Long-term operation of a double phase LAr LEM Time Projection Chamber with a simplified anode and extraction-grid design

Abstract

We report on the successful operation of a double phase Liquid Argon Large Electron Multiplier Time Projection Chamber (LAr LEM-TPC) equipped with two dimensional projective anodes with dimensions 10×10 cm 2 , and with a maximum drift length of 21 cm. The anodes were manufactured for the first time from a single multilayer printed circuit board (PCB). Various layouts of the readout views have been tested and optimised. In addition, the ionisation charge was efficiently extracted from the liquid to the gas phase with a single grid instead of two previously. We studied the response and the gain of the detector to cosmic muon tracks. To study long-term stability over several weeks, we continuously operated the chamber at fixed electric field settings. We reproducibly observe that after an initial decrease with a characteristic time of τ ≈ 1.6 days, the observed gain is stable. In 46 days of operation, a total of 14.6 million triggers have been collected at a stable effective gain of G ∞ ∼ 15 corresponding to a signal-to-noise ratio (S/N) 60 for minimum ionising tracks. During the full period, eight discharges across the LEM were observed. A maximum effective gain of 90 was also observed, corresponding to a signal-to-noise ratio (S/N) 400 for minimum ionising tracks, or S/N ≈ 10 for an energy deposition of 15 keV on a single readout channel.

Measurements of charging-up processes in THGEM-based particle detectors

“…where G F is the final gain, ∆G i is a gain drop value with corresponding "characteristic time" τ i . This function was found to better model the gain evolution than single exponential curve, or other empirical functions provided elsewhere [4,5]. The long-term gain evolution as it will be discussed later is a result of charge accumulation on a detector's "top rim" (figure 4), such process will typically have a single characteristic time, and it is expected to be fitted to an exponential curve.…”

WA105: a large-scale demonstrator of the Liquid Argon double phase TPC

“…The charge is then collected on anodes manufactured from a single multi-layer printed circuit board, providing symmetric charge sharing between the two orthogonal views and a space resolution of 3 mm on each view. Charge resolutions close to 8% have been obtained on both views, with an effective LEM gain close to 30 [4].…”

“…The charge is then collected on the readout anode with two orthogonal views, allowing for mm-level resolution on both views. The concept was proposed in [3] and several steps have been achieved since then with R&D mainly at CERN and ETH Zurich: from a 10×10×10 cm 3 (5 kg) prototype [4] to a 40×76×60 cm 3 (1 t) detector to a 1 m 3 mechanical mockup of the charge readout plane. The next stages envisage the construction of large-scale prototypes in the context of WA105.…”

The WA105-3x1x1 m3 dual phase LAr-TPC demonstrator

“…All designs have matured following many years of prototyping on smaller scale LAr detectors. For instance the pattern on the anode was optimised to ensure exact 50:50 charge sharing between both views and the best resolution on the particle energy loss per unit length [7]. LEMs of varying hole sizes, hole pitch, rim sizes or thicknesses have been operated to verify the geometry that provides the best and most stable gain in dual phase conditions [8].…”