The first opaque scintillator conception and formulation started active development from 2015-2016 upon LiquidO’s conception that motivated and contextualise the value of the entire development.
Within the dominant paradigm of large neutrino detectors with impeccable transparency, the advent of an opaque scintillator was considered an effective “failure” with the likely useless application. Indeed, a few experiments are known to have suffered from chemical unstable scintillators, especially upon the challenging metal loading, leading to major physics limitations or even the impractical continuation of data taking, such as CHOOZ. Moreover, several physics programmes proposed in the past have been largely hindered or slowed down dramatically by the complex reality of liquid scintillator loading stability.
There are two main categories of R&D for our developments:
- Opacified known transparent liquid scintillators – simplest.
- Natively opaque scintillators – more ambitious.
The LiquidO consortium has historically focused on the former, as the expected scintillator performance is well known allowing to focus on the LiquidO detector capability. However, we have simultaneously started some active conceptual and experimental efforts for the latter as we appreciate that the future of the ultimate LiquidO performance demands that.
The first opaque scintillator used in LiquidO relied on the first approach, whereby a traditionally transparent scintillator based on Linear-Alkylbenzene (or LAB), pioneered by M. Chen (LiquidO member), uses Diphenyloxazole (or PPO) as single wave-shifter. This configuration is one of the simplest and today’s most common transparent liquid scintillator cocktails used in large neutrino detectors. Experiments such as SNO+ (1kton) and JUNO (20ktons) rely on similar formulations. The opacity is obtained by doping the transparent aforementioned scintillator with paraffin wax (or wax), also commonly used in very small fractions in scintillator cocktails. The difference is that the new formulation will see the opacity formation upon loading larger fractions of paraffin ≥10%, thus causing light opacity upon the temperature controlled amorphous crystallisation of the paraffin. This scintillator formulation is called NoWaSH (”New opaque Wax Scintillator, Heidelberg”), pioneered for LiquidO by C. Buck et al. at the Max-Plank Institute. This formulation has been extensively and successfully used throughout all the prototyping LiquidO detectors 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. The possible implication of the phase-transition for the detector prototyping is still under study but has led so far to no evident issue for detector design other than a very slow cooling caused by the intrinsic reduction of convection upon crystallisation and the poor thermal conductivity of the wax. Despite simplicity and convenience, the NoWaSH formulation does not lead to any improvement of the light yield (order 10,000 photons per MeV) – this was never its goal. In fact, the light yield is linearly diminished by the non-scintillating paraffin concentration doping. This is typical and 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 and will be addressed in future formulations for optimised detector response at low energies in the MeV range.
The second branch of the opaque scintillators was conceived with the main design goal to prevent or minimise the aforementioned light yield quenching. One conceived formulation may enable even the potential boosting of light but this has not been demonstrated. This family of approaches is expected to open a completely new dimension of potential scintillators, led by LiquidO and called MicroCrystal whose concept was proposed by A. Cabrera and reported in arXiv:1807.00628 in mid-July 2018 entitled “A Hybrid Organic/Inorganic Scintillator for High-Performance Measurements” authored by S. Wagner, M. Grassi and A. Cabrera. The main idea is to add micrometre scale inorganic scintillating crystals so that both the light scattering and higher light yield light production are possible by the crystals, whose light yield can be factors higher than liquid scintillators – not the most luminous scintillator materials known in fact. In the highly heterogeneous configuration of a MicroCrystal system, a liquid may be used to immerse the crystals to ensure a better optical coupling – via the index of refraction matching – to ensure that maximal light transmission is possible. The liquid in question may also be an active organic scintillator, thus allowing a possible increase of aggregated light yield – to be demonstrated.
However, the NoWaSH scintillator combined with the MicroCrystal approach may grant extra mechanical support of the crystals – or any loading – upon paraffin crystallization leading to solidification. This may enable a new approach – ideally less sensitive to chemical instability – of detector metal doping thanks to the large variety of inorganic scintillators. Thus, the potential of MicroCrystal technology may be expected to endow LiquidO detectors with extra key detection capabilities (one or several) such as:
- higher density or different radiation detection capabilities (Pb, W, etc)
- enhanced neutron detection capabilities (B, Li, Cd, Gd, etc),
- non-native neutrino cross-sections (In, Cl, Pb, Fe, Ar, etc),
- exotic materials such as ββ-decaying emitters (Te, Mo, Se, Nd, Cd, etc).
Indeed, the MicroCrystal (or alike) approach may have the potential to open a vast range of fundamental physics and possible applications for LiquidO-based detectors, if successfully demonstrated. The first steps of this strategy have been successfully realised within the LiquidO consortium, while results have not yet been published, the concept is nonetheless public via arXiv.
Τhere are other promising ways to achieve opaque scintillator formulations beyond the NoWaSH and MicroCrystal systems being internally considered and discussed within the LiquidO collaboration and under active exploration. This remains an active front of open R&D development benefiting from the advent of the LiquidO detection paradigm. The LiquidO prototypes have been run on water-only filled so that Cherenkov-light only performance can be studied separately. Hence, LiquidO is a priori ready for oil-based and water-based scintillators, although only oil-based scintillators have been used for the sake of simplicity during prototyping.