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 module -- 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 module -- XENON100 -- is currently taking calibration data at LNGS in order to optimize the detector response. An upgrade to the current effort with a target mass of 1 ton -- XENON1T -- is already funded and in the planning phase.

XENON100

The scientific goal of the XENON100 experiment is to reach a WIMP sensitivity that it one order of magnitude better than its predecessor XENON10. This is achieved by an increase in target mass by one order of magnitude to 65 kg. At the same time, the gamma background is 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. This passive shield utilizes the excellent self shielding capabilities of xenon. Making the shield active by instrumenting it with PMTs, decreased the background even more.

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 is 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. We are leading the collaboration wide Monte Carlo Working Group.

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We are conducting Geant4 Monte Carlo simulation to predict the XENON100/XENON10 internal and external backgrounds at LNGS, as well as gamma calibration source simulations. Pictures of the XENON100 and XENON10 geometries are shown on the left.
XENON100 MC.

XENON10 MC.

XENON10 and Lab Activities

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.

XURICH TPC at UZH.
 
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 4x10-8pb (4x10-44cm2) 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 also been published (Phys. Rev. Lett. 101, 2008).

To measure the ionization and scintillation yield of nuclear 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 that is operated at our lab in Zurich. The picture shows the inner chamber, made of teflon and stainless steel grids. We are also constructing a new lab with a neutron generator in order to make measurements with nuclear recoils of a well defined energy.

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