1. DMSAG 14/8/06 Mark Boulay Towards Dark Matter with DEAP at SNOLAB Mark Boulay Canada Research Chair in Particle Astrophysics Queen’s University DEAP-1: 7 kg LAr cryostat at Queen’s, low-background, low-threshold detector (U/G in SNOLAB fall ‘06) Summary of backgrounds in DEAP-1 Plans for 1-tonne DM search with DEAP @ SNOLAB DEAP: Dark Matter Experiment using Argon PSD

2. DMSAG 14/8/06 Mark Boulay Direct WIMP detection in terrestrial experiment WIMPs can elastically scatter in detector producing nuclear recoils  40 Ar  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

3. DMSAG 14/8/06 Mark Boulay 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 “Noble” noble gas Liquid experiment can be continuously or periodically purified (advantage over crystals) Allows simple, inexpensive, scalable design

4. DMSAG 14/8/06 Mark Boulay 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

5. DMSAG 14/8/06 Mark Boulay What we’re up against… Backgrounds in DEAP Detector materials bulk U, Th, K Liquid argon target 39 Ar, Kr, U, Th Laboratory walls U,Th, K Radon daughter plate-out Surfaces and optical effects Cosmic rays fast neutrons  -  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) select and assay clean components argon purification and PSD Shielding for neutrons and  ’s SNOLAB (depth) Minimize surface plate-out, fit event vertices for fiducial volume DEAP-1 DEAP-3

6. DMSAG 14/8/06 Mark Boulay  -emitters (radon-daughters) plated out on detector surfaces are a dangerous background 210 Po on surface Decay in bulk detector tagged by  -particle energy Decay from surface releases untagged recoiling nucleus Cryostat wall LAr   WLS coating

7. DMSAG 14/8/06 Mark Boulay Backgrounds in DEAP-1 Source   (Hz) Nuclear recoil (events/year) Detector materials< 1 39 Ar 1.5 0 Lab walls< 1 Cosmic rays< 1 Rn plate-out<<1 Hz 17 mHz Surface ? ? Total (events/year) 5 x 10 7 5 x 10 5 need to measure need 10 8 PSD need position reconstruction to remove surface events need to reduce

8. DMSAG 14/8/06 Mark Boulay DEAP-1 at Queen’s 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

9. DMSAG 14/8/06 Mark Boulay ET 9390B PMT 5” 6” 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 DEAP-1 @ Queen’s

10. DMSAG 14/8/06 Mark Boulay DEAP-1 (7 kg LAr) at Queen’s

11. DMSAG 14/8/06 Mark Boulay DEAP-1 LAr calibration data from run @ Queen’s PE

12. DMSAG 14/8/06 Mark Boulay WIMP search region in DEAP-1 WIMP search region unshielded run on surface at Queen’s Need to shield detector and move UG to SNOLAB

13. DMSAG 14/8/06 Mark Boulay Purified argon gas, no source (alpha’s) (muon flux on light guides is 7.8 Hz@ 1/cm 2 /min) Measurement of surface alpha activity with argon gas

14. DMSAG 14/8/06 Mark Boulay Backgrounds CDMS Collab Meeting 15 Oct 2005 DEAP-1 endorsed by SNOLAB EAC

15. DMSAG 14/8/06 Mark Boulay Timeline for DEAP-1 (7 kg LAr) Commission shield, reduce surface contamination with Radon free glove box system, and calibrate (2 months) Deploy shielded detector in SNOLAB (fall 2006) Determine ultimate residual background level in WIMP search region (late 2006)

16. DMSAG 14/8/06 Mark Boulay Plans in Canada for large 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 Queen’s: M.Boulay, M.Chen, A.Hallin, A.B. McDonald Carleton: K. Graham plus significant SNOLAB involvement US involvement in 1-tonne experiment is likely essential

17. DMSAG 14/8/06 Mark Boulay 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 @ SNOLAB $2.5 M capital 500 PMTs

18. DMSAG 14/8/06 Mark Boulay Conclusions Liquid argon promising target for DM Inexpensive for large, sensitive DM search Single-phase simple design allows sufficient PSD and minimization of surfaces which could introduce backgrounds Determining constraints and measuring backgrounds for large argon experiment -combination gas, liquid phase measurements for measurements of surface radon, optical effects, bulk contamination Proposal fall 2006 to NSERC for $2.5M capital project for 2007 construction