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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Xenoscope (Towards a multi-ton xenon observatory for astroparticle physics)

Teaser

Summary

The Xenoscope project addresses the nature of dark matter in our Universe, and the fundamental nature of neutrinos. While we have ample evidence for the existence and distribution of dark matter at large scales, based on its gravitational influence on luminous matter, its composition at the microscopic level is not know. In the case of neutrinos, we know that they have mass, however the absolute mass scale, as well as their nature (Majorana versus Dirac particles) are unknown.

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

To design and construct a 50 t (40 t in the time projection chamber, TPC) detector, a number of critical technological challenges must first be addressed. Fundamental aspects are related to the design of the TPC, including the identification of new photosensors, the optimisation of the light and charge collection, and the minimisation of radioactive backgrounds. Xenoscope aims to address these aspects through a number of small, medium-size and a full-scale (in the z-coordinate of the TPC) prototypes. The goal is to specify the required input for the technical design of the 50 t detector.

Arrays of VUV-sensitive SiPMs will be studied as alternative light sensors to photomultiplier tubes, and the signal detection will be optimised for both low and high-energy readout, thus drastically increasing the dynamic range of a LXe-TPC. Low-background materials will be identified and characterized not only for the photosensors and their read-out, but for all the components of the detector. Finally, a full scale TPC in the z-dimension, 2.6 m in height, will be designed, built and operated. The main goal is to demonstrate electron drift and extraction into the vapour phase over the distance relevant for the final DARWIN geometry.

Work performed

We describe the work and achieved results in each working package:
WP1: TPCs with SiPM arrays
We designed and built the first xenon time projection chamber with SiPM array readout in the top photosensor plane, were we operate an array of 16 SiPM channels. We acquired data with the internal calibration source 83m-Kr, 57-Co and 137-Cs sources, and made all the preparations, including the activation at PSI, for the calibration with an internal 37-Ar source. We will publish the results achieved with the top SiPM array and the 37-Ar calibration at low energies. The next step is to replace also the bottom PMT with a SiPM array. The achieved position resolution in x-y is at the level of 1 mm, at 1-sigma.
WP2: Readout of PMTs and SiPMs
We developed the boards (cold electronics) to read out the SiPM arrays from Hamamatsu and from Fondazione Bruno Kessler (FBK), in collaboration with our electronics workshop in the department. We have also developed a warm, dual-channel PMT amplifier board that has a low gain and high gain amplification, for the high energy (relevant for the neutrinoless double beta decay search) and low energy regions (relevant for the dark matter search), respectively. This board was tested with PMTs operated with the MarmotX liquid xenon detector in our laboratory, and will be tested in situ with the XENONnT PMTs.
WP3: Background minimisation and MC simulations
We upgraded the shield structure of the Gator HPGe detector, which our group operates at the Laboratori Nazionali del Gran Sasso and quantified the new background rate. Since the upgrade, we screened detector components for the XENONnT experiment, currently under construction, and for the future DARWIN. Next we will try to reduce the background of the detector further, by increasing the gaseous nitrogen flow, and thus reducing the radon levels inside the shielding structure. We have implemented the DARWIN detector geometry into the Geant4 framework (in collaboration with the ERC funded group at Freiburg University, Ultimate) and we simulated the electronic recoil background from detector materials. We have also estimated the cosmogenic background from 137-Xe, which is produced by neutron capture on 136-Xe. The neutrons can be generated for example from interactions of high-energy cosmic muons. The background from the decay of 137-Xe is particularly relevant for the neutrino less double beta decay search.
WP4: A full scale TPC prototype
We designed the support structure, the cryostat, and completed the field simulations (based on the Comsol package) for the high-voltage feed-through. We are working on the cryogenic, gas and purification, as well as xenon storage systems for the full 2.6 m time projection chamber demonstrator.
WP5: Science reach
We have assessed the science reach of DARWIN for the pp, 7-Be and CNO solar neutrinos and for the neutrinoless double beta decay of 136-Xe. Regarding the solar neutrinos, we determined the levels of precision with which we can measure each solar neutrino type that we can detect with 3Ïƒ significance or better. Then, given those levels of precision, we investigated how well we can reconstruct the weak mixing angle and the electron neutrino survival probability with pp neutrinos at energies below 200 keV and to what extent the neutrino types enable us to discern between different solar metallicity models. In the former case, we find that we will be able to directly measure the pp flux down to low energies (~a few keV) and subsequently provide the first measurement of the electron neutrino survival probability below 200 keV and the first measurement of the weak mixing angle below 2.4 MeV. In the latter case, we expect to make a distinction between the high- and low-metallicity solar models with a median p-value of 0.03-0.15 in the lifetime of the DARWIN experiment. Regarding the neutrinoless double beta decay channel, we have determined the half-life sensitivity of the DARWIN detector, which will contain about 3.5 tons

Final results

Progress beyond the state of the art:

WP1: the first xenon TPC with SiPM readout; expected calibration with a new low-energy internal source, 37-Ar, produced at the Swiss Spallation Source at PSI Villigen. In the future we will operate a TPC with 2 SiPM read-out sensor planes (top ad bottom), and a TPC with 4-pi light readout.
WP2: successful development of a SiPM readout board with a cryogenic pre-amplifier; successful development of a cryogenic pre-amplified readout base for the 3-inch Hamamatsu VUV photomultiplier tube, which is the baseline photosensor for DARWIN
WP3: successful upgrade of the Gator low background counting facility, which is one of the world\'s most sensitive HPGe detectors. Reduction of the noise levels. First screening of DARWIN materials. In the future we expect to screen and identify the actual detector construction materials. Successful implementation of the detailed DARWIN geometry into the Geant4 framework, and first simulations of the electronic recoil background. We aim for a full background model of the detector, for both electronic and nuclear recoils.
WP4: the design of the 2.6 m TPC, together with the cryostat, and all the support systems (gas circulation and purification system, recuperation system, cryogenic system) is proceeding well. The final aim is to build and operate the 2.6 m xenon TPC and achieve an electron lifetime above 2 ms.