Objective: To improve and optimise material properties of detector targets and components
Lead: Michael Wurm (Johannes Gutenberg-Universität Mainz)
Task 3.1: Target properties and isotope loading of Water and Liquid Scintillator
Water Cherenkov and organic liquid scintillator detectors are widely-used target materials for current-day and future large-scale neutrino experiments and substantial R&D has been performed to understand the target properties. Both liquids can be loaded with metals or noble gases to improve detection properties, e.g. neutron tagging, or widen the physics scope, e.g. isotope loading for neutrino-less double beta decay (NLDBD) searches.
However, continuous further development is required to adjust water and scintillator detectors for their use in future large-scale experiments. The next generation of long-baseline oscillation experiments requires improved energy reconstruction capabilities. NLDBD detectors aiming to reach sensitivity for normal-hierarchy neutrino masses require large isotope loading fractions and excellent particle ID. Moreover, both Gd-loaded water and scintillators are increasingly used as external vetoes for direct dark matter and feebly interacting particle (FIP) searches, requiring special adjustments to these new use-cases.
Target optimisation and characterisation
Over the past years, several new lines of development have emerged in the scintillator sector. The most prominent ones are hybrid detectors (either water-based or slow liquid scintillators via low concentrations of primary fluors or intrinsically slow fluors) that aim at the simultaneous detection of Cherenkov and scintillation signals in a target medium optimised for transparency, offering both directionality and additional means of background discrimination based on the Cherenkov/scintillation ratio. Another area of interest is improved position resolution, multi-site background discrimination and tracking capabilities via fast liquid scintillator formulations which could greatly improve this discrimination, yielding vertex resolutions of just a few cm. On the opposite end of the spectrum, opaque scintillators are designed to restrict the propagation distance of the photons before being read out by wavelength shifting fibres coupled to silicon photomultipliers (SiPMs), promising superb vertex reconstruction. In addition, new wavelength shifters are explored to provide especially slow or fast scintillation or freely tunable emission spectra. Beyond conventional photomultiplier tubes (PMTs), new kinds of photosensors (wavelength shifting optical modules (WOMs), large area avalanche photodiodes (LAPPDs) are investgated to maximise benefit of the novel target properties. Efforts include the exploration of organic liquids as a target medium for time projection chamber (TPC) readout.
Target Loading
Several of these new technologies lend themselves to isotope loading at high concentration for NLDBD searches. Both water-based liquid scintillator and opaque scintillators are likely suitable for isotope loading beyond the current state-of-the-art and could achieve the several-% level, crucial to demonstrate that NLDBD experiments containing 10 tonnes of isotope or more are achievable. Moreover, water with large loading factors of gadolinium (>0.1%) will provide efficient neutron tagging for future dark matter searches.
Task 3.2: Noble Liquid target properties
Noble liquid technology has undergone impressive development in recent years in its applications to particle and astroparticle physics, as it can cover interactions with energies ranging from tens of eV to several GeV. The sub-keV regime represents a new frontier as it will extend the window of observation to light dark matter particles and astrophysics neutrinos via CEvNS (Coherent-elastic neutrino-nucleus scattering). Conversely, strategies to ascertain the directionality of low-energy particles will be essential for future massive detectors to discriminate between neutrino background and DM candidates. The potential of noble liquid techno- logy will be further extended by doping liquid argon (LAr) with xenon. This increases both photon and ionization yields with respect to LAr, acts as a wavelength shifter extending the attenuation length of photons, and makes scintillation faster. The Xe-Ar mixture target is suitable for kiloton-scale neutrino experiments, thanks to the enhanced and more homogeneous light collection. It can also be applied in ton-scale experiments, such as for direct search of light dark matter particles, relying on the ionization signal only, and of 0νββ signals, by doping LAr with 10-20% of Xe-136.
Understanding Microphysics of noble liquid (NL) response
The measured signal in noble liquid detectors is formed through a series of complex processes including energy transfer by an impinging particle to atoms, formation of excited and ionized states and phonons, charge drift in the liquid and through the liquid surface (in double-phase detectors) and, electroluminescence. A better understanding of all these processes underlying the response in scintillation and ionization, especially in the sub-keV range, is essential for fully exploiting the technology potential. This challenge can be addressed through small-scale dedicated laboratory setups which also may be exposed to neutron/electron beam/sources, enabling the investigation of responses to low-energy recoils under controlled conditions. The work plan for this deliverable is described in table 5.
Characterizing and Modelling NL light emission and transport
Once the NL energy response mech- anisms are understood, it is necessary to understand the emission and propagation properties of scintillation and electroluminescence light. Key parameters such as absorption and Rayleigh scattering lengths, refractive indices and group velocity, are not precisely established yet. Precision application of scintillation light in future large- scale neutrino detectors necessitates the development of fast-simulation approximated models and GPU-based parallelization using the determined parameters.
Characterizing Properties of Xe-Ar mixture
The primary challenge of implementing the Xe-Ar techno- logy is its thermodynamics, which has not yet been fully assessed. The maximum Xe content in the Xe-Ar mixture is determined by the solubility limit of solid Xe in LAr, which can only be inferred from a precise knowledge of the Xe-Ar phase diagram. At the same time, the long-term stability of the mixture, in terms of spatial uniformity, must be demonstrated via experimental characterization of the emitted scintillation light pulses. This also implies that circulation of the liquid, required for LAr purification, needs continuous movement of the liquid to avoid stratification, along with possible localized Xe solidification.