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 40 Ar c Direct WIMP detection with DEAP 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) Discrimination between nuclear recoils and EM events can be achieved using pulse shape discrimination

 Neutron backgrounds are reduced with clean materials and shielding  β,γ backgrounds are reduced with PSD Backgrounds  Ar-39 is the largest source of background  For DEAP-1, the expected βrate is ~3×10 7 for 7 kg-years  For a larger experiment, it’s ~10 10 for 3000 kg-years

DEAP-1 experiment at Queen’s University  A dark matter search with a sensitivity of cm 2  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 10 8

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 and Gas/liquid lines 7 kg LAr

dark box argon annulus NaI 22 Na Tagged Na-22 data run setup Na-22 source strength: 6 µCi = 222 kBq

DEAP-1 data Yellow: Prompt light region Blue: Late light region Fprompt = 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

R bn = rate of coincident events with high-Fprompt in ROI = R t × R n × 2 Δt = 20 kHz × 8 mHz × 80 ns = 1.3 × Hz (R bn (measured) = 2.9 × Hz) R γ = R t × ε G × ε E (rate of gammas in energy ROI in argon) R bn /R γ = R n × 2 Δt / (ε G × ε E ) = 6 × (achievable PSD with DEAP-1 on surface) (with ε G = 1 and ε E = 0.1) dark box argon annulus NaI 22 Na R t = rate of tagged events (singles) ~ 20 kHz R n = 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 on the surface.  This is the PSD required for the DEAP-1 WIMP search with a sensitivity of cm 2.  Will move DEAP-1 underground to SNOLAB in summer. Ad: A new post-doc position available in our group.