Work Package 4: Scaling-up challenges

Objective: To address challenges required to realise liquid detector technologies scalable for integration in large systems

Lead: Roberto Santorelli (CIEMAT)

Task 4.1: Radiopurity and background mitigation

The mitigation and calculation of background signals in rare event search experiments are among the forefront challenges for the next generation of liquid detectors. Traces of radioactivity present in the materials used to construct the detector or in its surroundings can be the dominant background for rare event search experiments. Low-background detectors require extensive radio-purity assays, employing diverse techniques and utilizing facilities worldwide. Development of new and more sensitive assay methods is essential to ensure complete radiogenic characterization of the primordial radionuclide decay chain and the cosmogenic activation of the materials. Besides the bulk contamination, surface contamination of detector materials can be a significant source of background, primarily due to radon diffusion and plate-out of radon daughters. Exposure to environmental Rn during fabrication, assembly, and installation of the experiment can result in the accumulation of Pb-210 on surfaces. Due to its 22-year half-life, Pb-210 can act as a nearly constant source of radiation, along with its daughters Bi-210 and Po-210, throughout the entire duration of an experiment. Moreover, alpha decays from Po-210 on the surfaces of the materials with high alpha-n cross sections can contribute to the neutron yield. The development of novel and effective strategies for material selection, as well as well-defined protocols for machining, storing, transporting, and assembling detector components, is necessary to mitigate the surface background.

To accurately measure and interpret the signals of interest, it is crucial to understand and model all the background contributions in an experiment. Neutrons and gamma rays can contribute to background even from distant sources, making the precise calculation of the radiogenic neutron flux induced by alpha-n reactions and the cosmogenic gamma flux indispensable for future experiments. Accurate knowledge of (α,n) cross sections is crucial to determine the neutron flux induced by alpha particles in the target or in the materials surrounding the active volume. Furthermore, comprehensive simulations are necessary to model the transport of gamma rays from cosmogenic activation, considering the specific experimental setup and shielding configurations. Advanced simulation techniques, coupled with improved (α,n) cross-section data are imperative to understand, control, and reject these sources of background.

Radioassay techniques at required sensitivity for next generation of rare-event search experiments

Significant efforts are planned in the coming years to overcome the current limitations in the sensitivity of radio purity assays for materials required for the construction of the next generation of detectors. R&D activities encompass both achieving higher sensitivity in measuring bulk material contamination using traditional methods such as Inductively Coupled Plasma Mass Spectrometry (ICPMS) and High Purity Germanium (HPGe) detectors, as well as refining more recent methods required for scrutinizing the entire decay chain of U-238 and Th-232 (e.g., Po-extraction). The forthcoming generation of experiments will require achieving sensitivities of ≈1 µBq/kg (for example for HPGe). This represents an improvement of at least an order of magnitude compared to the achievements thus far. Additionally, new ideas are being developed for the detailed measurement of surface contamination resulting from the plate-out of radon progeny, aiming to measure surface activity as low as approximately 10 µBq/m².

Mitigation through material selection/treatment and clean manufacture

Radon mitigation needs significant efforts in terms of surface cleaning, as well as storage and transportation of materials. Moreover, the fabrication of materials for low-background experiments necessitates stringent control over all the manufacturing process. Addressing these challenges requires specialized protocols to maintain low background conditions, regular monitoring, and maintenance of surfaces to ensure long-term mitigation of radon contamination. A further crucial aspect is the development of cleaning, transportation, and manipulation protocols aimed at reducing the native surface contamination of materials used in low background experiments, such as copper, stainless-steel, acrylic. In addition to radiogenic surface backgrounds, other material induced background will be considered e.g. surface defects leading to the field emission in electrodes of dual phase TPCs, surface treatments or new materials to reduce surface-trapped electron dwelling time. R&D on various approaches is planned, ranging from chemical and electrochemical methods to atmospheric and vacuum plasma techniques. At the same time, it is mandatory to minimize the Rn contamination inside the detector, requiring characterization of the Rn emanation of materials as a function of temperature and pressure.

New tools/materials for the evaluation/suppression of backgrounds

Neutrons are one of the main sources of background in rare-event search experiments. For the accurate calculation of radiogenic neutron production rates, it will be crucial to measure the (α,n) production cross-section in those materials relevant to the construction of low-background detectors for which experimental measurements are not available (e.g., argon). In parallel, significant research efforts are expected in the coming years to investigate innovative configurations of active veto technologies based on new materials for neutron background mitigation, e.g. R&D is currently ongoing aimed at producing a large amount (few tens of tons) of radiopure acrylic loaded with a gadolinium compound.

Task 4.2: Detector & target procurement/production & purification

A pure, many kiloton target of liquid scintillator has great sensitivity for sub- MeV neutrinos and other rare-event physics which often cannot be achieved by other detector mediums. While such detectors have high light yield, fast signals, and can be cost-effective, on some occasions large pure LS detectors might not be a viable choice due to no directional information, fluorescence quenching, and chemical safety. Some major R&D directions have emerged in recent years. Water-based Liquid Scintillator (WbLS) is a novel detection medium that delivers a hybrid event detection of particle interactions to combine the unique topology of Cherenkov light with the increased low-threshold scintillation light yield to improve energy and vertex resolution and obtain particle identification. As well, further detector enhancements, such as metal-doping, slow scintillation, dichroicon, and highly opaque, can be added on either LS or WbLS to extend their physics reaches for next-generation experiments.

Cryogenic noble liquids, especially argon and xenon, are used for many applications such as targets for dark matter searches, neutrino physics, medical imaging and in veto systems. Over the past decade, purification technologies for LXe have reached part-per-quadrillion levels of key impurities, and production and purification technologies for argon extracted from underground have progressed from prototypes to demonstration at the 50 kg scale towards the production at the few 100-tonne-scale.

Scale-up mass production

Several WbLS testbeds and LS experiments are now under development at numerous laboratories, in addition to research programs at several universities, worldwide. R&D required to progress this area includes establishing production and purification facilities supporting fundamental research and prototyping studies for proof of concept in different detector configurations, and to develop in-situ purifica- tion or circulation systems prolonging the detector lifetime by mitigating the colored quenching and radioactive background introduced from air ingression and material leaching.

For scaling-up noble liquid detectors, the proposed and in-preparation experiments are confronted with R&D challenges in procurement, storage and transport of large quantities of noble liquids. Ar dark matter searches and neutrino-less double beta decay experiments employing Ar veto detectors (i.e. LEGEND-1000) need to extract and purify 10-100 tonne quantities of underground argon, depleted of the 39Ar isotope relative to atmospheric Ar. For Xe the availability of large quantities of the target material is at present a bit difficult commercially due to the international situation, but may return back to normal in few years from now; actions are being undertaken to develop alternative production methods including the development of new extraction method from air using PSA methods instead of cryogenic distillation.

Purification and Methodology

For purification, the key challenges for liquid nobles are the need for under- standing of the impurities leading to the production of single isolated electrons. These constitute an irreducible background in the ionisation-only (“S2-only”) analyses that have leading sensitivity to light dark matter. For argon, the purification of the argon extracted underground, related to the facilities of Urania and Aria, is an enabling development to demonstrate the viability of kilotonne-scale future rare-event search detectors. For QA/QC of target purification it is vital to develop a method to measure electron lifetime/cryogen purity in-situ at large detectors, within the active volume. Existing methods for large LAr detectors employ miniature TPCs located outside the active volume, that have the disadvantage of not measuring the region used for data analysis. Other methods use samples of cosmic ray data, that are less frequent in deep underground labs. Developments are proposed to use a system of laser beams for in-situ measurement, based on the system operated in MicroBooNE but with several improvements. The aim is to test this concept in the ProtoDUNE detectors at CERN in the next years to develop the operational and analysis details of this new method.

Task 4.3: Large-area readouts

The scope of this Task covers the R&D on scalability of light and charge readout for current and future neutrino and dark matter experiments. All these liquid detectors plan to exploit scintillation or Cherenkov light signatures, which drives the need for an increase of photo-coverage to improve energy resolution and lower thresholds. The readout of the detectors will become challenging due to the increased number of readout channels and data volumes. In order to accomplish the scaling-up of the current detectors, mid-scale and large facilities will be necessary for assembling and testing the readout in protoypes before the final scaling is pursued.

The state-of-the-art in scale-up of liquid argon (LAr) TPCs is represented by the ProtoDUNE detectors at the CERN Neutrino Platform and ICARUS at the Fermilab Short-Baseline Neutrino Program. These single phase detectors feature several hundreds of tons of LAr mass, and encompass readouts based on wire planes or perforated PCBs for the ionization charge, and PMTs or X-ARAPUCAs for the scintillation light. For the dual phase detectors, DarkSide-50 has reached the 50 kg scale and employs a PMT readout. A demonstrator for the DUNE Near Detector (ND-LAr 2 ×2, 0.75 m ×0.75 m ×1.6 m) using pixelated charge readout and ArCLight (another light trap technology) is being tested at Fermilab. In addition to the previous TPCs, GERDA features a 64 m3 LAr veto read out by PMTs and SiPMs coupled to WLS fibers. Regarding the liquid xenon TPCs, the state of the art is the dual-phase ∼6-tonne XENON-nT and ~7-tonne LZ experiments with PMT readout.

The state of the art of liquid scintillator detectors is represented by the recently concluded Borexino experiment (278 ton) and the upcoming JUNO experiment (∼20 kton), both using PMTs as readout technology. For the water Cherenkov detectors, it is Super-Kamiokande, holding 22.5 kton and using PMTs as well, and the community is preparing for the Hyper-Kamiokande detector that will be up to 8 times bigger, using high quantum efficiency PMTs for the light readout.

The need for accomplishing the assembly and characterization of readout units at the final scale before deployment drives the creation of dedicated facilities equipped with the cryogenic infrastructure, slow controls and DAQ services capable of handling the required scales. A complementary and parallel effort is re- quired to address the large data volumes expected from these detectors by using readout electronics placed as close as possible to the sensors and digitizing the signal as far upstream as is feasible, in order to benefit from the higher signal-to-noise and multiplexing, which reduces the number of channels, simplifying the detector design and construction. Finally, there is a need for offering platforms for performing joint integration tests where all readout systems can be deployed simultaneously. A remarkable challenge is the extension of the photocoverage in the noble liquid TPCs. Since the cathode and field cage are subject to a very high voltage, any photodetector to be deployed on them must be electrically isolated, requiring power and signal transmission via non-conductive connections. The field cage structure will be slimmed down for new photodetectors to be embedded within the available space. A solution based on the use of power and signal over fiber systems will have to be tested on the large scale before it can be adopted.