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Bei Cai and Mark Boulay
For the DEAP-1 Collaboration
Queen’s University, Canada
Pulse Shape Discrimination (PSD) in Liquid Argon from
DEAP-1
APS meeting, April 14th-17th 2007, Jacksonville, Florida
DEAP--Dark Matter Experiment with Argon PSD
WIMPs can elastically scatter in detector producing nuclear recoils
c 40Ar
40Ar
c
Direct WIMP detection with DEAP
rAAr
Rate in terrestrial detector depends on WIMP mass and WIMP- nucleon interaction cross-section
Ionization radiation in LAr leads to formation of excited dimers Photons are emitted in the dimer de-excitation process Dimer molecules are in either singlet or triplet states, and the
lifetimes are well-separated:
~ 6 ns for singlet state (prompt)
~ 1.51 µs for triplet state (delayed) Fraction of dimers in singlet or triplet state depends on the
incident particle type
Scintillation in liquid Argon
M. Boulay and A. Hime, Astropart. Phys. 25 (2006) 179-182
Discrimination between nuclear recoils and EM events can be achieved using pulse shape discrimination
Neutron backgrounds are reduced with clean materials and shieldingβ,γ backgrounds are reduced with PSD
Backgrounds
Ar-39 is the largest source of background For DEAP-1, the expected βrate is ~3×107 for 7 kg-years For a larger experiment, it’s ~1010 for 3000 kg-years
DEAP-1 experiment at Queen’s University
A dark matter search with a sensitivity of 10-44 cm2
A 7 kg liquid argon detector using 2 PMTs Calibration with γ’s, n’s (tagged Na-22 and AmBe) Demonstration of PSD at the level of 1 in 108
Water shielding
SAES getter
Ar liquefying chamber
Dark box
The DEAP-1 detector
Argon chamber
ET 9390B PMT 5”8” long acrylic guide
11” x 6” (8” CF) tee
Acrylic vacuum chamber
Quartz windows
poly PMT supports
inner surface 97% diffuse reflector,Covered with TPB wavelength shifter
Neck connects to vacuum andGas/liquid lines
7 kg LAr
dark box
argon
annulus
NaI22Na
Tagged Na-22 data run setup
Na-22 source strength: 6 µCi = 222 kBq
DEAP-1 dataYellow: Prompt light regionBlue: Late light regionFprompt = Prompt Light / Total light
A neutron-like event
An electron-like event
Total about 145 hrs of data, prescale = 5
Na-22 data Fprompt distribution WIMP ROI
Preliminary
Current PSD: ~2.6×10-6
Can be improved with full-shielding and more advanced tagging system
Preliminary
Consistent with neutrons in the lab
Rbn = rate of coincident events with high-Fprompt in ROI
= Rt × Rn × 2 Δt = 20 kHz × 8 mHz × 80 ns = 1.3 × 10-5 Hz
(Rbn (measured) = 2.9 × 10-5 Hz)
Rγ = Rt × ε G × ε E (rate of gammas in energy ROI in argon)
Rbn/Rγ = Rn × 2 Δt / (ε G × ε E)
= 6 × 10 -9 (achievable PSD with DEAP-1 on surface)
(with ε G = 1 and ε E = 0.1)
dark boxargon
annulus
NaI
22Na
Rt = rate of tagged events (singles) ~ 20 kHz
Rn = rate of “high-Fprompt” events in argon, in energy ROI (singles) ~ 8 mHz
Coincidences in argon
Summary DEAP-1 has demonstrated 2.6×10-6 PSD with partial shielding
and single NaI coincidence system With full-shielding in place and more advanced tagging system,
we expect to achieve at least 10-8 on the surface. This is the PSD required for the DEAP-1 WIMP search with a
sensitivity of 10-44 cm2. Will move DEAP-1 underground to SNOLAB in summer.
Ad: A new post-doc position available in our group.