Objective: To develop novel approaches to charge readout
Lead: Elena Gramellini (University of Manchester)
Task 1.1: Pixels and multiple modality readouts
Pixelated noble liquid TPCs, which leverage correlated charge and scintillation signals, offer enhanced event reconstruction and lower detection thresholds for accelerator neutrinos, dark matter, and low-energy neutrinos. Compared to wire-based designs, pixel-TPCs provide intrinsic 3D resolution, greater robustness, and reduced noise. Studies show improved identification of νe and νμ in both high- and low-energy regimes. Realizing large-scale pixel TPCs requires R&D focused on low-power, high-resolution charge readouts and scalable architectures capable of hosting both charge and VUV light detectors.
Charge and Light Readout Concepts for kTon-Scale Pixelated TPCs
To address scalability, the pixel readouts concept employs low-power, self-triggering charge integrate/reset (CIR) blocks with local, asynchronous clocks and fault-tolerant data networks. Timestamped charge information is transmitted to recover universal event timing, with power consumption as low as 20 µW per ASIC and data rates under 100 MB/s at kiloton scale. To address charge and light readout, the SoLAr concept proposes monolithic charge and light readout via silicon-based anode tiles integrating SiPMs and charge pads. While Multiple Modality Pixels (MMP) aim to combine charge and VUV light detection at the pixel level using high-QE (≥80%) photosensitive coatings, including amorphous selenium and other novel materials. Prototype development focuses on cryogenic operation, 128/178 nm sensitivity, and integrated performance benchmarking, with potential ∼m³ demonstrator planned for SNS and CERN neutrino platform deployment.
Task 1.2: Charge to light conversion, electroluminescence, and charge amplification
Charge amplification and conversion to light in dual-phase TPCs have been key to reaching unprecedented sensitivities in the search for dark matter candidates in argon and xenon targets. In a dual-phase TPC, ionisation electrons produced when a particle deposits energy in the detector are extracted to the gas phase, allowing signal amplification similarly to gaseous detectors. Typically, a wire mesh or grid is positioned above the liquid-gas interface so that electroluminescence (EL) light (termed the S2 signal) is developed in a uniform electric field between the liquid surface and the electrode. This amplification of the ionisation electron signal allows measurement of interaction energies below 1 keV, as compared to current single-phase liquid TPCs with a 30×higher energy threshold. There is scope for improvement with micro-pattern gaseous detectors (MPGD) positioned in the gas phase to achieve further charge multiplication and secondary scintillation. Gaseous electron multipliers (GEMs) and thick GEMs (THGEMs) are some of the most recent developments within the field of MPGDs; these have proved important for their simplicity and effectiveness, and can be made radio-pure. R&D plans in the next 3–5 years include optimization studies of manufacturing very large area GEMs for cryogenic operation, increasing light production from avalanche electron multiplication in hole multipliers operated in cryogenic Xe and Ar vapour, the development of cryogenic resistive materials to improve detector stability, exploration of electrochemically etched meshes and grids, and development of new amplification structures to maximize the active volume. The last point is of particular importance for dual-phase xenon TPC with a mass of several ∼10 t given the price of Xe.
Single phase TPCs offer an attractive alternative to mitigate some of the constraints of dual-phase TPCs, in which amplification in the gas phase above the liquid restricts possible detector layouts, requires precise control of the liquid level between electrodes, and ever-higher cathode voltages are needed for subsequent generations of larger experiments. Single-phase TPCs can tackle this challenge by splitting the detector volume in several sub-TPCs with shorter drift volumes, but currently there are no reliable and scalable approaches to amplify electrons by producing S2 light or by charge amplification in the liquid. Amplification is needed to lower the energy threshold for particle detection to a level competitive with dual-phase TPCs, and thus research on amplification stages for single-phase detectors has attracted great interest. There are also hybrid approaches which aim at creating gas pockets at specific locations in the liquid (“bubbles”), which would allow to build dual-phase detectors, where the gas phases can be placed anywhere in the active volume.
Granular S2 light readout of ionisation charge
An active area of R&D in LAr TPCs is to capture the amplified secondary scintillation light by high spatial resolution and speed cameras, for example the TPX3 cameras employed by the ARIADNE 1-ton and ARIADNE+ 15-ton liquid argon TPCs operated at the CERN neutrino platform. The raw data from TPX3 cameras are natively 3D and zero suppressed, thus providing a straightforward event reconstruction as illustrated in Figure 1. With appropriate lenses on the front of the camera a large area can be imaged (i.e. 1 m ×1 m at 4 mm/pixel resolution) creating a very cost effective system for large scale detectors. Near-future R&D proposes optimisation and characterisation of next-generation TPX4 cameras within liquid argon TPCs, to explore alternative approaches by imaging S2 light with an array of SiPMs, and a combined development of SiPM arrays and cameras to achieve mm2 tracking with sub-percent energy resolution. The possibility of detecting S1 light with cameras will also be explored. (The S1 signal is the light produced during the primary interaction of a particle in the liquid.) This R&D line aims to study the scaling of this technology by integrating large amplification structures (i.e. glass THGEMs) and granular S2 light camera systems in the ProtoDUNE cryostat at CERN. This would provide confirmation of the applicability of the new technologies for the kton-scale planned neutrino experiments.
Optimisation and characterisation of charge amplified structures
R&D plans include the development of novel EL structures for dual phase detectors – e.g. floating THGEMs, thin pillars on aligned meshes or, novel micro-structures; the development of amplification stages producing EL in LXe, and such stages producing EL and/or charge multiplication in LAr; and, mixed phase detectors relying on bubble creation. The foreseen application for these liquid detectors are DM experiments and the measurement of CEνNS (Coherent elastic neutrino-nucleus scattering), hence the resulting prototype detectors relying on EL will need to demon- strate single electron sensitivity, the capability to discriminate between energy deposits due to nuclear and electron recoil, and then the scalability of the adopted technologies.