1. 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

2. 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

3.  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

4.  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

5. DEAP-1 experiment at Queen’s University  A dark matter search with a sensitivity of 10 -44 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

6. Water shielding SAES getter Ar liquefying chamber Dark box The DEAP-1 detector

7. Argon chamber

8. 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

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

10. DEAP-1 data Yellow: Prompt light region Blue: Late light region Fprompt = Prompt Light / Total light A neutron-like event An electron-like event

11. Total about 145 hrs of data, prescale = 5 Na-22 data Fprompt distribution WIMP ROI Preliminary

12. Current PSD: ~2.6×10 -6 Can be improved with full-shielding and more advanced tagging system Preliminary Consistent with neutrons in the lab

13. 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 × 10 -5 Hz (R bn (measured) = 2.9 × 10 -5 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 × 10 -9 (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

14. 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 cm 2.  Will move DEAP-1 underground to SNOLAB in summer. Ad: A new post-doc position available in our group.