The XENON program
Dark Matter Direct Detection Experiments
An experiment that is designed to successfully detect new particles with extremely low interaction probabilities - the Dark Matter WIMPs -- has to fulfill several criteria:
• The experiment has to be performed in a low background environment, i.e. in underground laboratories located under mountains or in deep mines to shield from cosmic rays.
• Internal radioactivity (from detector components, housing, etc.) must be minimized.
• The main background contributions are due to gamma-rays and electrons from beta decays. An effective method to discriminate between these "electron" events and the expected "nuclear" signals (by WIMPs and background neutrons) is extremely important.
• Heavy detector materials are superior since the (spin-independent) WIMP interaction cross section is proportional to the mass number squared.
• The next generation Dark Matter detectors will have an effective mass of ~1 ton. Detector technologies must therefore be scaleable to large masses.
The XENON Dark Matter Search - Principle of Operation
The XENON detector fulfills all of these criteria. It is a dual phase liquid/gas Xenon time projection chamber (LXeTPC), operated at a temperature of about -95 C. The liquid target volume is bounded by a cathode mesh on the bottom, a gate mesh on the top, and a surrounding Teflon (PTFE) structure with field shaping rings for reflectivity and field uniformity, respectively. Two arrays of photomulipliers (PMTs) view the target volume from above and below. A moderate electric field is applied across this volume. When a WIMP interacts in the volume, it produces a primary scintillation light (S1) signal in the liquid Xe as well as ionization electrons, which are drifted to the gate mesh. A high electric field between gate mesh and anode extracts the electrons from the liquid into the gas phase, where they produce an amplified proportional scintillation (S2) signal. The ratio of the two signals, S1/S2, is different for electron and nuclear events, providing an effective background discrimination method.
Working principle of the XENON detector.
Moreover, a three-dimensional reconstruction of the interaction vertex is possible using the information of the upper PMT array (x,y) together with the time difference between S1 and S2 (z). This allows us to chose an inner fiducial volume, where the background is much reduced due to the self-shielding properties of liquid xenon.
The noble gas xenon is an excellent target material: It can be purified very effectively to a fraction of parts per billion (ppb). Its high atomic mass (A=131) promises higher WIMP detection probability, and the high atomic number (Z=54) results in short attenuation lengths for gamma-rays and hence effective self shielding. Compared to other Dark Matter approaches, TPC's based on liquid xenon (and other noble liquids such as argon) appear to be the technique with the most straightforward scaleability to the ton target mass level.
All parts of the current XENON100 experiment have been screened to have a very low internal activity, and the whole setup is additionally shielded with water, lead, polyethylene, and ultrapure copper to reduce the external background of gamma-rays and neutrons. The experiment is located at the Gran Sasso Underground Laboratory in central Italy, at a depth of 3100 m water equivalent. This reduces the cosmic ray muon flux by a factor of one million.
More information can be found here.