Xenoscope is a project focused on essential, cutting-edge research towards the DARWIN experiment, a next-generation observatory in astroparticle physics.   At its core, DARWIN will operate a multi-ton liquid xenon (LXe) Time Projection Chamber (TPC), with the direct detection of dark matter as primary science goal. This detector will achieve an unprecedented sensitivity that will be limited by the irreducible background of neutrino interactions. However, neutrinos themselves are also an interesting physics chanel for DARWIN. With a lower energy threshold than current neutrino experiments and its ultra low background level, DARWIN will even be sensitive to low energy solar neutrinos (pp, ⁷Be), as well as to the neutrinoless double beta decay of ¹³⁶Xe, which has an abundance of 8.9 % in natural Xe. Other rare-event searches with DARWIN will include the coherent neutrino scattering of ⁸B and galactic supernova neutrinos and the observation of axions and axion-like-particles.

Funded by an ERC advanced grant, the research for the Xenoscope project started in October 2017 with the goal of specifying the required input for the technical design of a 50 t detector to be realised by the DARWIN collaboration.

Along with the R&D activities for DARWIN, the Monte Carlo (MC) simulations play an important role within the Xenescope project. For a detector in a design phase, like DARWIN, the simulations become crucial since they provide:

1. the requirements, in terms of background, to optimise the detector geometry and the materials in order to achieve the physics goals of the experiment.
2. the background reduction techniques, like adding extra shielding.
3. the science reach via the sensitivity studies.

Several members of our group actively work in the Simulation Working Group of DARWIN, which has successfully developed a Geant4-based MC framework. This tool includes a detailed geometry of DARWIN detector as well as a post-processor that allows to model detector effects like the energy resolution. Among the achievements of the Simulation Group we find the identification of titanium as an excellent material for the cryostat and two physics studies: sensitivity to the neutrinoless double beta decay of Xe136 (arXiv) and prospects for the detection of low energy solar neutrinos (to be published soon). Future goals of the Simulation group are to reevaluate the sensitivity for WIMPs and the prospects for the observation of the coherent neutrino-nucleus scattering.

TPC with SiPMs

The optimisation of the light readout in TPCs is one of the main goals of the Xenoscope project. In this context we are operating a small-scale TPC at UZH, called Xurich II, equipped with state-of-the-art VUV-sensitive silicon photomultipliers (SiPMs) as top sensors. A total of 16 SiPM channels arranged in a layout that optimises light collection allow the reconstruction of the x-y position of interactions with a resolution of 1.5 mm. We characterised this novel detector concept with 83m-Kr and 37-Ar calibration sources over a wide range of drift fields. The results of this campaign can be found here: arXiv. This detector is the first dual-phase xenon TPC operated with SiPMs.

In the future, a TPC with a 4-pi photosensor coverage will be constructed for the first time and low-background materials will be identified and characterized, not only for the photosensors, but for all components of the TPC.

Photosensor testing

The DARWIN TPC will require large photosensor arrays. At UZH, we are testing novel photosensors at low temperatures, as potential replacement to the traditional photo-multiplier tubes used in ultra-low background experiments. The Silicon Photo-Multiplier (SiPM), a solid-state technology, is seen as a good candidate due to its low-voltage operation, low intrinsic radioactivity, compactness and low cost.

We characterized two sensors, from Hamamatsu Photonics (arXiv, IOP Science) and Fondazione Bruno Kesler (FBK SiPM - Report 04-2020 (PDF, 4 MB)). Up to now, all tested SiPMs exhibit Dark Count Rates too high for DARWIN. The testing of photosensors is therefore an ongoing topic.

Another technical challenge for SiPMs is their readout and the signal pre-amplification. The main shortcoming is a high heat dissipation of the pre-amplifiers. To that end, we are investigating the lowest allowable pre-amplification factor and testing various low-power pre-amplifiers at cryogenic temperatures.

Background mitigation

Another important objective of the R&D is to address material background reduction, i.e. the identification of materials with ultra-low radioactivity levels by means of a comprehensive screening campaign with Gator.

The final objective of Xenoscope is to design, build and operate a full-height LXe TPC as a prototype for the ~40 tonne target LXe DARWIN observatory. The main goals are to address the key requirements for drifting electrons over 2.6 m and to determine the required fast recirculation and purification rate of LXe. The facility will be also used to study high voltage components to be used in DARWIN, as well as different electrode materials.

The initial phase of the project consisted on the construction of a 50 cm TPC fully inmersed in liquid xenon. This purity monitor collected the signals produced by electron populations generated by means of a flashing xenon-lamp onto a photo-cathode plate located at the bottom. The operation of the purity monitor finished successfully and the results of the research are published here.

The modular design of the detector makes it easier to expand. In a second stage, the purity monitor was upgraded to a 2.6 m long dual-phase TPC. At this stage there are two electric fields: one in the drift region of the electrons, in the LXe phase, followed by an extraction field in the gas phase. Several upgrades in the system were performed; such as the installation of the high voltage system, liquid-gas level control and a top SiPM array to measure the secondary electroluminiscence signal induced by the electrons in the gas phase.

Once built and operational, the Xenoscope - DARWIN Demonstrator will become a platform for other DARWIN collaborators to test their own innovative research in conditions close to that of the DARWIN observatory.

The design and construction of the Xenoscope - DARWIN Demonstrator has been made in close collaboration with our mechanical workshop, and it is currently located in the assembly hall of the physics institute of the University of Zurich.

The Xenoscope DARWIN demonstrator is complemented by the full-diameter ULTIMATE DARWIN demonstrator, in construction at the University of Freiburg.