Combined Optical and Electronic Readout For Event Reconstruction in a GEM-based TPC

Optically read out Time Projection Chambers (TPCs) based on Gaseous Electron Multipliers (GEMs) combine 3D event reconstruction capabilities with high spatial resolution and charge amplification factors. The approach of reconstructing particle tracks from 2D projections obtained with imaging sensors and depth information from Photomultiplier Tubes (PMTs) is limited to specific cases such as straight particle trajectories. A combination of optical and electronic readout realized by a semi-transparent anode placed between a triple-GEM stack and a camera in an optically read out TPC has been realized and used to reconstruct more complex particle tracks. High spatial resolution 2D projections combined with a low number of charge readout channels enable accurate 3D event topology reconstruction. Straight alpha tracks as well as more complex cosmic events have been reconstructed with the presented readout concept. Relative depth information from electronically read out charge signals has been combined with drift time information between primary and secondary scintillation pulses to absolute alpha track reconstructions.