# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 3 Dual-Phase Experiments
### Optical Readout and the ARIADNE Program  
Optical readout of dual-phase LArTPCs offers an alternative approach to the existing and previously described charge readout methodology. As noted previously in Section 3, Townsend multiplication occurring in the THGEM/LEM holes not only produces additional electrons, but also secondary scintillation light (hereafter denoted by \"S2\"), and studies have shown that-depending on the exact amplification field across the THGEM/LEM-many hundreds of S2 photons can be produced for every drift electron [85; 86; 87; 88; 89]. Optical readout therefore relates to the detection of these S2 photons. Although there are a wide variety of photo-sensitive detectors available with which to observe the S2 light, one of the key requirements for use in a neutrino detector must be the capability for high resolution tracking and position reconstruction, as previously noted in Section 3. With this in mind, in recent years devices such as commercial SiPMs [85; 89] and CCDs (\"charge-coupled devices\") [90] have been used to optically read out small-scale dual-phase LArTPCs, and linearly graded SiPMs (\"LG-SiPMs\") have also shown potential following initial tests conducted in a gaseous TPC [91]. (LG-SiPMs are a relatively new type of SiPM that natively produces 3D position information using a combination of 2D microcell sensor structure and nanosecond timing resolution [92].) Much of the development of optical readout for dual-phase LArTPCs in the neutrino sector-including the aforementioned CCD and LG-SiPM experiments, and more recently the use of EMCCDs (\"electron-multiplying CCDs\") and TPX3 (\"Timepix3\") technology-has been performed by the Argon Imaging Detection Chamber (ARIADNE) R&D program [93], hosted at the University of Liverpool. The centrepiece of the ARIADNE program is the dual-phase ARIADNE LArTPC itself [94], depicted in Figure 49. At the centre of the detector is a 54 \(\times\) 54 \(\times\) 80 cm field cage, enclosing an active LAr mass of 330 kg and defined by 79 stainless steel field-shaping rings, with 1 cm centre-to-centre spacing. The rings are electrically connected via attached resistor chain elements. A view of the field cage interior is shown in Figure 50 (left). The cathode, consisting of a tensioned stainless steel mesh held within a stainless steel frame, is positioned below the field-shaping rings and supplied with bias via a 2 m long HV feedthrough. The feedthrough design is identical to that used by other LArTPCs previously described, and contact between it and the cathode occurs via the feedthrough's spring-loaded tip touching the inside of a stainless steel torus welded to the cathode frame [94]. The cathode grid has an optical transparency of 70%, allowing the placement below it of four TPB-coated 8-inch diameter Hamamatsu R5912-MOD PMTs. As with other LArTPCs, the primary purpose of these PMTs is to detect prompt LAr scintillation light, the relative time of which denotes the event start and therefore allows the absolute \(z\) position to be determined. In ARIADNE, the PMTs also provide supplementary S2 light detection capability alongside the primary optical readout devices. The overall light collection efficiency of the PMTs is enhanced by the placement of TPB-coated reflector panels around the outside of the field cage [94]. The extraction grid is identical in design to the cathode, and located above the top field-shaping ring, 5 mm below the surface of the liquid. Bias to the extraction grid is provided via a dedicated HV feedthrough. The THGEM--shown in Figure 50 (right)-is positioned 1 cm above the extraction grid, within the gas phase, and is constructed from a 1 mm thick 54 \(\times\) 54 cm FR4 panel with copper top and bottom surfaces, penetrated by 500 \(\upmu\)m diameter holes with 800 \(\upmu\)m pitch and 50 \(\upmu\)m dielectric rims, covering an area of 53 \(\times\) 53 cm [94].