of the position of annihilation, when the same happens directly on the detector surface. We also present a comparison between experimental data and GEANT4 simulations and previous data obtained with a silicon imaging detector. These results are being used to determine the geometrical and process parameters to be adopted by the silicon annihilation detector to be installed in AEg̅IS.