# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 2 Single-Phase Experiments  
All single-phase LArTPC detectors are broadly based on the same design as the original concept, and therefore share the same detection and operating principles, as described below and depicted in Figure 1. When a charged particle traverses the detector's active LAr volume, it both excites and ionises the argon. Such particles can enter the detector directly-for example, cosmic muons or charged particles from a beamline-or be produced by interactions within the LAr. An example of the latter is the elastic scattering of a neutrino from an atomic electron: on its own, the neutrino does not produce any ionisation, but the scattered electron does, therefore allowing the presence of the neutrino to be inferred. Excited Ar atoms de-excite on an extremely fast timescale, leading to the emission of \"prompt\" scintillation light, and the ionisation of the Ar atoms results in the production of free electrons along the particle's path. These electrons are drifted in a uniform electric field (the \"drift field\") towards one or more anode planes.  
In detectors that utilise wire-based anode planes (which is the most common scheme used for charge readout), each plane is biased at a certain voltage that dictates its response to the electrons. One that is biased with a negative or zero voltage acts as an \"induction\" plane: bipolar voltage signals are induced on any wires that the electrons pass near to, with the amplitude of the signal being proportional to the distance between wire and electron, but the electrons will not be captured on the wires. However, if the plane is biased at a positive voltage, the electrons will be attracted to, and therefore collected on, the closest wire-thereby producing unipolar current signals on this so-called \"collection\" plane. When using multiple wire planes, the wires on each one are arranged in parallel, but orientated at different relative angles between planes, resulting in the \"crossing points\" between wires on different planes being unique and unambiguous-that is, no wire on a particular plane crosses over a wire on a different plane more than once [11]. This means that the 2D position at which an electron is incident on the planes, and therefore the 2D projection of the ionisation point, can be localised to a single crossing point that only depends on the specific wires that signals are produced on. The third dimension of the ionisation point can be calculated using the time difference between the start of the event (determined via the detection of the aforementioned prompt scintillation light) and the signal production on the wires. When performed over all wire signals produced during a single event, a series of 3D ionisation points can therefore be determined, corresponding to the original particle track [11]. Additionally, the total charge accumulated on the collection plane is directly related to the number of electrons produced by the original particle's ionisation, and this in turn is related to its energy.  
Although wire-based anode planes are the most established type of charge readout in single-phase LArTPCs, alternate methods have been developed with great success, as described below.