The 3x1x1 m3 active volume dual-phase prototype, the first large-scale detector of this kind — a small replica of the design planned for future DUNE far detector modules — is in the final stages of construction at CERN. It will be immersed in 17 m3 of liquid argon, suspended in the cryostat under an insulating lid. The Dual-Phase TPC Working Group is currently assembling, testing and installing the components, and plans to start data taking with cosmic rays this fall.
Here’s how it works in a nutshell. Drifting charges are extracted vertically upward from the liquid argon in the large lower portion of the detector into a gas phase near the top of the cryostat, where Large Electron Multipliers (LEMs) amplify the signals. LEMs are essentially printed circuit boards (PCBs) drilled with half a million holes, inside of which large electric fields multiply the electrons. Another PCB, the anode, collects the multiplied charge above the LEMs and provides symmetric x-y readout of the event with a fine granularity of 3 mm. The extraction, amplification and readout all take place in a single integrated frame called the Charge Readout Plane (CRP), designed for a fully active 3×1 m2 area.
As detectors get larger, so do the potential problems with electronic noise. The ability to amplify the drifting electrons is a game-changer; in this detector design, the signals are well above the noise, and the spatial resolution is significantly higher than in the single-phase design, as is the sensitivity to astrophysical neutrinos and other low-energy events. The drift distances are much longer, too, leaving the main body of the TPC as a single, homogenous drift area free of obstructions.
The group has installed and cabled the CRP, and dipped it in liquid nitrogen to confirm the frame’s mechanical robustness and the electrical continuity of the signals in the cold. Next they will install the drift cage and the light-readout system.
The 3x1x1 m3 detector is an important predecessor to ProtoDUNE-DP, and CERN will continue to play a very active and important role in development and construction of this technology for DUNE. Besides the scientific benefit the 3x3x1 will provide, it serves as an engineering prototype.
The detector installation sequence turns out to be rather simple and straightforward, and most of the detector components can be produced and tested by industry. These features are paramount when planning to construct large detectors.