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The DEAP-1 Detector at SNOLAB

The DEAP-1 Detector at SNOLAB. Chris Jillings, SNOLAB/Laurentian U. For the DEAP/CLEAN Collaboration. The DEAP-1 Detector. Nuclear recoil. Electron recoil. DEAP-1 at Queen’s. Demonstrated a pulse shape discrimination between electron recoils and nuclear recoils at ~4x10 -8.

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The DEAP-1 Detector at SNOLAB

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  1. The DEAP-1 Detector at SNOLAB Chris Jillings, SNOLAB/Laurentian U. For the DEAP/CLEAN Collaboration

  2. The DEAP-1 Detector Nuclear recoil Electron recoil

  3. DEAP-1 at Queen’s Demonstrated a pulse shape discrimination between electron recoils and nuclear recoils at ~4x10-8 Detector stability (120-240 pe) Measured at 511 keV 2.9 2.8 2.7

  4. DEAP-1 moved to SNOLAB in 2007 • Runs underground: • December 4, 2007 to January 2008 • v1 clean chamber: July 4, 08 to Dec 6, 08 • v2 clean chamber: March 19, 09 to Dec 10, 09 • v3 clean chamber: March 25, 10 • PSD improved to ~ 10-8 • Light yield increased using HQE PMTs • Backgrounds in WIMP energy ROI greatly reduced

  5. Clean v1 chamber Glove box preparation of inner chamber (reduce Rn adsorption/implantation on surfaces)

  6. 222Rn in DEAP-1 (Gen 1) alpha 222Rn introduced from gas bottle, settles to about 25 decays per day

  7. DEAP-1 Gen 2 chamber • DEAP-1 inner chamber redesigned, teflon as reflector instead of TiO2 paint • Radon trap installed for filling Gen 2, no Rn spike and ~10 times cleaner Gen 1 chamber

  8. Stability (Generation 2) Gen 2 data taken with new DAQ Stable to 10% over 150 days

  9. Gen 3: Improved light yield Arbitrary unit v2, ~2.5 pe/keV v3, ~4.7 pe/keV 60 keV gammas from 241Am in AmBe neutron calibration runs

  10. Hamamatsu R5912 HQE PMTs • Qualified two of each candidate 8” PMT • Evaluate gain, relative efficiency, dark rate, timing, late pulsing, after pulsing, prepulsing, magnetic field sensitivity .... PMT in testing facility at Queen’s 5912 SPE R5912 HQE will be used for DEAP-3600. <75 ppb U/Th for R5912

  11. Background rates in DEAP-1 versus time 120-240 pe region v3 data being analyzed

  12. Background Questions • Given the efforts at surface cleaning between Gen 2 and Gen 3 yielded small results, is there a source of low-energy backgrounds we are missing? • Is the WIMP-region background caused by radon in the bulk? • Or quantitatively: what is the event rate in the WIMP region induced by radon in the bulk? • A sample of radon extracted from approximately 100 litres of air, after corrections for efficiencies, should add Bq levels of radon.

  13. Radon Spike From Air Procedures and equipment from SNO. NaOH Inlet Water trap (coils at -60C) ChromaSorb Trap at -110C (ethanol slush) DEAP Rn tube Lucas cell

  14. Radon Spike • Use high-flow trap with chromasorb at -110C to trap 222Rn. • Oxygen, nitrogen and argon pass through trap. • Transfer radon with cryopumping to small trap • Volume expand radon into Lucas cell and Rn tube. • Count Lucas cell to measure Rn spike. • Next day: install on inlet to argon system. • As long as only a few standard cc’s of contaminant gas, our SAES purifier will purify. • Concentrating the radon in 1m3 of air is not considered a “source” by SNOLAB.

  15. PSD Underground • PSD is a huge data-reduction effort • Depends low-noise electronics • We have 27 TeraBytes of MIDAS data.

  16. Sample PSD Data

  17. Background To PSD • The detector high-Fprompt background rates have some probability of being coincident with a valid tag as described in the DEAP-1 Surface paper (arXiv:0904.2930). • Depends on rate of tags and the time window imposed in analysis. • We expect:

  18. Analyzed PSD

  19. Future PSD • Surface, Gen 1 and Gen 2 data u/g had the same light yield. Analysis of Gen 3 PSD will allow the relationship between energy and PSD to be explored as well as effects of photon counting. • Would like few x 109 events background free. • Requires • Optimized tagging • Stronger source

  20. New 22Na source Place source in bicron BC-490 plastic scint in mold. Double-tag: 1- positron in plastic 2- back 511keV 2 1 1 cm PMT Source in design stages. Early testing with BC-490 successful.

  21. Neutron-Shielding/M.C. Tests • A series of runs were taken with the SNO AmBe neutron source behind various thicknesses of plastic • Model test • Neutron spectrum from AmBe source (Neutron energy spectrum from AmBe source depends on the grain sizes.) • neutron shielding Monte-Carlo calculations • CLEAN nuclear-recoil quenching factor. • Analysis ongoing Frame holds from 0.25” to 2” HDPP Fixed source holder

  22. Some Notes About Analysis • Switching PMT and base circuit forced change in baseline algorithm. • PMT SPE mean charge was determined using a mean charge over a restricted integration window. We have developed fits to Polya functions. • Software noise-reduction techniques developed. • Re-analysis of all SNOLAB data just underway. • Goal: submit manuscripts for publication in timely way.

  23. DEAP-1 to DEAP-3600 • Light yield in DEAP-1 + Monte Carlo  Light yield in DEAP-3600 > 8 pe/keV (with R5912 HQE PMTs) • Stability of DEAP-1 suggests that continuous purification of Argon not needed in DEAP-3600 (but it is available) • PSD data are consistent with surface results: PSD model used holds up. • Detailed analysis of Gen 3 PSD underway. This is important because PSD depends on statistics of photon counting and energy. • PMT/Electronics for DEAP-3600 prototyped on DEAP-1 • We are likely to go to a tapered base to improve signal linearity. • Measured backgrounds in DEAP-1 allow for DEAP-3600 with reduced FV to be useful. • Re-assembly of DEAP-1 in J-drift after with cleaned plumbing and new chamber.

  24. Next 12 Months • Move to J drift • Gen 4 acrylic chamber • Better control of neck events • Wash all argon plumbing lines • Small improvements to cooling system • Hotter source for PSD with improved time tag

  25. SNOLAB • SNOLAB has provided • Services (IT, logistics …) • LN2, • technical staff, • engineering support, • URAs, • funds for new source development • extra shifts, …

  26. People • DEAP-1 slides shown here are drawn from work by many including people at… • LU/SNOLAB (incl 9+ URAs in past three years) • Queen’s • Alberta • TRIUMF • Carleton • Yale • U. North Carolina • U. New Mexico • LANL

  27. Position reconstruction Reconstructed position (cm) Size of DEAP-1 Very good position reconstruction, useful for identifying surface background events

  28. Background rates in DEAP-1 (120-240 pe)

  29. Alpha backgrounds • Are very high energy • Non-linear energy response must be calibrated out.

  30. Clipping of Prompt Light Average alpha Average low energy recoil scaled to alpha energy Protection diodes clip the pulse Clipping is necessary to observe alphas and low energy recoils in the same run for DEAP-1 (clipping will be rare in DEAP 3600) New energy scale required for alphas

  31. Energy Non-linearity • Each PMT sees a >50% change in light based on event vertex position • With clipped pulses, the effective gain may be highly non-linear over this range • Methods to deal with this: • Correct for clipping (currently gives ~10% energy resolution) • Develop independent alpha energy scale (currently gives ~3% energy resolution)

  32. Radon Daughter Coincidence Tags 238U chain 232Th Chain • Timing coincidences for alpha decays give calibration points for the alpha energy scale

  33. Radon 220 Coincidences 220Rn Fit T½ = 0.15 ± .02 s Real T½ = 0.15 s 216Po

  34. Polonium 214 Coincidences 214Bi Fit T½ = 163 ± 27 us Real T½ = 164 s 214Po

  35. Correcting for Nonlinearity

  36. Correcting for Nonlinearity Corrected Prompt = Total Prompt _ 1 – 0.05 PromptZ + 1.33 PromptZ2 PromptZ = Prompt0 – Prompt1 Prompt0 + Prompt1

  37. Calibrated Alpha Spectrum Daughter Constrained Fit Unconstrained Fit 222Rn 267 ± 14 325 ± 54 218Po 267 ± 14 214 ± 20 214Po 41 ± 7 42 ± 9 210Po 35 ± 18 2 ± 58 220Rn 68 ± 7 123 ± 35 216Po 68 ± 7 54 ± 9 212Po 20 ± 10 20 ± 10 Χ2/dof 83/60 67/58 All widths are set at 2.9%

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