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XENON group at the University of Zurich

The XENON program

XENON is a direct dark matter detection experiment using liquid xenon as the detector medium. The goal is to detect the small charge and light signal after a dark matter particle interacts with a xenon nucleus in a liquid-gas two-phase time projection chamber (TPC): The prompt light signal (S1) is detected with two array of photomultipliers. The ionization electrons are separated from the Xe ions and drifted upwards by a strong electric field. A second electric field extracts the charges from the liquid into the gas phase where they generate secondary scintillation light (S2) which is proportional to the charge signal. The secondary signal -- delayed to the S1 by the electron drift time -- is detected with the same two photodetector arrays.

The TPC design allows the precise 3-dimensional reconstruction of the interaction vertex which can be used to reduce the background contamination by fiducial volume cuts. Furthermore, the ratio S2/S1 has a different value for electron recoils (background) and nuclear recoils (signal) and can be used for background discrimination.

The first phase - XENON10 - has been successfully operated in dark matter mode at the Gran Sasso Underground Laboratory (LNGS) until 2007 leading to some of the best limits on WIMP dark matter so far.
The second phase - XENON100 - has achieved the projected sensitivity (and world-best limits on WIMP scattering at the time of publication). The detector is being used to test novel ideas for calibrations of the dual-phase TPCs.
The third phase of the XENON dark matter search project, XENON1T, is the largest xenon TPC ever built. It is in science operation at the LNGS underground laboratory since autumn 2016, and already obtained the first, world-leading result with a short 30-day run.

XENON1T

XENON1T is one of the most ambitious ongoing dark matter search projects. It is a dual-phase LXe TPC with a total mass of ~3 t and a fiducial mass of ~1 t. In order to detect the VUV scintillation light from particle interactions with the xenon target, two arrays of 3-inch Hamamatsu R11410 photomultiplier tubes will be installed on the top and bottom of the TPC. We aim for a background of less than one event in 2 t-years exposure, leading to a sensitivity to spin-independent WIMP-nucleon scattering cross sections of 2×10-47 cm2 for 100 GeV/c2 WIMPs, two orders of magnitude below the sensitivity of XENON100 and a factor of 40 below the current best limits from the LUX and PandaX experiments.

In order to reduce the ambient backgrounds to negligible levels, the XENON1T detector is placed inside a large water shield of 10 m height and 9.6 m diameter, instrumented with 84 8-inch Hamamatsu R5912 PMTs which provide an active Cerenkov veto and a shield against environmental radiation. The assembly of the water tank was already completed in Hall B at LNGS, and most other subsystems are currently under construction. The XENON1T detector with its Cherenkov muon veto, together with a picture taken during the TPC assembly are shown below:


The XENON1T experiment at LNGS


TPC assembly

Within the XENON1T project, we are co-leading several collaboration-wide working groups, working on Monte Carlo simulations, detector calibrations with UV light and radiactive sources, radioactive screening of detector components, characterization and testing of the photosensors, design of the the time-projection chamber, construction of the field cage, and cabling.

The next step in the XENON dark matter search program is the XENONnT project. It will double the amount of xenon in the sensitive volume, which would allow to increase the fiducial mass and simultaneously provide a significant reduction of radioactive background. The XENONnT TPC with the inner cryostat vessel will be constructed while XENON1T is taking data, and will be installed in the same outer vessel and the water shield as XENON1T.

XENON100

The scientific goal of the XENON100 experiment was to reach a WIMP sensitivity that is one order of magnitude better than its predecessor XENON10. This was achieved by an increase in target mass by one order of magnitude, up to 62 kg. At the same time, the gamma background was reduced by a factor 100. This was achieved by a careful selection of all detector materials for radiopurity and by an improved detector design.

In order to reduce the background even further, the target volume is completely surrounded by a layer of liquid xenon (almost 100 kg in total). This shield utilizes the excellent self shielding capabilities of xenon, and, being instrumented with PMTs, provides further background reduction.

The pictures show the XENON100 detector with some members of the UZH group during its installation, the top and bottom PMT arrays, as well as the closed detector in its shield:

 

Our group was involved in PMT testing, calibration and operations at LNGS, in the construction of various detector and shield hardware components, in material screening with a high-purity Ge spectrometer, in data processing and analysis, in Monte Carlo simulations of the expected gamma, alpha, and neutron backgrounds, in the WIMP analysis for spin-dependent and spin-independent interactions, as well as in the design, construction and Monte Carlo simulations for the next phase.

The results of WIMP searches with the 225 live days of data acquired with XENON100 have been published in (Phys. Rev. Lett. 109, 181301, 2012), for spin-independent interactions, and in (Phys. Rev. Lett. 111, 021301, 2013), for spin-dependent case.

XENON10

The picture shows the XENON10 detector in its low-background shield. Note the difference to XENON100, for which the cables, feed-throughs and cryo-cooler have been moved outside the shield.


XENON10 in shield
XENON10 limits		  
First results on spin-independent WIMP-nucleon interactions have been published in 2008 (Phys. Rev. Lett. 100, 2008). No dark matter candidates have been detected, and the sensitivity to WIMP-nucleon cross sections extends to about 4×10-8pb (4×10-44cm2) at a WIMP mass of 30 GeV/c2. The figure shows the excluded region in the WIMP cross section-WIMP mass parameter space (region above the red curve), along with the CDMS-II 2005 results (blue curve), and predictions from the constrained MSSM (filled regions).
Limits for spin-dependent WIMP nucleon interactions have been published in (Phys. Rev. Lett. 101, 2008).

R&D and Lab Activities

To measure the ionization and scintillation yield of electronic recoils at low energies, and to study and develop important processes necessary for future big LXe detectors, we have built a small dual-phase xenon chamber, Xurich I that was operated at our lab in Zurich. The picture shows the inner chamber, made of teflon and stainless steel grids. The results of the studies have been published in (Phys. Rev. D 87, 115015, 2013). The same TPC has been employed in R&D study of the energy calibrations with 83mKr source, which is now conventional for LXe detectors; the results have been published in (Review of Sci. Instr. 81, 7, 2010).
Xurich I TPC at UZH

In order to perform the light and charge yield measurements for nuclear recoils with high precision we realized a dual-phase LXe time-projection chamber (TPC), Xurich II, which is currently being commissioned. The target volume is enclosed in a PTFE cylinder of 30 mm height and 30mm diameter. The scintillation light is detected by 2 PMTs (2-inch Hamamatsu R9869), one in the liquid xenon below the cathode grid and one in the gas phase above the anode grid. The contribution of multiple neutron scatters is minimized by reducing the TPC size and the amount of surrounding materials, in particular PTFE, which has relatively high cross-section for neutron interactions. The electric field uniformity within the TPC has been optimized via extensive simulations with COMSOL, resulting in the maximum variations below 4%.
Xurich II TPC

Gator: low level counting

Our group operates a 2 kg high-purity germanium (HPGe) spectrometer in a low background environment underground at LNGS. It is used to screen all XENON detector and shield components for their U/Th/K/Co content. The results were used to construct a full background model of XENON10, XENON100, and are guiding the construction of the next phase, XENON1T. The pictures show the HPGe detector during its installation, as well as the final setup.
Gator during installation.
Gator in shield.

XENON in the news