# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 3 Dual-Phase Experiments  
Dual-phase LArTPCs are a relatively recent addition to the neutrino sector, having only been given detailed consideration and development since the early 2010s. However, the underlying dual-phase concept itself is by no means a new one, having been used for many years in dark matter searches [64]. The xenon-based ZEPLIN-II [65] and ZEPLIN-III [66] experiments pioneered the design and operation of dual-phase TPCs in the early 2000s, and more recent developments include LUX [67], LUX-ZEPLIN [68] (both also using xenon) and the argon-based DarkSide program [69]. One of the key differences between implementations of dual-phase technology in the neutrino and dark matter sectors is that neutrino experiments require high resolution position reconstruction across a relatively  
Figure 37: The readout strip signals of the 32 \(\times\) 32 cm two-layer perforated PCB anode operating in the CERN 501 LArTPC, for a cosmic muon (**left**) and an EM shower (**right**). Each “pixel” in these images corresponds to the signal size (ADC units, colour axis) on a given strip (channel number, \(x\) axis) at a given time (clock ticks, \(y\) axis). The bottom of each image (time = 0) corresponds to the anode position, with the cathode at the top. In both images, the induction plane strips are on the right (channel numbers \(\geq\)63) and the collection plane strips are on the left (channel numbers \(<\)63). Both taken with permission from [62].  
large spatial region, i.e., a particle track, whereas dark matter searches call for high precision calorimetry of more point-like energy depositions.  
The general detection principle of dual-phase TPCs with charge readout is shown in Figure 38. Electrons produced during ionisation are drifted upwards towards the surface of the liquid scintillator phase. Just below the surface is positioned an \"extraction grid\", and just above it in the gas phase is a \"Thick Gaseous Electron Multiplier\" or \"Large Electron Multiplier\" (abbreviated to THGEM or LEM respectively). (The names \"THGEM\" and \"LEM\" both refer to the same type of device, and when describing the specific hardware used by each experiment henceforth, we will use the name that is predominantly used by that experiment's publications.) THGEMs/LEMs [70] are a further development of Gaseous Electron Multipliers (GEMs) [71], and are a type of micropattern detector that typically consists of an insulating layer with conducting material on the top and bottom surfaces that can be individually biased, and many thousands of holes drilled through the entire structure. The extraction grid and bottom THGEM/LEM surface are biased so as to produce an electric field between them (referred to as the \"extraction field\") that is stronger than the drift field, resulting in the electrons being accelerated sufficiently to escape the liquid and enter the gas. The top THGEM/LEM surface is also biased in such a way that the electrons are subject to an \"amplification field\" across the THGEM/LEM, further accelerating them through the holes and inducing electron avalanches. This results in electron multiplication and the production of secondary scintillation light. (The entire process is known as \"Townsend multiplication\".) The proof of concept of a dual-phase argon TPC operating with a GEM and THGEM readout was demonstrated in [72,73] respectively.  
Figure 38: The general detection principle of dual-phase LAr detectors with charge readout. This schematic depicts the assembly in the dual-phase DUNE far detector, but the same principle is applicable to any such design. A description of the operation is given in the main text. Taken from [74].  
In dual-phase detectors which utilise charge readout, the multiplied electrons are finally collected by a wire-based anode plane positioned above the THGEM/LEM. As the extraction grid, THGEM/LEM and anode plane must all be present together in order for charge readout to occur, they are often grouped into a single assembly known as a \"charge readout plane\" (CRP).