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Progress on DEAP
DEAP: Dark Matter Experiment using Argon PSD
•Scintillation PSD with LAr
•DEAP-1:
7 kg LAr cryostat, low-background,
low-threshold detector (U/G in SNOLAB fall ‘06)
•Summary of backgrounds in DEAP-1
Materials assays
Shielding design
•Schedule for DEAP-1
•Plans for 1-tonne DM search with DEAP @ SNOLAB
Mark Boulay
Canada Research Chair in Particle Astrophysics
Queen’s University
Mark Boulay
SNOLAB V 21/8/06
DEAP-1 R&D Collaboration
Queen’s University
M. Boulay, M. Chen, A. Hallin, J. Lidgard, R. Matthew,
A.B. McDonald, K. Nicolics, P. Skensved
Carleton University
K. Graham
Case Western Reserve University
M. Dragowsky
Los Alamos National Laboratory
A. Hime, D. Mei, K. Rielage, L. Stonehill, J. Wouters
SNOLAB
F. Duncan, I. Lawson, C.J. Jillings
Yale University
D. McKinsey, J. Nikkel
Mark Boulay
SNOLAB V 21/8/06
Direct WIMP detection in terrestrial experiment
•WIMPs can elastically scatter in detector producing nuclear
recoils
40Ar
c
40Ar
c
•Rate in terrestrial detector depends on WIMP mass and
WIMP-nucleon interaction cross-section
•Low-energy recoils with E ~ 10 keV, low threshold
•Easy to detect nuclear recoils, experimental challenge is
to detect small number of WIMP nuclear recoils in a sea of
backgrounds
Mark Boulay
SNOLAB V 21/8/06
Scintillation in liquid argon
•ionizing radiation leads to formation of excited dimers in argon (Ar*2)
•dimers are produced in either singlet or triplet excited states
•decays have characteristic times, and can result
in photon emission
•~ 2 ns for singlet state (prompt)
• 1.6 us for triplet state (delayed)
•Fraction of dimers in singlet versus triplet state depends on
ionization density along track, and thus on incident particle
type
Net effect is a difference in the photon emission versus time
curve for g/b events and for nuclear recoils
Mark Boulay
SNOLAB V 21/8/06
http://arxiv.org/astro-ph/0411358
scintillation pulseshape analysis for
discrimination of e- vs
nuclear recoils
-> no electron-drift
DEAP : Dark-matter Experiment with Argon PSD
Mark Boulay
SNOLAB V 21/8/06
Idea is to use scintillation photons only for
discrimination in DEAP…
…allows for simple and clean detector design
and a more easily scalable experiment
Preliminary simulations and data show promise for
using this technique to mitigate backgrounds
DEAP experimental program focused on determining and
measuring background requirements for large (1-tonne)
experiment
Mark Boulay
SNOLAB V 21/8/06
Some advantages of LAr
•Inexpensive : 10 kg = 25$ of LAr
•Good light yield, 40000 photons/MeV = good resolution
•Used extensively, very large experiments underground
•Easily accessible temperature (~85 K)
•Same requirements as LN for cryogenic components
•Liquid experiment can be continuously or periodically
purified
Allows simple, inexpensive, scalable design:
O(2000$ per kg) fiducial mass
Mark Boulay
SNOLAB V 21/8/06
Discrimination in liquid argon
<pe> = 60
preliminary
O(1in 105)
consistent
with random
coincidence
with intrinsic
background
(preliminary)
LANL cryostat
<pe> = 60 corresponds to 10 keV with 75% coverage
Mark Boulay
SNOLAB V 21/8/06
DEAP-1 detector
A 7 kg LAr cryostat to:
• develop high light yield and low threshold detector
• demonstrate PSD at low threshold (10 keV)
• develop low background detector and verify
background calculations
• measure residual surface backgrounds
• define requirements for large (1-tonne)
experiment
Mark Boulay
SNOLAB V 21/8/06
DEAP-1 design
Quartz windows
Neck connects to vacuum and
Gas/liquid lines
11” x 6” (8” CF) tee
poly PMT supports
6” acrylic guide
ET 9390 PMT 5”
Acrylic vacuum chamber
inner surface 97% diffuse reflector,
Covered with TPB wavelength shifter
Mark Boulay
SNOLAB V 21/8/06
DEAP-1 (7 kg LAr) at Queen’s
Mark Boulay
SNOLAB V 21/8/06
DEAP-1 LAr calibration data from run @ Queen’s
PE
Mark Boulay
SNOLAB V 21/8/06
WIMP search region in DEAP-1
Need to
shield
detector and
move UG
to SNOLAB
WIMP search region
unshielded run on surface at Queen’s
Mark Boulay
SNOLAB V 21/8/06
Mark Boulay
SNOLAB V 21/8/06
What we’re up against…
Backgrounds in DEAP
Detector materials
bulk U, Th, K
select and assay
clean components
Liquid argon target
39Ar, Kr, U, Th
argon purification
and PSD
Laboratory walls
U,Th, K
Shielding for
neutrons and g’s
Cosmic rays
fast neutrons
SNOLAB
(depth)
Radon daughter
plate-out
Minimize surface
plate-out, fit event
vertices for
fiducial volume
Surfaces and
optical effects
Mark Boulay
b-g bkgs are reduced with PSD
Neutron and nuclear recoils
bkgs are reduced with clean
materials and shielding
Surface bkgs are reduced using
vertex positioning (large expt)
DEAP-1
DEAP-3
SNOLAB V 21/8/06
a-emitters (radon-daughters) plated out on detector surfaces are a
dangerous background
LAr
Cryostat
wall
210Po
a
Decay in bulk detector
tagged by a-particle
energy
on surface
Decay from surface releases
untagged recoiling nucleus
a
WLS coating
cf. SNO NCDs: residual surface
contamination of 1/m2/day
0.1 mHz for 1-tonne expt
(irreducible Radon emanation)
Mark Boulay
SNOLAB V 21/8/06
Measurement of surface alpha activity with argon gas
(muon flux on light
guides is 7.8 Hz@
1/cm2/min)
(alpha’s)
Purified argon gas, no source
Mark Boulay
SNOLAB V 21/8/06
Summary of materials assayed for DEAP-1
SNOLAB Ge counter
(see Ian Lawson’s talk)
232Th
238U
40K
(ppb)
(ppb)
(ppm)
stainless
(chamber,flanges,
neck)
1.9 ± 0.14
0.51 ± 0.04
218 ± 69
Al (80/20)
dark box
332 ± 12
762 ± 58
2956 ± 1216
Al sheet (dark
box)
248 ± 1
788 ± 13
none
rubber gasket
6049 ± 147
1547 ± 133
828± 45
circuit boards
(PMT bases)
5309 ± 135
1439 ± 162
1005 ± 57
235U
welding rods
76 ± 6
39 ± 25
0.4 ± 3
60Co
9390UL PMTs
(Electron Tubes)
31 ± 11
28 ± 19
60 ± 16
Material
Mark Boulay
other
60Co
235U, 60Co
SNOLAB V 21/8/06
Mark Boulay
SNOLAB V 21/8/06
Backgrounds in DEAP-1
Source
b-g
(Hz)
Nuclear recoil
(events/year)
Detector materials
≤1
≤1
39Ar
1.5
0
Lab walls
<1
<1
Cosmic rays
<1
<1
Rn plate-out
<<1 Hz
17 mHz
Surface
?
?
Total
(events/year)
5 x 10 7
5 x 10 5
need 108 PSD
Mark Boulay
need to
reduce
need to
measure
need position reconstruction to
remove surface events
SNOLAB V 21/8/06
DEAP-1 shielding design
60 cm water “cubes”
reduces (alpha,n) UG (from rock wall)
to < 1 per year
evaluating requirements for g shielding,
radon
Mark Boulay
figure F. Duncan
SNOLAB V 21/8/06
DEAP-1 proposed location at SNO
space limits shielding design
Mark Boulay
SNOLAB V 21/8/06
Timeline for DEAP-1 (7 kg LAr)
• Commission shield, reduce surface contamination with
radon free glove box system, and calibrate on surface
(2 months)
• Submit first RTP to SNO/SNOLAB (End of August 2006)
• Deploy shielded detector in SNOLAB (fall 2006)
• Determine ultimate residual background level in WIMP
search region
Mark Boulay
SNOLAB V 21/8/06
Plans in for large (fiducialized) liquid argon detector
• Will use DEAP-1 to define background reduction
needed for tonne-scale experiment
• NSERC project grant proposal for fall 2006
($2.5M CAD capital+operating total project cost)
for construction start 2007
• Currently Queen’s+Carleton+SNOLAB
(Boulay, Chen, Hallin, McDonald, Graham,
Duncan, Lawson, Jillings)
Mark Boulay
SNOLAB V 21/8/06
Conceptual design for DEAP-3
~10 cm position resolution
allows reduction of 0.5 mHz
surface contamination
1-tonne fiducial LAr for
sensitive WIMP search
$2.5 M capital
500 PMTs
Needs 5 m diameter liquid shielding tank, explore
possibility of overlap with DEAP-1
Mark Boulay
SNOLAB V 21/8/06
Conclusions
• Liquid argon promising target for DM
• Inexpensive for large, sensitive DM search
• DEAP-1 built, commissioning shield for U/G
deployment fall 2006
• Will further evaluate PSD and backgrounds UG
• Proposal fall 2006 to NSERC for $2.5M capital
project for 2007 funds
(5 m diameter footprint)
Mark Boulay
SNOLAB V 21/8/06