The XENON10 dark matter search
T. Shutt
Case Western Reserve University
T. Shutt, SNOLAB, 8/22/6
1
The XENON10 Collaboration
Columbia University
Elena Aprile (PI), Karl-Ludwig Giboni, Sharmila Kamat,
Maria Elena Monzani, , Guillaume Plante*, and Masaki Yamashita
Brown University
Richard Gaitskell, Simon Fiorucci, Peter Sorensen*, Luiz DeViveiros*
University of Florida
Laura Baudis, Jesse Angle*, Joerg Orboeck, Aaron Manalaysay*
Lawrence Livermore National Laboratory
Adam Bernstein, Norm Madden and Celeste Winant
Case Western Reserve University
Tom Shutt, Adam Bradley, Paul Brusov, Eric Dahl*, John Kwong* and Alexander Bolozdynya
Rice University
Uwe Oberlack , Roman Gomez* and Peter Shagin
Yale University
Daniel McKinsey, Richard Hasty, Angel Manzur*, Kaixuan Ni
LNGS
Francesco Arneodo, Alfredo Ferella*
Coimbra University
Jose Matias Lopes, Luis Coelho*, Joaquim Santos
T. Shutt, SNOLAB, 8/22/6
2
Promise of liquid Xenon.
• Good WIMP target.
• Readily purified (except 85Kr)
• Self-shielding - high density, high Z.
• Can separate spin, no spin isotopes
129Xe, 130Xe, 131Xe, 132Xe, 134Xe, 136Xe
• ~ Low-background PMTs available
• Rich detection media
— Scintillation
— Ionization
— Recombination discriminates between
electron (backgrounds) and nuclear (WIMPs,
neutrons) recoils
Scalable to large masses
T. Shutt, SNOLAB, 8/22/6
3
XENON: Dual Phase, LXe TPC “Calorimeter”
• Very good 3D event location.
• Background discrimination based
on recombination
PMTs
5 µs/cm
• Modular design: 1 ton in ten 100
kg modules.
• XENON10 Phase: 15 kg active
target in Gran Sasso Lab as of
March, 2006. Shield under
construction. Physics runs start:
June 2006.
• XENON100 Phase:
design/construction in FY07 and
FY08 ($2M construction).
Commission and undeground start
physics run with 2008.
Time
XENON Overview
T. Shutt, SNOLAB, 8/22/6
-
Es
~1 µs
---
~40 ns
LXe
Ed
WIMP
A. Bolozdynya, NIMA 422 p314 (1999).
4
Scintillation Efficiency of Nuclear Recoils
Columbia and Yale
Columbia RARAF
2.4 MeV neutrons
p(t,3He)n
Borated
Polyethylene
Lead
LXe
L ~ 20 cm

Aprile et al., Phys. Rev. D 72 (2005) 072006
T. Shutt, SNOLAB, 8/22/6
BC501A
Use pulse shape
discrimination and ToF
to identify n-recoils
5
Nuclear and electron recoils in LXe
Case
Columbia+Brown
ELASTIC Neutron Recoils
INELASTIC 131Xe
80 keV  + NR
Neutron
ELASTIC Recoil
INELASTIC 129Xe
40 keV  + NR
AmBe n-source
137Cs  source
5 keVee energy threshold = 10 keV nuclear recoil
T. Shutt, SNOLAB, 8/22/6
6
Charge and light yields
Charge yield - nuclear recoils
Columbia+Brown; Case
Qui ckTime™ and a TIFF (LZW) decompressor are needed to see thi s pi ctur e.
Aprile et al., astro-ph/0601552,
submitted to PRL
122 keV Electron Recoil, Charge and Light Yields
Light Yield (ph/keV)
80
Wph-1
60
zero
field
40
zero
recombination
20
0
0
T. Shutt, SNOLAB, 8/22/6
20
40
60
Charge Yield (e/keV)
W-1
W0
80
-1
7
Recombination fluctuations
Scintillation-based energy
Case
Combined energy
40 keV (n–
inelastic)
Neutron Recoils
• Recombination independent energy: E = W0  (ne- +n)
—
—
Improves energy resolution
Restores linearity.
• Recombination fluctuations fundamental issue for discrimination.
• New energy definition itself cannot improve discrimination
T. Shutt, SNOLAB, 8/22/6
8
Discrimination at low energy
Nuclear recoil data
(Case)
Electron recoil data
Electron Recoil Band
Centroid
99% Rejection (MC)
Nuclear Recoil Band
Centroid
Nuclear Recoil Event: 5 keVr
• Charge yield increase for BOTH nuclear recoils and
electron recoils at low energy.
• E> 20 keVr: recombination fluctuations dominate.
• Monte Carlo:
— >~99% discrimination at 10 keVr.
This is value used in XENON10/100/1T proposals
T. Shutt,
See:SNOLAB,
T.Shutt,8/22/6
et al.,
astro-ph/0608137
9
Some comments on Ar and Xe: atomic physics surprises
• Xe:
—
—
—
Drop in recombination for
low energy nuclear recoils
Energy independence of
nuclear recoil recombination.
Drop in recombination for
very low energy electron
recoils
• Ar:
—
—
Huge pulse-shaped discrimination needed
because of 39Ar.
Very small apparent “Lindhard” factor.
• Xe scintillation discrimination: a cautionary
tale
LAr
LXe
(Yale)
LAr
nr
er
XY Position Reconstruction in 3 kg prototype
“S2” signal
122 keV 
(57Co)
Reconstructed edge
events at 122 keV
Resolution
≈ 2 mm.
• Chisquare estimate from
Monte Carlo - generated S2
map
T. Shutt, SNOLAB, 8/22/6
5 mm radial cut reduces gamma events in
80 keV
nuclear recoils region.
Inelastic
(131Xe)
110 keV
inelastic (19F)
+ NR
40 keV
Inelastic
129
Neutron ( Xe)
Elastic Recoil + NR
80 keV
Inelastic
(131Xe)
+ NR
40 keV
Inelastic
129
Neutron ( Xe)
Elastic Recoil + NR
11
XENON10: Cryostat Assembly
Pulse tube
cryocooler
Re-condenser
PMTs (top 48)
LXe Active
Gas Region
PMTs (bottom 41)
Vacuum Cryostat
T. Shutt, SNOLAB, 8/22/6
12
XENON10: Detector Assembly
89 Hamamatsu R5900 (1” square)
20 cm diameter, 15 cm drift length
22 kg LXe total; 15 kg LXe active
LN Emergency Cooling Loop
PMT Base (LLNL)
Top PMT Array, Liquid Level Meters, HV- FT
Bottom PMT Array, PTFE Vessel
Grids- Tiltmeters-Case
Liquid Level Meter-Yale
T. Shutt, SNOLAB, 8/22/6
13
Summary: XENON10 Backgrounds
Monte Carlo studies of Radioactivity (Background Events) from:
•
Gamma / Electron

Gammas inside Pb Shield
•
•
•
PMT (K/U/Th/Co)
Vessel: Stainless Steel (Co)
Contributions from Other Components

Xe Intrinsic Backgrounds (incl. 85Kr)
External Gammas - Pb Shield
Rn exclusion
Detector Performance/Design
•
•
Gamma Discrimination Requirements
Use of xyz cuts instead of LXe Outer Veto
•
Neutron Backgrounds




Internal Sources: PMT (a,n)
External: Rock (a,n): Muons in Shield
Punch-through neutrons: Generated by muons in rock
•
NOTE: Active Muon Shield Not Required for XENON10 @ LNGS

Neutron flux from muon interaction in Pb shield << Target Level


T. Shutt, SNOLAB, 8/22/6
[Background Modeling U. FLORIDA / BROWN/COLUMBIA]
14
XENON10 Shield Construction - LNGS
Red-Shield Dimension Blue-Ex-LUNA Box Dimension
Clearance to Crane Hook (after
moving crane upwards) 20 mm
Brown Design / LNGS Engineering
40 Tonne Pb / 3.5 Tonne Poly
Crane Hook Low-Activity (210Pb 30 Bq/kg) inner Pb &
Normal Activity (210Pb 500 Bq/kg) Outer Pb
Construction Underway: Contractor
COMASUD – Mid May Expect
2630 mmCompletion of Installation
2410 mm
LNGS (Ex-LUNA) Box Dimensions are
critical constraints for shield - expansion of
shield to accommodate much larger
detector difficult
Inner Space for XENON10 detector
900 x 900 x 1075(h) mm
Clearance
Box 450 mm
T. Shutt, SNOLAB, 8/22/6
3500 mm
4400 mm
200 mm
Clearance
Box 450 mm
15
XENON10: Punch-Through Neutron Backgrounds
•
High Energy Neutrons from Muons in
Rock
—
—
—
—
—
Poly in shield is not efficient in
moderating High Energy Muon-Induced
Neutrons
Depth is “standard” way to reduce high
energy neutron flux
(LNGS effective depth is 3050 mwe)
Brown Monte Carlos show that:
Goal WIMP 1.3 evts/10 kg/mth: LNGS
depth comfortably achieves goal
Goal WIMP 1.3 evts/100 kg/mth: HE
Neutrons evts ~1/6 rate of dark matter.
Much reduced comfort margin.
(Continue to study mitigation strategies)
Mei and Hime, astro-ph/0512125, calculate
HE neutron/dm ~1/2–1/3 for Ge detectors
at Gran Sasso Depth
Ratio HE Neutron BG / WIMP Signal
DM Goal
(Rates for Current
XENON10 Shield
Design)
NR
Signal
Rate Xe
@ 16
keVr
High Energy Neutron
Relative Flux (from
muons)
Gran
Sasso
Homestake
3.0 kmwe
4.3 kmwe
x6.5
x1
x1/5
Soudan
2.0 kmwe
1.3 evts/10kg/mth
σ ~ 2x10-44 cm2
300
µdru
x1/10
x1/60
x1/300
1.3 evts/100kg/mth
σ ~ 2x10-45 cm2
30
µdru
x1.1
x1/6
x1/30
1.3 evts/1000kg/mth
σ ~ 2x10-46 cm2
3
µdru
x11
x2
x1/3
Additional margin could be achieved using anti-coincidence signal from muon
veto (none currently in place for XENON10) around shield (c.f. CDMSII
simulations indicate factor 15 reduction may be possible by tagging pions also
generated in muon showers that generate HE neutrons). However, it will be
important to verify that such a strategy works before relying on it. (CDMSII:
T. Shutt, SNOLAB, 8/22/6
Reisetter,
U. Minn. / R Hennings-Yeomans, CWRU)
For 2x10-46 cm2 also evaluating
water/active shielding wrt all types of
background
16
Kr removal
•
85Kr
—
—
- beta decay, 687 keV endpoint.
Goals for 10, 100, 1000 kg detectors: Kr/Xe < 1000, 100, 10 ppt.
Commercial Xe (SpectraGas, NJ): ~ 5 ppb (XMASS)
Kr
recovery
10 Kg-charocoal column
system at Case
feed
purge
• Chromatographic separation on charcoal column
Cycle: > 1000 separation
Xe
200 g/cycle, 2 kg/day
25 Kg purifed to < 10 ppt
T. Shutt, SNOLAB, 8/22/6
17
XENON10 expected background
• Dominant background: Stainless Steel Cryostat & PMTs
—
Stainless Steel :100 mBq/kg 60Co
• ~ 4x higher than originally assumed, but faster assembly
—
PMTs - 89 x 1x1” sq Hamamatsu 8520
• 17.2/<3.5/12.7/<3.9 mBq/kg, U/Th/K/Co
• Increased Bg from high number of PMTs / trade off with increased position info. = Bg
diagnostic
• Expected background: ~ 0.4 cnts/kg/keV/day (before discr.)
Depth (cm)
Electron recoil background
(Brown) 5-25 keVee
• Analytical estimate
—
—
—
Original
XENON10 Goal
<0.14 /keVee/kg/day
Current estimate:
2-3 x original goal
T. Shutt, SNOLAB, 8/22/6
Radius (cm)
Single, low-energy Compton
scattering
Very forward peaked.
Probability of n scatters while
traversing distance L:
1  L 
Pn (L)    e 
n!   
n
L
18
XENON10: Underground at LNGS
T. Shutt, SNOLAB, 8/22/6
19
XENON Box: March 10, 2006
T. Shutt, SNOLAB, 8/22/6
XENON Box: March 7 2006
20
Test Mounting Of Detector (June 22)
T. Shutt, SNOLAB, 8/22/6
21
XENON10 Example Low Energy Event
•
•
•
•
Low Energy Compton Scattering
Event
S1=15.4 phe ~ 6 keVee
Drift Time ~38 μs = 76 mm
(Max depth 150 mm)
Bulk gamma calib shows avg S1
2.3 phe/keVee
0.9 phe/keVr
Trigger n>=4 in 80 ns window
— Able to trigger on S1 for 10
keVr with >90% eff
— Also catching S2 triggers
(with pretrigger look-back)
Noise on separate PMT chans
<<0.1 phe equiv
T. Shutt, SNOLAB, 8/22/6
22
Status of XENON10
• 15 kg detector (~8 kg
fiducial) now operational in
Gran Sasso
• First low background
operations in shield started
8/12.
• Calibrations ongoing
—
Dark Matter Data Plotter
http://dmtools.brown.edu
Activated Xe soon
• 164 + 236 keV lines
• 80 keV beta.
—
—
Neutron
External gammas
• Background close to
expectation
• Results soon.
T. Shutt, SNOLAB, 8/22/6
SUSY
Theory
Models
SUSY
Theory
Models
23
Scaling LXe Detector: Fiducial BG Reduction /1
•
Compare LXe Detectors (factor 2 linear scale up each time)
15 kg (ø21 cm x 15 cm) -> 118 kg (ø42 cm x 30 cm) -> 1041 kg (ø84 cm x
60 cm)
—
Monte Carlos simply assume external activity scales with area (from PMTs and cryostat) using
XENON10 values from screening
Low energy rate in FV before any ER vs NR rejection /keVee/kg/day
Gross Mass 15 kg
x2 linear
118 kg
x2 linear
1041 kg
x10 reduction
>102 reduction
(Brown)
Fiducial Mass
T. Shutt, SNOLAB, 8/22/6
dru = cts/keVee/kg/day
24
Scaling LXe Detector: Fiducial BG Reduction /2
• Assuming ER are rejected at 99% for 50% acceptance of NR
—
Diagonal dashed lines show background x exposure giving 1 event leakage
—
If rej. 99% -> 99.5% and acc. 50% -> ~100% then all σ are better by 4x
Low energy rate in FV before any ER vs NR rejection /keVee/kg/day
10-43
cm2
XENON10 - 400 mdruee
7 live-days x 8 kg fid
σ=4
“118 kg” - 40 mdruee
30 live-days x 20 kg fid
σ = 4 10-44 cm2
Gross Mass 15 kg
x2 linear
118 kg
x2 linear
1041 kg
x10 reduction
1 leakage event
(rej 99%)
>102 reduction
1 week
1 month
σ = 2 10-46 cm2
“1041 kg” - 0.2 mdruee
1200 live-days x 100 kg fid
1 year
4 years
Reference: Current CDMS II
90% CL σ = 2 10-43 cm2 for 100 GeV WIMP
T. Shutt, SNOLAB, 8/22/6
dru = cts/keVee/kg/day
(Brown)
Fiducial Mass
25