The XENON Dark Matter Project: Status of the XENON100 Phase. Elena Aprile Columbia University

Transcription

1 The XENON Dark Matter Project: Status of the XENON100 Phase Elena Aprile Columbia University 8th UCLA Symposium Marina del Rey, Feb 22, 2008

2 The XENON Dark Matter Phased Program Detect WIMPS through their elastic scattering with Xe nuclei ultimate goal σ SI ~10-46 cm 2 with XENON1T Xe two-phase TPC, 3D position sensitive, self-shielded by an active LXe veto event-by-event discrimination (>99.5%) by simultaneous charge & light detection energy threshold ~5 kevr with optimized PMT readout (>3 pe/kev) XENON10 first demonstration of the concept in underground laboratory (LNGS) Reached sensitivity of ~10-43 cm 2 for 100 GeV WIMP XENON100, currently under commissioning at LNGS Goal: gamma background reduction by ~100 and fiducial mass increase by ~10 w/r XENON10 sensitivity reach ~2x10-45 cm 2 by the end of 2008 Supported by NSF and foreign contributions XENON100 physics run starts Summer 2008 Pathfinder for XENON1T to be proposed for

3 XENON Sensitivity Reach: Present and Near Future Nature 448, July 19, % 95% XENON100 1 event/kg/yr XENON1T 10 events/ton/yr CMSSM (Roszkowski, Ruiz, Trotta)

4 The Two-Phase XeTPC Concept Prompt (S1) light signal after interaction in active volume; charge is drifted, extracted into the gas phase and detected as proportional light (S2) Challenge: ultra-pure liquid + high drift field; efficient extraction + detection of e - PMT array S1 S2 WIMP Gas hν E ext Liquid hν hν t drift E d WIMP S1 drift time S2 gamma E R e - hν drift time Gamma PMT array PMT array S2 S1 WIMP S2 S1 gamma electron

5 XENON100 Phase: New detector in existing shield at LNGS: 170 kg LXe (70 kg target), active LXe veto (factor 3-4 BG reduction vs. passive LXe) 242 low-activity 1 inch R8520 PMTs, double-walled stainless steel vacuum cryostat cryocooler (170 W) and feed-throughs outside the shield New dedicated material screening facility at LNGS 170 kg ~ 15 cm radius, 30 cm drift (150 µs)

6 counts/kg/d/kev New XENON100 material screening facility Dedicated ultra-low background, 100 % efficient (2 kg) HPGe-spectrometer Shield: 5 cm of OFRP Cu; 20 cm Pb (inner 5 cm: 3 Bq/kg 210 Pb), air-lock system and Rn purge screen all XENON100 detector/shield components for a complete BG model additional screening the LNGS low level counting facility Gator Soudan background 44 kg days Gator LNGS background 30 kg days (October 2007)

7 XENON100 Materials Radioactivity Measurements All materials were screened at LNGS with high purity Ge-spectrometers Material Th-232 (mbq/kg) U-238 (mbq/kg) K-40 (mbq/kg) Co-60 (mbq/kg) amount of material Stainless Steel kg Teflon kg PMT 0.21 mbq/pmt 0.25 mbq/pmt 9.30 mbq/pmt 0.59 mbq/pmt 242 pieces Polyethylene kg Gamma-BG from Poly shield is simulated after the addition of 5 cm of copper inside top/back/side walls and 5 mm of copper on bottom of Poly cavity

8 XENON100 Gamma Background Simulations done with actual mass model of XENON100, including the passive shielding material Simulations uses U/Th/K/Co activities as measured for PMTs, HV bases, HV resistors for drift field, SS, Cu, cables, Teflon, Poly and Pb The main quantity of interest is the rate of single scatters as a function of position in the sensitive volume. With the single scatters rate as a function of position information we can compute the optimal fiducial volume (of a given mass) which minimizes the rate. Total gamma rate is dominated by PMTs, but Poly contribution is non negligible. We are optimizing Cu lining of Poly to reduce its background. Single scatters rate in XENON100 in the energy region of kev with energy threshold 20 kev in the Active Veto Material rate [mdru] Stainless steel 1.95 ± 0.22 Teflon PMT 4.73 ± 0.60 Polyethylene 2.52 ± 0.29 Total 9.52 ± 0.67

9 XENON100 Gamma Background the power of LXe stopping power for penetrating radiation is exploited! rate of single scatters (0-100 kev) before S2/S1 discrimination

10 XENON100 Neutron Background Neutrons from natural radioactivity in detector and shielding materials: (α, n) reaction of α s from natural α-emitters (U, Th) with light target nuclei spontaneous fission (U 238 ) detailed MC, taking into account the measured U/Th activities of all the materials modified code SOURCES-4A (Wilson et al., Carson et al., Lemrani et al. ) to calculate neutron fluxes and energy spectra from U/Th contamination in several materials The cross-sections of (α,n) reaction updated to recent experimental results Uranium in Stainless Steel Uranium in Polyethylene Thorium in Stainless Steel

11 Rate of Neutrons (Single Scatters in Fiducial Volume of 50 kg) Internal Neutron Background dominated by Teflon NOT PMTs! Single scatters: 45%, Multiple scatters: 55% Neutron rates in XENON100 in the energy region of kev Material Stainless steel PMTs Teflon Copper Poly Total Neutron rate [mdru] 2.93e e e e e e-3 The total neutron bg from detector materials and shielding in XENON100 is 0.9 single neutron recoils/year

12 Expected WIMP rates vs Neutron Background = cm 2 = cm 2 = cm 2 = cm 2

13 Expected sensitivity of XENON100

14 Status of XENON100: the Cryostat and Cooling Tower high-purity, double walled stainless steel cryostat assembled and tested at LNGS in Nov-Dec 07 performance of new Cryocooler (Iwatani PC150) assembly tested in Dec 07

15

16 It is nice to work even on a Gran Sasso All cables/cooling system outside shield SS mass alone reduced from ~150 kg to ~60 kg! Start filling 170 kg of high purity Xe on Feb 25, 08

17 Status of XENON100: the TPC Assembly

18 TPC Electric Field and Mesh Configuration 40 field shaping rings (inside and outside) 3 Mesh Structure: Top mesh: 5mm pitch, 171um wires Anode, Bottom mesh: 2.5mm pitch, 125um wires S2 Resolution : 7% (anode shifted by 0.5 pitch)

19 Status of XENON100: PMTs Photomultipliers: Hamamatsu R Al 98 on top - 80 on bottom - 64 in active LXe shield QE~23% for top array; QE~33% for bottom array XENON10 PMTs for LXe shield

20 The new high QE PMTs for XENON100 LED Calibration System Measured SPE response for all tubes

21 XENON100 Data Acquisition 242 FADC channels: CAEN V modules 100 MHz continuous buffer (zero deadtime) analog sum output for S2 trigger Zero Length Encoding via FPGA -->enables rate > 60 Hz (XENON10 < 5 Hz) Threshold S1 and S2 trigger capability high energy veto automatic baseline adjustment deadtime measurement

22 Status of XENON100: Installed LNGS Shield modification/improvement completed Jan 08 Detector moved underground in its shield Feb 08 Filling with Xe start on Feb 25 Purification w/circulation to reach ppb purity XENON100 XENON10

23

24 Kr Purification System for XENON100 XENON100 Refrigerator In XENON100 Out XENON10 Purification Refrigerator Tower Heater

25 Summary 2008 is a very busy but exciting year for the XENON Dark Matter Search the XENON100 phase combines all the good features of XENON10 with a dramatic reduction of the internal background sources with a new engineering approach and with careful screening of all detector components with the reduced background, the increased mass and the active LXe veto we should be ready to explore the 10-8 pb region within XENON100 is an essential step before we can confidently pass to the XENON1T MC studies of XENON1T have started and a proposal will be prepared for Fall08