First operation of a double phase LAr Large Electron Multiplier Time Projection Chamber with a 2D projective readout anode

We have previously reported on the construction and successful operation of the novel double phase Liquid Argon Large Electron Multiplier Time Projection Chamber (LAr LEM-TPC). This detector concept provides a 3D-tracking and calorimetric device capable of adjustable charge amplification, a promising readout technology for next generation neutrino detectors and direct Dark Matter searches. In this paper, we report on the first operation of a LAr LEM-TPC prototype equipped with a single 1 mm thick LEM amplifying stage and a 2D projective readout anode. The active area of the detector is 10×10 cm2 and the drift length is 21 cm. Cosmic muon events were collected, fully reconstructed and used to characterize the performance of the chamber. The obtained signals provide images of very high quality and the energy loss distributions of minimum ionizing tracks give a direct estimate of the amplification. We find that a stable gain of ∼ 30 can be achieved with this detector configuration corresponding to a signal-over-noise ratio larger than 200 for minimum ionizing tracks. The decoupling of the amplification stage and the use of the 2D readout anode offer several advantages which are described in the text.