# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 2 Single-Phase Experiments
### Pixelated Charge Readout and the ArgonCube Program
In both cases, the positions of the dots on the planes correspond to the pixels that were hit, and the colours of the dots represent the (estimated) charge incident on the pixels. The distance of the dots from the pixel planes is inferred from the relative pixel trigger times. These images represent the raw pixel outputs-no noise filtering or amplification has been performed. Both adapted from [56]. Figure 30: The 832-pixel anode plane developed by LArPix. A variety of pixel shapes and sizes were used, in order to evaluate the optimal pixel characteristics. The overall area covered by the pixels is 10 cm in diameter. A second plane, with 512 pixels covering a 4.8 \(\times\) 9.6 cm area was also produced and operated. Taken from [56]. and is shown in Figure 32, is in the process of being assembled, with the cryostat and first module having been completed and commissioned in 2019 [54]. Once complete, this detector will be operated on the NuMI beamline at Fermilab in order to fully characterise its functionality and operation in a similar environment to that which will be experienced by the DUNE near detector. Design and physics studies are also underway for the eventual extension of the modular design to the scale of the DUNE near detector [54], as depicted in Figure 33, and proposals have also been made for a kiloton-scale iteration, which is envisioned as one of the DUNE far detector modules [57]. Figure 33: **(Left)** A 3D model of one module of the proposed LArTPC sub-system of the DUNE near detector. The basic design is that of two 0.5 m wide horizontal single-phase drift volumes arranged back-to-back, with a shared cathode orientated vertically in the middle and pixelated charge readout anode planes on the corresponding outer faces. Each module will possess its own dedicated LAr recirculation, HV and photon detection systems. **(Right)** The envisioned design of the LArTPC sub-system of the DUNE near detector, consisting of 20 modules arranged in a 4 \(\times\) 5 grid encompassing an active volume measuring 3 \(\times\) 4 \(\times\) 5 m. Both taken from [58]. Figure 32: A 3D model of the ArgonCube 2 \(\times\) 2 demonstrator, showing the positions of the four TPC modules (one partially removed). The demonstrator has an overall active volume of 1.2 \(\times\) 1.3 \(\times\) 1.3 m [58]. Taken from [54]. An alternative approach to signal digitisation in pixelated charge readouts has been recently proposed by the Q-Pix consortium [59]. As with LArPix, each pixel in the Q-Pix design is connected to an ASIC containing a self-triggering circuit: the charge collected on the pixel is integrated on a charge-sensitive amplifier, the integral is compared to a pre-set threshold, and once this threshold is exceeded the amplifier is reset (in this design, via a Schmitt trigger). However, in Q-Pix a local clock is also present in the ASIC, and when the threshold is exceeded, the timestamp on this clock is read out and stored. As long as charge continues to build on a pixel, the circuit will continue to output timestamps, and the time difference between successive timestamps (referred to as the \"reset time difference\" (RTD)) therefore represents the time required for the pixel to collect a fixed and known amount of charge-the \"time-to-charge\" [59]. The pattern of RTDs across many pixels on a charge-capturing anode plane can then be related to the spatial and temporal shape of the incident charge, and therefore what type of event produced it. For example, a short series of small RTDs produced by many pixels indicates that a lot of charge was incident in a short space of time (i.e., the pixels' amplifiers were reset multiple times in quick succession), corresponding to a spatially wide but temporally localised ionisation of the LAr, i.e., a particle track. On the other hand, background events or noise-which generally produce low-rate, highly localised ionisation-will be observed as large RTDs in only a few pixels across the plane.