# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 2 Single-Phase Experiments
### Sbnd
The wires in a single APA are installed in a stainless steel frame that forms a \(2\times 3\) grid of \"windows\", and so each of the drift volumes' outer faces therefore consists of two such APAs, as previously shown in Figure 13. The APAs on each face are manufactured and wired independently, before being electronically coupled so that they effectively function as a single structure. To ensure flatness of the wires across each APA's full height and width and to support them away from their soldered mounting points on the perimeter of the frames, specially designed combs are installed on the frames' cross-beams [36]. As with the use of a membrane cryostat, the design, manufacturing and testing of the SBND APAs has acted as prototyping for the DUNE experiments, which are expected to use wire planes of similar design and size. Like MicroBooNE, SBND utilises a combination of cold and warm electronics in its readout system [30]. Initial signal collection, amplification and digitisation is performed by ASICs located on cold front-end motherboards that are mounted on the APAs within the LAr environment, with one motherboard collating signals from 128 wires. The design of these motherboards is in part based on similar cold electronics devices developed specifically for the single-phase ProtoDUNE detector, but commercial off-the-shelf components are also used in SBND [30]. The signals from the front-end motherboards are carried via the cryostat's four signal feedthrough flanges to \"warm interface electronics crates\" (WIECs), one of which is mounted directly to the external side of each feedthrough. These crates collate the signals from up to 24 front-end motherboards (3072 wires) before passing them to the off-detector back-end acquisition and recording systems. Figure 14: Conceptual design of the SBND membrane cryostat (green), with the TPC structure suspended from the removable lid (orange). Taken from [35]. The SBND \"photon detection system\" (PDS) [34] also offers an opportunity for R&D and prototyping of technologies and designs that could prove important for future large-scale LArTPCs. The basis of the PDS is a system of 120 8-inch diameter Hamamatsu PMTs (60 per drift volume), 96 of which are coated with TPB-making them sensitive to both VUV and visible wavelengths. These operate in a similar way to the PMT assemblies previously described at ICARUS T600 and MicroBooNE: detecting the prompt scintillation light for the purpose of event triggering. The remainder of the PMTs are uncoated, and are therefore only sensitive to direct visible wavelength photons. On their own, these uncoated PMTs therefore do not directly observe any VUV scintillation light, but a second component of the PDS is a total of 38 m\({}^{2}\) of TPB-coated reflective foil mounted on both sides of the CPA [34]. This wavelength-shifts any VUV scintillation light that is emitted towards the CPA, and then reflects it at visible wavelengths in the direction of the PMTs, allowing it to be detected and therefore significantly increasing the per-event light yield compared to using only the TPB-coated PMTs. In addition to the PMTs, two relatively new devices called ARAPUCA and X-ARAPUCA are being tested as part of the SBND PDS [34]. The ARAPUCA (\"Argon R&D Advanced Program at UniCAmp\") device [37] is a form of light trap consisting of a box with a highly reflective interior, and an aperture that combines a dichroic filter and two wavelength shifters-one on the external surface that shifts incident light so as to allow it through the filter, and the other on the interior that shifts it into the opaque range of the filter, thus trapping it inside. The box is itself directly coupled to a photon-sensitive sensor (such as a SiPM), and detection of the light relies on the photons reflecting at the correct angle to be incident on the sensor before they are absorbed. The X-ARAPUCA [38] is a direct advancement of the ARAPUCA, and contains an internal acrylic slab with embedded wavelength shifter, to which the photo-sensitive sensor is coupled to.