# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 2 Single-Phase Experiments
### Vertical Drift
An alternate design, shown in Figure 36 (right) includes a second PCB, positioned 10 mm from the first one and consisting of a single layer with 8.7 mm wide strips orientated at 45\({}^{\circ}\) to those on the first PCB. With appropriate biasing, this additional PCB acts as a second induction layer, brings the design to equivalence with the three-plane wire-based APAs described previously [62]. A perforated PCB anode has several advantages over wire-based APAs [62]. Large area PCBs can be produced commercially and in bulk, helping to reduce the costs, manpower and time associated with manufacturing and assembly. This is particularly important to consider for a DUNE far detector implementation, which is envisioned to incorporate a total of 160 3 \(\times\) 3.4 m perforated PCB anode assemblies across the two \"charge readout planes\" (CRPs) [61; 62], as shown in Figure 35. The perforated PCB anode design itself is more robust than tensioned wires, and it may also be possible to integrate the front-end readout electronics within the top PCB layer-in a similar way to how the previously described pixelated charge readout electronics are bonded directly to the pixels that they service. The remainder of the readout components are expected to parallel the systems which have been successfully operated by the ProtoDUNE experiments [61]. A proof-of-principle test of a two-layer perforated PCB anode has recently been performed using cosmic muon data taken on a 501 LArTPC at CERN [62]. The anode used was 32 \(\times\) 32 cm in area, but otherwise identical to the proposed VD design, as were the readout electronics. The readout strip signals, examples of which are depicted in Figure 37, show clear and precise particle tracks, with high signal-to-noise ratio for both MIP and shower-like topologies. A similar test of the three-layer iteration, also on the 501 detector, is due to take place in early 2021 [63]. Figure 36: (**Left**) A two-layer perforated PCB anode, with the layers cut into strips orientated at 90\({}^{\circ}\) to each other and perforated by 2.5 mm diameter holes. One layer acts as an induction plane, and the other as a collection plane. (**Right**) A three-layer perforated PCB anode, in which the additional layer consists of strips orientated at 45\({}^{\circ}\) to the other two, located on a separate PCB and acting as another induction plane. In both images, the arrows denote the direction of strip readout on each layer. Both taken with permission from [62]. Taking full advantage of the various characterisation studies of different light collection devices already performed for the (Proto)DUNE experiments, the VD concept is envisioned to use a system of ARAPUCAs and associated silicon photomultipliers (SiPMs) as its PDS [62]. However, unlike previous single-phase detectors which locate these assemblies adjacent to the anode planes, the VD design calls for them to be embedded within the cathode, within the previously noted 58 mm gap between the meshes [62]. This location does require the development (currently ongoing) of new \"power over (optical) fibre\" (PoF) readout and operation electronics for the SiPMs, since there will be insufficient access or space for standard electronics, but the overall PDS design is anticipated to perform just as well as one in which the ARAPUCAs are mounted on the anodes, while using fewer individual devices [62]. An \"extended\" system, using additional ARAPUCAs positioned on the field cage looking inwards, would allow the VD PDS to have 4\(\pi\) coverage, greatly improving the detection of low energy events in the detector, and xenon-doping of the LAr (up to the level of \(\approx\)19 ppm) has also been proven to increase the uniformity and amount of collected light, by increasing the optical scattering length and significantly reducing the effects of N\({}_{2}\) contamination [62; 63].