# Review of Liquid Argon Detector Technologies in the Neutrino Sector
## 3 Dual-Phase Experiments
### Optical Readout and the ARIADNE Program
Figure 51: Final installation of the fully assembled ARIADNE TPC into the cryostat, performed at the University of Liverpool. Taken from [93]. Figure 52: Examples of events imaged by the ARIADNE detector using EMCCDs, showing **(top left)** a 5 GeV/c EM shower from a beam interaction, **(bottom left)** a 3 GeV/c antiproton candidate from a beam interaction, and **(right)** a cosmic muon interaction. All taken from [94]. Though successful in proving the feasibility of the optical readout concept, operation of the ARIADNE detector using EMCCDs has demonstrated certain limitations with this technology. The most pressing are the necessity for reliable and unambiguous correlation with PMT signals in order to achieve full 3D position reconstruction (since the EMCCDs can only provide a 2D projection view of any given event), and the relatively low readout rate of the EMCCDs themselves [94]. Beginning in 2019 and currently ongoing, the focus of the ARIADNE program has moved towards TPX3 based optical readout. This technology is a derivative of Medipix [96]-a suite of photo-sensitive pixel sensors originally developed to demonstrate how technologies developed for the LHC particle detectors could find application outside High Energy Physics. The TPX3 chip [97; 98; 99] itself consists of a photo-sensitive silicon sensor bump-bonded to an ASIC. The sensor is divided into a 256 \(\times\) 256 array of 55 \(\upmu\)m wide pixels, with each pixel being connected to its own dedicated circuitry in the ASIC. Incident photons are converted to electron/hole pairs, and the electrons are accumulated by each pixel's capacitor in the ASIC. The time taken for this accumulation to exceed a preset pixel-specific threshold is recorded (at 1.6 ns resolution) from an internal clock in the ASIC. This time is known as the \"time of arrival\" (ToA), and once the threshold is exceeded, that pixel is said to have been \"hit\". The capacitor is then discharged, and the additional time required after the ToA for the accumulation to fall back below threshold is called the \"time over threshold\" (ToT). This is proportional to the amount of accumulated charge, and therefore the number of photons incident on the pixel. Each hit pixel therefore provides four pieces of information upon readout: its \(x\) and \(y\) coordinates on the sensor surface, the ToA and ToT. In the context of imaging a three-dimensional particle track, the track's shape along the \(z\) axis is related to the hit pixels' relative ToAs, and the summation of their ToTs is directly related to the total amount of LAr ionisation, and therefore the originating particle's energy. TPX3 technology therefore offers complete 3D position reconstruction and calorimetry from a single, self-contained device. Pixels that do not accumulate enough charge to exceed their thresholds are not read out--i.e., the data are naturally zero-suppressed, allowing for extremely fast readout rates of \(\approx\)80 MHits/s [100]. As with any semiconductor device, the TPX3 chip is susceptible to thermal noise-typically \(\approx\)500 e\({}^{-}\) equivalent across the entire sensor surface [99]. However, in certain circumstances (such as in the presence of low energy particles or a low THGEM/LEM amplification field) there may not be enough incident S2 photons, and therefore electron/hole pairs, to overcome this minimum value. The S2 light must therefore first be amplified, and this is achieved using an image intensifier. Such devices generally consist of a photocathode (which converts the incident light into electrons), followed by one or more stages of electron multiplication, and a final phosphor layer that converts the electrons back into light. The commercial Photonis Cricket image intensifier [101] used by the ARIADNE experiment has a photocathode quantum efficiency of \(\approx\)30% at 420 nm, a peak phosphor emission at 420 nm (matching well with the TPX3 chip's 90% quantum efficiency at this wavelength), and an overall photon gain of 1.1 \(\times\) 10\({}^{6}\) output photons per incident photon [100].