Monolithic optical detectors, either water–Cherenkov detectors or liquid scintillator detectors, are a well-established technique in neutrino physics. Using water-based liquid scintillators (WbLS) is an approach that exploits Cherenkov and scintillation signals simultaneously; i.e., water is loaded with 1% to 10% liquid scintillator [95,96,97,98]. The approach is similar to previously used highly diluted organic scintillators [70,71] or an approach using slow scintillators. However, since water is the main component in WbLS, the WbLS approach offers benefits such as low costs and strongly reduced fire and environmental hazards. The WbLS technology is foreseen in the Eos and ANNIE experiments [99,100] and could be deployed in planned experiments such as AIT-NEO, Theia, and beyond [20,101,102] or in the context of medical particle-beam therapy [103]. The technology is expected to allow improvements in many fields, such as high-energy, nuclear, geo-, and astrophysics such as neutrino mass ordering, CP violation in the leptonic sector, solar neutrinos, diffuse supernova neutrinos, neutrinos from supernova bursts, neutrinos from the Earth’s crust, nucleon decay, and neutrinoless double beta decay with sensitivity towards normal neutrino mass ordering [104,105,106,107,108].

In a water-based liquid scintillator detector, it is possible to use the Cherenkov signals to provide directional and topological information while maintaining the good energy resolution of liquid scintillators. This separation between Cherenkov and scintillation light has been demonstrated in the CHESS setup [109,110]. Water-based liquid scintillators are expected to have good particle-identification capabilities (PID) following a discrimination strategy based on the particle-dependent Cherenkov/scintillation light ratio. This PID can improve the discrimination of alpha/beta particles and might allow some discrimination of beta/gamma particles. The PID performance of WbLS will be investigated by Eos [99]. It arises from two sources: the time profile of scintillation light emitted due to a recoiling proton may differ from electron-like events due to quenching effects, and the ratio of Cherenkov to scintillation light differs between heavier and lighter particles. Additionally, recent developments have demonstrated the capability to identify neutron/gamma particles through the pulse shape discrimination of the scintillation light [111]. Another additional benefit exists with respect to metal loading. In WbLS, loading can happen in the aqueous phase, which is easier to achieve than the direct loading of the organic scintillator.

The separation of Cherenkov and scintillation light signals can be achieved by two means. One idea is to separate fast-Cherenkov and slow-scintillation light time-wise [112]. This is in particular achieved via fast photon detectors, e.g., a Large Area Picosecond Photo-Detector (LAPPD™) [113,114,115]. Another idea is spectral separation between long-wavelength photons, which are dominated by the Cherenkov light, and short-wavelength photons, which can be – dependent on the scintillator fraction in the detector medium – dominated by scintillation light. Here, photodetectors with strong wavelength-dependent efficiency or dichroicons, Winston-cone-style light concentrators built out of dichroic reflectors, can be used [116,117].

Organic solvents, as used in liquid scintillators, are immiscible with water. This is mainly caused by the differences in the polarities of the molecules. To produce water-based liquid scintillators, the hydrophobic (lipophilic) scintillator component has to be brought into a stable suspension with the hydrophilic (lipophobic) water phase. An ampliphilic surface-active agent (surfactant) consisting of molecules with lypophilic and hydrophilic groups can be used to emulsify the organic solvent into the water solvent by reducing the tension between the organic solvent and the water [118]. The degree of tension reduction depends on the concentration of the surfactant. Its concentration can in turn also affect the optical and stability properties of the medium. A typical surfactant molecule possesses hydrophilic groups on one end and hydrophilic groups at the opposite end of the molecule. They can therefore form a hydrophilic shell around a hydrophobic droplet of scintillator inside the water, a so-called micelle. Micelles of large size can cause substantial translucence. Likewise, a high concentration of micelles can give rise to opacity.

Early studies investigated the possibility of producing water-based liquid scintillators from linear-alkyl-benzene-sulfonate (LAS), a derivate of the well-known linear-alkyl-benzene (LAB) [95]. Its light yield was found to have a dependence on the scintillator concentration of (127.9 ± 17.0) photons/MeV/%LS and an intercept value of (108.3 ± 51.0) photons/MeV, indicating a non-linear behavior at low concentrations [119]. For scintillator fractions between 1% and 10% in water, a clear dominance of Cherenkov light over scintillation light in the rising part of the light pulse could be seen in a fit of the data. From about 5% loading upwards, the fraction of scintillation light starts to dominate the peak of the pulse by more than an order of magnitude. A measurement of the relative proton light yield of a 5% WbLS showed it to be approximately 3.8% lower than that of a pure LAB+PPO reference [120].

An alternative approach to WbLS uses 13% Triton™X-100 as surfactant combined with 86% water and 1% LAB including 100 g/L PPO, as well as 10 mg/L vitamin C for pH control [98]. Here, a light yield of about (198 ± 5) photons/MeV is reported. The distribution of the sizes of micelles peaks at 2.8 nm. An important step in the production is the filtering of the WbLS, typically at the order of 10 nm to 100 nm.