XENON Dark Matter Project

XENON100 Experiment

XENON100 is a new dark matter search experiment, aiming to increase the fiducial liquid xenon target mass to 100 kg with a 100 times reduction in background rate, compared to the XENON10 experiment. XENON100 was funded by NSF to Columbia and Rice universities, together with European funding agency's support to University of Zurich, University of Coimbra and Gran Sasso National Laboratory. UCLA joined the XENON100 effort in April 2008. The current XENON100 detector is in operation at Gran Sasso Underground Laboratory. Its scientific reach is a spin-independent WIMP-nucleon cross section of 2 x 10-45 cm2 by the end of 2009. The most recent status of XENON100 can be found in talks presented by XENON100 members in various conferences.

XENON100 TPC

XENON100 Detector

XENON100 TPC

The XENON100 detector uses the same principle of operation and many design features successfully tested in the XENON10 prototype. It is a position-sensitive XeTPC, with the sensitive LXe volume viewed by two arrays of total 178 photomultiplier tubes (PMTs), to detect simultaneously the primary scintillation signal (S1) and the ionization signal via the proportional scintillation mechanism (S2). The active target is enclosed in a PTFE cylinder of 15 cm radius and 30 cm height. This PTFE cylinder reflects scintillation light with high efficiency, and optically separates the LXe target from the surrounding LXe which is necessary to separate the TPC with its electric field from the walls of the vessel. 64 PMTs turn this outer LXe volume into an active LXe veto, with a total mass of 105 kg, including LXe layers above the top and below the bottom PMT arrays. More photo about this detector during it's assembly and installation can be found in our gallery.

Background Prediction

Fiducial Cut

The materials used in the XENON100 TPC and shield were carefully selected for low intrinsic radioactivity. The sensitive target can be "fiducialized" to keep only the inner core free of background. Monte Carlo simulations, based on these measured radioactivity, predict an electron recoil background rate to be less than 3 mDRU (mDRU = 10-3 evts/keVee/kg/day) in the 30-kg inner fiducial target. Such a low background rate enables us to search for WIMPs with a raw exposure of 6000 kg-day with no gamma background event. The nuclear recoil background produced by neutrons will be even less.

Krypton Removal

Kr Column

LXe, as a condensed noble gas, is readily purifiable for most radioactive impurities. The one notable exception is Kr, presents in commercial Xe gas at the ppm (part per million) level. Beta decays of 85Kr (687~keV end point, 10.76 years of half life) presents a serious background for a dark matter search. The gas used in XENON100 was processed by the Spectra Gases Company to reduce the Kr concentration to 5~ppb, while XENON100 requires another two or three orders of magnitude lower Kr concentration. We have purchased a small-scale cryogenic distillation column made by Taiyo-Nippon Sanso Co. The column has been commissioned at LNGS and has been already used to purify part of the XENON100 gas.

Sensitivity Projection

A WIMP search in the current XENON100 will be first performed in 50 kg fiducial target with 40 live-days of exposure, to reach a WIMP-nucleon spin-independent cross section at 6 x 10-45 cm2 for 100 GeV/c2 WIMPs. By taking data for another half a year and searching for WIMPs in 30-kg fiducial target will improve the sensitivity by a factor of three (2 x 10-45 cm2).