An experiment that is designed to successfully detect new particles with
extremely low interaction probabilities - the Dark Matter WIMPs -- has to fulfill several criteria. Some of them are:
- The experiment has to be performed in a low background environment, i.e.
in underground laboratories located under mountains or in deep mines.
- Internal radioactivity (from detector components, housing, etc.) must
- Since the main background contribution are due to gammas and
electrons, an effective method to discriminate between these "electon"
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 A.
- In the future, Dark Matter detectors with an effective mass
of one ton are desired. Modern approaches of smaller detectors should therefore be somehow scaleable to higher masses.
The XENON principle
The XENON detector fulfills all of these criteria. It is a dual phase liquid Xenon
time projection chamber (LXeTPC): The target volume is placed within the strong electric field of the TPC. Two arrays of photomulipliers (PMT) below and above the field are used to detect both, the direct scintillation light (S1) in the liquid Xe and ionization, via proportional scintillation in the Xe gas phase (S2). The liquid-gas border is located between the upper end of the TPC and the top PMT array. The ratio of the two signals, S1/S2, is different for electron and nuclear events, providing an effective
background discrimination method.
Moreover, a three-dimensional reconstruction of the interaction vertex is
possible using the information of the two densly packed PMT arrays (x,y) together with the time difference between S1 and S2 (z). This allows for geometric fiducial volume cuts further suppressing the background.
The noble gas xenon is a perfect target material: It can be purified very effectively on the parts per billion (ppb) level, its rather high mass
promises higher WIMP detection probability and provides effective self shielding.
And, compared to other Dark Matter approaches, liquid xenon (and liquid noble
gases in general) detectors seem to be the technique that most likely can be scaled
to the one ton target mass level.
Working principle of the XENON detector.
All parts of the XENON experiment have a very low internal activity and the whole
setup is additionally shielded with polyethylene and lead to reduce the external background from neutrons and gamma-rays. The experiment is located at the
Gran Sasso Underground Lab at a depth of
3100 m water equivalent. This reduces the muon flux by 10^6.
updated: 09/23/2007 by Marc Schumann