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.