The XENON Project

A 1 tonne Liquid Xenon experiment for a sensitive Dark Matter Search

Elena Aprile

Columbia University

Elena Aprile Dark Matter 2002

The XENON Collaboration

• Columbia University: E. Aprile (Principal Investigator)

T. Baltz, A. Curioni (graduate student), K-L. Giboni, C. Hailey, L. Hui,

M. Kobayashi and K. Ni (graduate student)

• Brown University: R. Gaitskell

• Princeton University: T.Shutt

• Rice University: U. Oberlack

• LLNL: W. Craig

Elena Aprile Dark Matter 2002

Current and Projected Limits of WIMP Searches

• Projection for CDMS Soudan (7kg Ge+Si) is ~1 event / kg / yr. Similar limits projected for competing experiments in Europe.

• It will take a substantial increase in target mass and superior background discrimination power to reach a sensitivity of ~1 event / 100kg / yr.

For a Xe target with <10 keV recoil energy threshold this rate corresponds to a WIMP-nucleon of ~ 10-46 cm2

• A 1 tonne XENON experiment approaches this sensitivity, assuming 3.9x 10-5 cts /kg /d /keV background rate, 99.5% discrimination and 10

keV recoil energy threshold .

Elena Aprile Dark Matter 2002

Liquid Xenon for a Dark Matter Detector

Many favorable properties, from high Z (54) and density (3g/cc) for a compact instrument of flexible design, to high ionization (~60,000 e/ MeV) and scintillation yields if highly purified, to only stable isotopes ...etc but a low energy threshold is essential for a sensitive WIMP detector.

Expected rates of WIMP interactions in Xe and other targets as a function of recoil energy

threshold for a 100 GeV WIMP with a = 10-9 pb (from R. Schnee).

Elena Aprile Dark Matter 2002

Ionization and Scintillation in Liquid Xenon

I/S (electron) >> I/S (non relativistic particle)

Elena Aprile Dark Matter 2002

The XENON Experiment : Design Overview

• The XENON design is modular. An array of 10 independent 3D position sensitive LXeTPC modules, each with a 100 kg active Xe mass, is used to make the 1-tonne scale experiment.

• The TPC fiducial LXe volume is self-shielded by a few cm thick layer of additional LXe. The active scintillator shield is very effective for charged and neutral background rejection.

• One common vessel of ~ 60 cm diameter and 60 cm height is used to house the TPC teflon and copper rings structure filled with the 100 kg Xe target and the ~50 kg Xe for shielding.

Elena Aprile Dark Matter 2002

The XENON TPC: Principle of Operation

• 30 cm drift gap to maximize active target long electron lifetime in LXe demonstrated

• 5 kV/cm drift field to detect small charge from nuclear recoils internal HV multiplier (Cockroft Walton type)

• Electrons extraction into gas phase to detect charge via proportional scintillation (~1000 UV /e/cm) demonstrated

• Internal CsI photocathode with QE~31% (Aprile et al. NIMA 338,1994) to enhance direct light signal and thus lower threshold demonstrated

• PMTs readout inside the TPC for direct and secondary light need PMTs with low activity from U/Th/K

Elena Aprile Dark Matter 2002

The XENON TPC Signals: Nuclear Recoil Discrimination

• Redundant information from charge (secondaryl light) signal (S2) and primary scintillation light (S1) signal from PMTs and CsI photocathode

• Background (,e,produce electron recoils with S2/S1 >>0 • WIMPs (and neutrons) produce nuclear recoils with S2/S1~0• Depth of interaction Z from timing and XY from c.o.g of PMT signals. • 3D event localization for effective background rejection via fiducial volume cuts

Elena Aprile Dark Matter 2002

The Columbia LXeTPC for Gamma-Ray Astrophysics

30 kg active Xe mass

20 x 20 cm2 active area

8 cm drift gap with 4 kV/cm

Charge and Light readout for

calorimetry and 3D imaging

1st LXeTPC demonstration in near space

Elena Aprile Dark Matter 2002

Compton Imaging of MeV ray Sources with a LXeTPC

Elena Aprile Dark Matter 2002

Background Considerations for XENON

and induced background85Kr (1/2=10.7y): 85Kr/Kr 2 x 10-11 in air giving ~1Bq/m3

Standard Xe gas contains ~ 10ppm of Kr10 Hz from 85Kr decays in 1 liter of LXe.Allowing <1 85Kr decay/day i n XENON energy band <1 ppb level of Kr in Xe

136Xe 2 decay (1/2=8 x 1021y): with Q= 2.48 MeV expected rate inXENON is 1 x 10-6 cts/kg/d/keV before any rejection

• Neutron induced backgroundMuon induced neutrons: spallation of 136Xe and 134Xe take 10 mb and

Homestake 4.4 kmwe estimate 6 x 10-5 cts/kg/d before any rejection reduce by muon veto with 99% efficiency

(,n) neutrons from rock: 1000/n/m2/d from (,n) reactions from U/Th of rock appropriate shield reduces this background to 1 x 10-6 cts/kg/d/keV

Neutrons from U/Th of detector materials: within shield, neutrons from U/Th ofdetector components and vessel give 5 x 10-5 cts/kg/d/keV lower it by x10 with materials selection

Elena Aprile Dark Matter 2002

Background Considerations for XENON

-rays from U/Th/K contamination in PMTs and detector components dominate the background rate. For the PMTs contribution we have assumed a low activity version of the Hamamatsu R6041 ( 100 cts/d ) consistent with recent measurements in Japan with a Hamamatsu R7281Q developed for the XMASS group (Moriyama et al., Xenon01 Workshop).

Numbers are based on Homestake location and reflect 99.5% background rejection but no reduction due to 3D imaging and active LXe shield.

Elena Aprile Dark Matter 2002

Charge readout with GEMs: a promising alternative

• High gain in pure Xe with 3GEMs demonstrated

• Coating of GEMs with CsI

• 2D readout for mm resolution

See Bondar et al.,Vienna01

Elena Aprile Dark Matter 2002

Proposed Strategy for XENON

• 10 kg prototype with all design parameters of 100 kg module.• Demonstrate electron drift over 30 cm and charge/light readout under

high field operation. Test with electron/nuclear recoils.• Study Kr removal techniques (distillation, cryopumping).• Develop and test low activity PMTs (Hamamatsu metal channel

and/or Burle MCP based).• Test multi GEMs charge readout in LXe.• Studies of low activity detector materials.• Finalize the design of a 100 kg LXeTPC after 2yrs of R&D.• Construction phase of 1st XENON module starts in 3rd year.• Enlarge collaboration and start underground operation by 2005.

Elena Aprile Dark Matter 2002

Summary

• Liquid Xenon is an excellent detector material well suited for the large target mass required for a sensitive Dark Matter experiment.

• The XENON experiment is proposed as an array of ten independent, self shielded, 3D position sensitive LXeTPCs each with 100 kg active mass.

• The detector design, largely based on established technology and >10 yrs experience with LXe detectors development at Columbia, maximizes the fiducial volume and the signal information useful to distinguish the rare WIMP events from the large background.

• With a total mass of 1-tonne, a nuclear recoil discrimination > 99.5% and

a threshold of 10 keV, the projected sensitivity for XENON is 0.0001 events/kg/day in 3 yrs operation, covering most SUSY predictions.