The XENON Dark Matter Project:
Status of the XENON100 Phase
Elena Aprile
Columbia University
8th UCLA Symposium
Marina del Rey, Feb 22, 2008
The XENON Dark Matter Phased Program
• Detect WIMPS through their elastic scattering with Xe nuclei
• ultimate goal !SI~10-46 cm2 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 cm2 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 cm2 by the end of 2008
Supported by NSF and foreign contributions
• XENON100 physics run starts Summer 2008
• Pathfinder for XENON1T to be proposed for 2009-11
XENON Sensitivity Reach: Present and Near Future
Nature 448, July 19, 2007
95%
68%
XENON100
1 event/kg/yr
XENON1T
10 events/ton/yr
CMSSM (Roszkowski, Ruiz, Trotta)
x
x
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 ePMT array
PMT
array
S1
h!
Gas
Eext
drift time
h!
Liquid
tdrift
h!
ER
S2
WIMP
eh!
Ed
WIMP
S1
S2
drift time
Gamma
PMTarray
array
PMT
! S2 $
#" &%
S1 WIMP
! S2 $
#" &%
S1 gamma
electron
gamma
XENON100 Phase: 2007-2008
• 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)
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 210Pb), air-lock system and Rn purge
• screen all XENON100 detector/shield components for a complete BG model
counts/kg/d/keV
• additional screening done @ the LNGS low level counting facility
Gator Soudan background
44 kg days
Gator LNGS background
30 kg days (October 2007)
XENON100 Materials Radioactivity Measurements
All materials were screened at LNGS with high purity Ge-spectrometers
Material
Th-232
U-238
K-40
Co-60
(mBq/kg)
(mBq/kg)
(mBq/kg)
(mBq/kg)
Stainless Steel
2.4
1.9
10.5
8.5
Teflon
0.44
0.87
6.2
PMT
0.21
0.25
9.30
mBq/PMT
Polyethylene
2.69
amount of
material
65.4 kg
8.49 kg
0.59
242 pieces
mBq/PMT mBq/PMT mBq/PMT
3.54
5.88
0.90
1542 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
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 1. – 30. keV
with energy threshold 20 keV in the Active Veto
Material
rate [mdru]
Stainless steel
1.95 ± 0.22
Teflon
0.32 +- 0.04
PMT
4.73 ± 0.60
Polyethylene
2.52 ± 0.29
Total
9.52 ± 0.67
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
XENON100 Neutron Background
• Neutrons from natural radioactivity in detector and shielding materials:
(!, n) reaction
of !’s from natural !-emitters (U, Th) with light target nuclei
spontaneous fission (U238)
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 (!,n) reaction updated to recent experimental results
•
•
•
•
Uranium in
Stainless Steel
Uranium in
Polyethylene
Thorium in
Stainless Steel
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 4.5 – 26.9 keV
Material
Neutron rate [mdru]
Stainless steel
2.93e-4
PMTs
3.18e-4
Teflon
16.26e-4
Copper
0.11e-4
Poly
4.87e-4
Total
2.74e-3
The total neutron bg from detector materials and shielding
in XENON100 is 0.9 single neutron recoils/year
Expected WIMP rates vs Neutron Background
= 8·10-43cm2
= 9·10-44cm2
= 1·10-44cm2
= 2·10-45cm2
Expected sensitivity of XENON100
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
It is nice to work even on a Sunday @ 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
Status of XENON100: the TPC Assembly
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)
Status of XENON100: PMTs
•
•
•
•
Photomultipliers: Hamamatsu R8520-06-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
The new high QE PMTs for XENON100
• LED Calibration System
• Measured SPE response for all tubes
XENON100 Data Acquisition
242 FADC channels: CAEN V1724
32 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
Status of XENON100: Installed underground @ 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
XENON10
XENON100
Kr Purification System for XENON100
XENON100
Refrigerator
In
Tower
Out
XENON100
XENON10
Purification
Refrigerator
Heater
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 2008-9
• 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