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In particle physics, LiquidO is a class of particle detection technologies utilizing opaque media for light detection. It uses stochastic light confinement to detect subatomic fundamental particles, charged and neutral, when they deposit energy in the medium. It has a resolution of up to a few millimeters, and can enable the identification of particles with energies lower than 1 MeV.

In some cases, when a moving particle deposits energy in a medium, light is emitted, due to mechanisms such as Cherenkov or scintillation. The defining characteristic of LiquidO detection is that the medium used to produce this emitted light is opaque, which affects the propagation of light within the detector. Traditional liquid detectors, such as the ones used in the discovery of the neutrino, have relied on the transparency of the medium to allow light to propagate freely to a detector. LiquidO, by contrast, requires a certain minimum level of opacity.

LiquidO was conceived in the context of neutrino fundamental research]] for neutrino detection, but it may be applied to other fields where the detection of fundamental particle radiation is necessary, such as Particle, Astro-Particle and Nuclear Physics as well as high energy Astrophysics and medical physics.

As of 2022, most LiquidO detectors rely on scintillation as their light-production mechanism, a process known as opaque scintillation. In many prior scintillation experiments, opacity would have been considered an undesirable feature of the detector medium. For example, the CHOOZ experiment ended data collection due the increased opacity of its scintillator caused by chemical instability.

Naming Convention & Origin
The name LiquidO is composed of two elements "Liquid" + "O", where the last "o" should be emphasised upon pronunciation. The former term makes reference to the historical context of liquid media used for light emission, such as Cherenkov and scintillation radiation considered during LiquidO's conception. The latter makes reference to the core opacity ingredient behind the technology.

While the technology naming suggests a liquid medium only, LiquidO may in principle work in any type of medium state (solid, liquid or gas) providing that the opacity constraint is practically met. In practice, LiquidO R&D has already been successfully demonstrated in the context both solid and liquid states.

The name LiquidO is reserved for fundamental physics related projects, including its own R&D developments. While projects leading to innovation with an imminent industrial transfer goal(s) are tagged with the suffix -OTech; i.e. a short version for opaque technology.

Original Developments
The LiquidO detection technology and technique was conceived in 2012-2013, by A.Cabrera (CNRS/IN2P3, APC/IJCLab/LNCA, Paris/Orsay/Chooz, France), in the context of neutrino detection during the scientific developments of the Double Chooz reactor neutrino fundamental physics experiment. The original goal at the time was an attempt to achieve antimatter positron (e+; signal) identification to reduce the large cosmogenic backgrounds of the experiment. The studies demonstrated that indeed the positron identification was impractical (impossible so far) in a transparent detector like Double Chooz, thus seeing the birth of LiquidO as accidental outcome. The first conceptual and experimental developments were led by A.Cabrera along with (alphabetically ordered) H.de Kerret (CNRS/IN2P3), M.Obolensky (CNRS/IN2P3), F.Yermia (CNRS/IN2P3, Subatech, Nantes, France) in France. The concept was soon confidentially discussed with (time ordered) F.Suekane (Tohoku University, Sendai, Japan) and C.Buck (Max-Plank Institute, Heidelberg, Germany). Those scientists shaped much of the start of LiquidO in the period 2012-2015.

Until 2016, all LiquidO developments were kept confidential, while two ERC-CoG grant proposals were attempted for the funding the initial R&D. The first ERC was submitted in 2014 (classed "A" upon evaluation) but not funded in the final evaluation stage, thus truncating any stand-alone R&D programme strategy. Hence, in 2016, the LiquidO scientific collaboration consortium was founded by A.Cabrera for the R&D development of the technology as well as its possible physics programme. The first R&D funding was granted via the Chaire Blaise Pascal fellowship of F.Suekane (Tohoku University, Sendai, Japan), as visiting professor in the APC laboratory (Paris, France) between 2016-2018 in the context of CNRS/IN2P3. At the time, an EU Marie Curie fellow (M.Grassi, under the supervision of A.Cabrera) enabled much of the simulation studies providing LiquidO's proof of principle. In 2017, the first LiquidO CNRS/IN2P3 national collaboration was formed (6 national laboratories: APC, CENBG, CPPM, LAL, LNCA, Subatech) enabling extra R&D funding support and the start of coherent demonstrating prototyping strategy (detailed below), supported by the LiquidO international consortium. In the 2016-2018 period, the main focus remained on prospective application to fundamental neutrino and ββ detection and research with some preliminary work addressing both collider calorimetry and detection of radioactive gases.

In 2018, the first ever LiquidO presentation in a scientific international conference was done at the Neutrino Oscillation Workshop 2018 (NOW-2018, Ostuni, Italy). This release benefited from the just obtained first experimental proof-of-principle, using the Micro-LiquidO prototype detector, led by LiquidO consortium with main contributions by the CNRS/IN2P3 teams. The release of 2018 was preceded by some preliminary results shown at the Neutrino-2018 conference (Heidelberg, Germany). However, the first official released of the technology awaited until June 2019 in dedicated CERN Detector seminar, which was orchestrated to the completion of the first publication writeup and release.

The first LiquidO publication (A.Cabrera et al) was submitted as arXiv:1908.02859 (7 August 2019) followed by its complementary (C.Buck et al) arXiv:1908.03334 (9 August 2019) publication with the first opaque scintillator developed for the LiquidO prototyping - details below. LiquidO publication, edited by A.Cabrera, J.Hartnell (Sussex University, UK) and P.Ochoa-Ricoux (University of California, UCI, US), was submitted to Nature's publisher proposing their Communication Physics for submition. The final publication was released on the 21 December of 2021, after some COVID related delay.

Detection Principle
LiquidO fundamental particle detection technology relies on the exploitation of opaque media for light detection. Here opaque medium refers to material(s) causing very strong scattering of light (order 1 millimeter mean-free-path) with minimal absorption. The main mechanisms used by LiquidO is Mie and Raleigh elastic scattering, however internal reflections may also be used. A common example of high scattering medium is milk (emulsion of small droplets of fat in water), as opposed to water (transparent). While solid and gas media are possible, most LiquidO R&D so far relies on liquid media with oil and water as main basis. The main goal is to ensure a lossless (i.e. elastic) light scatter, as opposed to reflections commonly used in technology for detector segmentation. Even excellent reflective materials (≥98%) may lead to large losses of light due to the large amount of reflections to contain light within a reduce volume. LiquidO exploits materials that can produce light (i.e. optical photons) upon the energy deposition of fundamental particles, such as scintillation and Cherenkov light emission. However, LiquidO may exploit other light emission mechanisms, as it detection functioning is not attached the light mechanism exploited.

The light produced is bended by the strong scattering, thus forming "light balls" around every energy deposition, leading to phenomenon pioneered by LiquidO referred as the stochastic confinement of light since each optical photon describes random walk trajectories relative to its production point. Scattering has historically been minimised, or even avoided, in traditional detectors since this leads to an increase of the attenuation length, once folded with some unavoidable light absorption effects. LiquidO detection efficiency is the outcome of the competition between detection and absorption, so light must be detected at original using wavelength-shifting fibres before light is absorbed. An optimised LiquidO detector is characterised by its excellent collection efficiency, typically ≥90%), which is the probability for any generated optical photons to hit a fibre before absorption. The overall detection mechanism is typically limited by the trapping efficiency of fibres and the quantum efficiency of the photodetectors used.

Thanks to stochastic light confinement LiquidO enables the imaging of fundamental particle energy depositions without the need of any mechanical segmentation, typically leading to detector volume sub-divisions into pixels or any sub-structure alike. The size of the light ball is proportional to the mean-free-path of the scattering length and the amount of light per ball, hence the energy deposition. The pitch of the fibre lattice traversing the medium must be proportional to the light ball dimension to ensure optimal sampling of the event topology is possible.

Opaque Scintillation R&D
The first opaque scintillator conception and formulation started active development from 2015-2016, after LiquidO conception and motivation. Within the dominant paradigm of large neutrino detector with impeccable transparency, the advent of an opaque scintillator was considered as an effective R&D failure with likely useless application. Indeed, a few experiments has suffered from chemical unstable scintillators, specially if metal loading was done, leading to major physics limitations or even the impractical continuation of data taking, such as CHOOZ. There are two types main R&D avenues for the developments:


 * 1) Opacified known transparent liquid scintillators
 * 2) Natively opaque scintillators

The first opaque scintillator used in LiquidO relied on the first approach, whereby a traditionally transparent scintillator based on LAB using a single wave-shifter (such as PPO) is doped by paraffin (i.e. wax) so that opacity is possible upon the amorphous crystallisation of the latter. This scintillator is called NoWaSH (New opaque Wax Scintillator, Heidelberg) and has been extensively and successfully used throughout all the prototyping LiquidO detector chain so far. The paraffin temperature dependent crystallisation around room temperature enables the convenient control of the scattering mean-free-path. Hence, the same detector can be tested with both opaque and less opaque configurations regulated by a temperature bath between 0-40 °C. Once crystallised, the scintillator solidifies, hence LiquidO is tested in both liquid and solid states at once. Despite simplicity and convenience, the NoWaSH formulation does not lead to any improvement of the light yield (order 10,000 photons per MeV). In fact, the light yield is linearly diminished by the non-scintillating paraffin concentration doping. Indeed, this is a typical unavoidable behaviour of the scintillator loading leading to both transparency and light yield decrease. While LiquidO is rather immune to the loss of transparency, the light yield quenching is a highly undesirable feature.

Hence, the second branch of the opaque scintillators was conceived where the main goal is not only to prevent light yield quenching but possibly to boost it. This approach is expected to open for a completely new dimension of potential scintillators, led by LiquidO via the MicroCrystal dubbed concept, but there may be other ways. The main idea is to add micrometer scale inorganic scintillating crystals, so that both the light scattering and higher light yield light production (up to factors higher) is possible by the crystals. In this highly heterogenous configuration a liquid may be used to immerse the crystals to ensure a better optical coupling, via index of refraction matching, to ensure the maximum light yield is possible. The liquid in question may also be an active organic scintillator, thus allowing a larger increase light yield, if necessary. A NoWaSH scintillator may grant the extra mechanical support of the crystals, upon paraffin crystallization. The MicroCrystal formulation, thanks to its possible compensation (or even boost) of the common loss of light yield may additionally enable detector metal doping thanks to the large variety of inorganic scintillators. Thus, a MicroCrystal technology is expected to endow any LiquidO detector with extra key capabilities (one or several) such as:


 * higher density or radiation detection capabilities (Pb, W, etc)
 * enhanced neutron detection capabilities (B, Li, Cd, Gd, etc),
 * non-native high neutrino cross-sections (In, Cl, Pb, Fe, Ar, etc),
 * and even exotic materials such as ββ-decaying emitters (Te, Mo, Se, Nd, Cd, etc).

Indeed, the MicroCrystal approach is likely to open for a vast range of fundamental physics and possible applications for LiquidO-based detectors. The first steps of this strategy has been successfully realised within the LiquidO consortium, while results has not yet been published, the concept is nonetheless public.

Other ideas (not published) for opaque scintillation formulation beyond both NoWaSH and MicroCrystal approaches are being developed within the LiquidO consortium. This remains an active front of open R&D development benefiting by the advent of the LiquidO detection paradigm.

Related Technology R&D
Beyond the opaque scintillators, the construction and the detection optimisation of LiquidO implies a major effort in engineering beyond today's state of the art technology in terms of mechanical structure(s), radiopurity, instrumentation, electronics, DAQ, temperature control as well as novel active veto detection strategies. The LiquidO consortium has been involved, for a few years, in developing novel engineering solutions addressing:


 * 1) conceptual design of novel optical fibre technology,
 * 2) optical coupling between the fibres and the readout, including possible specialised light collector system,
 * 3) few meter detector full engineered scaling solutions,
 * 4) sub-100ps fast front-end readout,
 * 5) pulse optimisation for sub-ns waveform digitisation and reconstruction,
 * 6) readout and trigger schemes, including high trigger rate and large data-volume capabilities,
 * 7) precise large volume temperature control and monitoring,
 * 8) novel active veto-detector technology,
 * 9) radiopurity compliant design of all detector elements and
 * 10) customised optimal data analysis techniques, including full event topology reconstruction.

Some of those developments are still ongoing and solutions will be adopted and implemented in the different prototype and project detectors. The outcome of those development will follow dedicated technical publication(s) in due time.

LiquidO Consortium
As of today, the LiquidO consortium consist of more than 70 scientists in 22 academic institutions (universities and/or laboratories) over 10 countries in Europe, the America's and Asia. The consortium is led by A.Cabrera (CNRS/IN2P3) and F.Suekane (Tohoku University, Sendai, Japan), as spokespersons.

While not directly member, Électricité de France (EDF) has direct scientific participation and contributions to one of the LiquidO related projects (see below). EDF provides the most powerful European experimental site(s) for neutrino research and innovation at the Chooz nuclear reactor, where the past CHOOZ (data taking: 1995-1999) and Double Chooz (data taking: 2008-2018) experiments took place.

Prototype Strategy
LiquidO prototyping, led by its consortium, follows an incremental staged strategy where detectors size gradually incresed towards a realist LiquidO detector, including the engineering solution:


 * 1) Micro-LiquidO (~0.1 L with 3 readout fibres including a 3" PMT; scale ≤5 cm). Status: completed (2018-2019) and published. Located at the LP2I laboratory (Bordeaux).
 * 2) Mini-LiquidO (~10 L with 64 readout fibres including a 3" PMT; scale ≤15 cm). Status: data taking (2019-2022). Located at the LP2I laboratory (Bordeaux).
 * 3) Mini-γ-LiquidO (~100 L with  320 readout fibres; scale ≤0.5 cm). Status: under construction (from 2021). Located at the IJCLab laboratory (Orsay).

While the Micro-LiquidO and Mini-LiquidO focused on the LiquidO detection R&D demonstration, the Mini-γ-LiquidO prototype detector aims to focus on the engineering R&D with some novel physics results, thus concluding the LiquidO basic demonstration phase. All other forthcoming detectors planned - not listed - are related to projects for physics or applications goals. The maximal scaling of LiquidO is expected to follow similar scale as the NOvA detector (i.e. order 10 kton), given the similar technological composition (fibres, scintillators, etc), while the exact performance would need careful re-evaluation and quantification.

Derived Physics Projects
LiquidO technology is current exploited in several R&D projects whose main goal is fundamental scientific research in neutrino and high energy physics and astrophysics. The main topics being considered are the following:


 * Reactor Neutrino Detection - fundamental research and innovation.
 * Solar Neutrino Detection - fundamental research.
 * Geo-Neutrino Detection - fundamental research.
 * Supernovae Neutrino Detection: Core-Collapse and Remnant - fundamental research.
 * Accelerator Neutrino Detection in the range 0.1-10 GeV - fundamental research.
 * ββ Decay Searches - fundamental research.
 * Proton Decay Searches - fundamental research.
 * Collider Calorimeter(s) (electromagnetic and hadronic) - fundamental research.
 * Support for Axion Detection - fundamental research.

including some topics with a more imminent innovation application such as:


 * Medical Physics Detection - fundamental research and innovation.
 * Environment Radiation Detection in the range 0.1-10 MeV - fundamental research and innovation.
 * Radioactive Gases Detection - fundamental research and innovation.

In 2021, two innovation projects exploiting LiquidO's ability for positron (e+) anti-matter detection and ID, using its annihilation pattern, have been approved for funding and realisation from 2022. Those projects are:


 * AntiMatter-OTech - funded by the EU's EIC-Pathfinder-2021 - the main leading consortium is led by several academic institutions in France (IJCLab and Subatech laboratories in Orsay and Nantes), Germany (Mainz University in Mainz), Spain (CIEMAT laboratory in Madrid), UK (Sussex University in Brighton) along with the industrial partner Électricité de France (EDF, France) - largest nuclear reactor holder in Europe.


 * LPET-OTech - funded by the French Agence National de Recherche (ANR) - led by the institutions from CNRS (IJCLab, IPHC, Subatech laboratories) and INSERM (LaTIM) in France.

Several fundamental physics projects are under funding request(s) and exploration, typically under the leadership of different LiquidO PIs.

Community-wise Communication
Since its released, LiquidO has been presented in the context of several European fundamental physics consortia such as ApPEC, NuPECC and ECFA as well as IFCA, so that other scientists and the community in general may benefit from LiquidO's developments in other physics frontiers.