# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 3 Dual-Phase Experiments
### Optical Readout and the ARIADNE Program
As many photo-sensitive detectors have high quantum efficiency in the visible wavelength range (and generally much lower efficiency at VUV wavelengths),a TPB-coated glass pane is mounted above the THGEM in order to shift the S2 light to wavelengths that are more conducive to detection. Figure 49: A schematic depiction of the Argon Imaging Detection Chamber (ARIADNE) detector, with major components labelled. The TPB-coated reflector panels have been omitted for clarity. Taken from [93]. The internal detector components described thus far are contained within the ARIADNE cryostat-a double-walled construction utilising a combination of Mylar super-insulation and a vacuum jacket to maintain the interior cryogenic environment [94]. A vacuum-containing beam window penetrates both walls of the cryostat at the halfway height of the field cage, to allow incident beam particles to reach the active LAr with minimal material interaction. The top flange of the cryostat is a separate entity with its own vacuum jacket, and houses penetrations for the various feedthroughs and ports required for detector operation. Of particular note are the four DN200 CF chimneys and associated optical viewports, arranged in a 2 \(\times\) 2 array around the flange's central axis, and onto which are mounted the primary optical readout devices (as shown in Figure 49). When coupled with the appropriate optics, this position gives the devices an unobstructed view of the entire active THGEM area [94], and the external location allows for easy access and installation of the readout devices. A view of the final installation of the fully assembled TPC into the cryostat is shown in Figure 51.  
Uniquely among the LArTPC detectors described in this paper, the main purification system of ARIADNE is located entirely within the cryostat, rather than externally [94]. The system consists of a filtration cartridge containing molecular sieves and copper meshes that serve to remove O\({}_{2}\) and H\({}_{2}\)O impurities, connected to a custom-designed bellows pump that circulates the LAr between the cartridge and the surrounding internal cryostat volume. The ARIADNE detector was first operated in 2018, using four Andor iXon 888 EMCCD cameras as the primary optical readout devices [94]. This followed from successful small-scale demonstration of the technology [95], which constituted the world first optical imaging of cosmic muons interacting in a dual-phase LArTPC. The EMCCDs have \(\approx\)80% quantum efficiency at 430 nm, and are single-photon sensitive. They also possess built-in readout electronics, thereby significantly reducing the complexity of the detector readout systems, and can be operated with a variety of sensor size and pixel binning options-the choice of which affect both the per-pixel resolution and the maximum readout rate. During their operation on ARIADNE, a position resolution of 1.1 mm per pixel and a readout rate of 60 Hz were used [94]. The ARIADNE detector was initially placed on the CERN T9 beamline, where it was exposed to a variety of particle species covering a wide range of  
Figure 50: (**Left**) A view of the interior of the ARIADNE field cage, looking upwards from the position of the cathode. The field cage is defined by 79 stainless steel field-shaping rings, and the drift field is provided via the resistor chain, components of which are housed on the nine PCBs attached to the rings. Taken from [93]. (**Right**) The THGEM used by ARIADNE, consisting of a a 1 mm thick FR4 panel with copper top and bottom surfaces, penetrated by 500 μm diameter holes with 800 μm pitch and 50 μm dielectric rims. The total hole coverage is 53 \(\times\) 53 cm. Taken from [94]. momenta, and this was followed by cosmic particle data-taking at the University of Liverpool. Figure 52 shows examples of beam and cosmic interactions observed by ARIADNE using the EMCCDs, with the former representing the world first optical imaging of such events in LAr.