Detection Principle

LiquidO fundamental particle detection technology relies on the exploitation of opaque media for light detection. Here opaque medium refers to the use of material(s) causing very strong elastic scattering of light, order 1millimetre mean-free path with minimal absorption. The main mechanisms used by LiquidO’s scattering are the Mie and Raleigh light scattering, however, internal reflections may also be used. A common example of a highly scattering medium is milk – an emulsion of small droplets of fat (≤5%) in water – as opposed to water, being an easy transparent medium. While gas media is possible, most LiquidO R&D so far relies on liquid and solid media with oil or water as the main basis.

The main goal is to ensure a lossless (i.e. elastic) light scatter, as opposed to inelastic scattering or even reflections, which have long been commonly used in technology for detector segmentation. Even excellent reflective materials (≥95%) may lead to large losses of light due to a large number of reflections to contain light within a reduced 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 its detection functioning is not attached to the light production mechanism exploited. So, scintillation and Cherenkov are exploited so far simply because they are the most abundant and common in nature and existing technology.

Nature’s Communication Physics 4 273 2021 LiquidO Consortium (A. Cabrera et al.): The energy depositions in a LiquidO detector from a simulated e+ with 1 MeV of kinetic energy plus the associated annihilation gammas are used to illustrate the detection principle and imaging capability. a shows a two-dimensional x–y projection of the energy depositions in the scintillator, while b shows the full three-dimensional extent. The green lines running in parallel with the z-axis represent the fibres in the simplest configuration of the detector geometry. The energy deposition of the e+ is shown by the orange point and the Compton scatters of the 0.51 MeV back-to-back annihilation γ’s by light and dark blues. One of the γ’s (dark blue) turns upwards to run approximately parallel with the fibres and this is reflected by its shorter extent in the x–y projection. The simulation of the light hitting each fibre in a 1-cm-pitch lattice is shown in two scenarios: on c an opaque scintillator with a 5 mm scattering length is simulated, whereas on d the scintillator is transparent. The colour of each point represents the number of photons hitting a fibre at that x–y location. With a transparent medium the resulting image from the fibre array is almost completely washed out. In stark contrast, the light confinement around each energy deposition with the opaque scintillator allows preservation of the event’s precious topological information and the formation of a high-resolution image.

The light produced is therefore effectively bent by the strong scattering, thus forming light balls around every energy deposition, leading to the phenomenon pioneered by LiquidO referred to as the stochastic confinement of light. This is the trivial consequence that each optical photon describes a random walk trajectory relative to its production point. This is illustrated using a sub-atomic positron (e+; antimatter) energy deposition imaging from our Nature’s Communication Physics 4 273 2021. Scattering during light detection has historically been minimised, or even avoided, in traditional detectors since this leads to the effective 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 as close as possible to the origin using wavelength-shifting optical fibres before the light is absorbed. This implies a high-density array of fibres, whose arrangement may lead to different topological features. An optimised LiquidO detector is characterised by its excellent collection efficiency (typically ≥90%, depending on the fibre array density), which is the probability for any generated optical photons to hit a fibre before absorption. The overall detection mechanism is however typically limited by the trapping efficiency of fibres (≤25%) and the quantum efficiency of the photosensors (≤60%) used for the ultimate light detection, once trapped within the fibres.

Thanks to its stochastic light confinement, LiquidO enables the imaging of fundamental particle energy deposition(s) without the need for 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, the density of the fibre array and the amount of light per ball, hence the energy deposition and light yield. The pitch of the fibre lattice traversing the medium must be at least proportional to the light ball dimension to ensure optimal sampling of the event topology is possible.