LiquidO: A Novel Neutrino Detection Technology

LiquidO is an R+D project for the development of a new neutrino detection technology which uses opaque liquid scintillator (LS), like milk, with a very short scattering length and an intermediate absorption length. This is contrary to the traditional approach that uses transparent LS volumes, with scattering lengths of up to tens of meters, as neutrino targets surrounded by sensitive sensors that collect light. Reducing the scattering length down to the scale of millimeters causes the light to be confined to a few cm volume near its creation point. To extract the light a lattice of wavelength-shifting fibers runs through the scintillator. 

These figures feature LiquidO detection principle using the simplest configuration, where the fibers all run along the z-axis. The energy depositions in three-dimensional space from a simulated positron (e+) with 1 MeV of kinetic energy are shown in figure (b), while figure (a) shows the two-dimensional x-y projection. A simulation of the light propagation is shown in figure (c) and (d), where the color of each point represents the number of pho- tons hitting a fiber. An opaque scintillator is simulated in (c) and in (d) the scintillator is transparent. The formation of the light-ball around the position of each Compton electron can be seen clearly for the opaque scintillator, whereas that pattern is almost completely washed out in the transparent case. A finely segmented detector would be required to measure this topology using a transparent scintillator. In contrast, the LiquidO technique effectively self-segments due to stochastic light confinement. 

The LiquidO technique provides high-resolution imaging that enables highly efficient identification of individual particles event-by-event. Additionally, the exploitation of an opaque medium gives LiquidO natural affinity for using dopants at unprecedented levels. 

This new technology represents a breakthrough with respect to the traditional neutrino detection with liquid scintillator, essential for future neutrino physics experiments requiring kton-mass detectors and isotope high loading, as neutrinoless double-beta decay experiments. 

A small prototype called ”micro-LiquidO” was built and tested with a 1 MeV monochromatic electron beam, validating the basic principles behind the new paradigm. A larger prototype of ~10 liters volume is currently taking data with the same beam; it has already shown its capacity for tracking muons and its calorimetric capability.

LiquidO is an international collaboration that includes research institutes and universities from France, UK, Germany, US and Japan. CIEMAT participates in this R&D effort since 2017, contributing to the prototyping program and with the development of small test-benches for testing the coupling fiber-SiPM. Recently, the group, together with IJCLab (France), U. Mainz (Germany), U. Sussex (UK) and EDF (France), has been awarded with an European Innovation Council (EIC-Pathfinder) grant for the application of this new technology to the nuclear industry to open the possibility of direct monitoring of nuclear reactions inside nuclear power plant cores.