The DEAP1 Detector at SNOLAB Chris Jillings SNOLABLaurentian

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

Nuclear recoil Electron recoil The DEAP-1 Detector

DEAP-1 at Queen’s Demonstrated a pulse shape discrimination between electron recoils and nuclear recoils at ~4 x 10 -8 Detector stability (120 -240 pe) Measured at 511 ke. V 2. 9 2. 8 2. 7 ar. Xiv: 0904. 2930

DEAP-1 moved to SNOLAB in 2007 • Runs underground: – – December 4, 2007 to January 2008 v 1 clean chamber: July 4, 08 to Dec 6, 08 v 2 clean chamber: March 19, 09 to Dec 10, 09 v 3 clean chamber: March 25, 10 • PSD improved to ~ 10 -8 • Light yield increased using HQE PMTs • Backgrounds in WIMP energy ROI greatly reduced

Clean v 1 chamber Glove box preparation of inner chamber (reduce Rn adsorption/implantation on surfaces)

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

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

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

Arbitrary unit Gen 3: Improved light yield v 2, ~2. 5 pe/ke. V v 3, ~4. 7 pe/ke. V 60 ke. V gammas from 241 Am in Am. Be neutron calibration runs

Hamamatsu R 5912 HQE PMTs • Qualified two of each candidate 8” PMT • Evaluate gain, relative efficiency, dark rate, timing, late pulsing, after pulsing, prepulsing, magnetic field sensitivity. . Manufacturer Model No. Eff. Relative to PMT in testing facility at Queen’s R 1408 Hamamatsu R 1408 (SNO) 1. 00 Hamamatsu R 5912 1. 30 Hamamatsu R 5912 HQE 1. 40 Photonis XP 1806 1. 18 Electron Tubes ET 9354 KB 1. 20 5912 SPE R 5912 HQE will be used for DEAP-3600. <75 ppb U/Th for R 5912

Background rates in DEAP-1 versus time 120 -240 pe region v 3 data being analyzed

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.

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

Radon Spike • Use high-flow trap with chromasorb at -110 C to trap 222 Rn. • 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 1 m 3 of air is not considered a “source” by SNOLAB.

PSD Underground DAQ Scope Sampling Data Rate Ev/sec 500 MHz <~150 /s 10 sec V 1720 & 250 MHz MIDAS 16 sec ~350 /s Data Rate Mbyte/sec Bottleneck 1 Scope readout 8 Source strength • PSD is a huge data-reduction effort • Depends low-noise electronics • We have 27 Tera. Bytes of MIDAS data.

Sample PSD Data

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 (ar. Xiv: 0904. 2930). • Depends on rate of tags and the time window imposed in analysis. • We expect: Run PSD Entries Expected # pile-up events Surface 17 M 0. 26 U/G 2008 (scope) 22 M 0. 16 (preliminary) U/G 2009 (MIDAS) 70 M 0. 13 (preliminary) Total 109 M 0. 45

Analyzed PSD

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

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

Neutron-Shielding/M. C. Tests • A series of runs were taken with the SNO Am. Be neutron source behind various thicknesses of plastic • Model test – Neutron spectrum from Am. Be source (Neutron energy spectrum from Am. Be 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

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.

DEAP-1 to DEAP-3600 • Light yield in DEAP-1 + Monte Carlo Light yield in DEAP 3600 > 8 pe/ke. V (with R 5912 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.

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

SNOLAB • SNOLAB has provided – – – – Services (IT, logistics …) LN 2, technical staff, engineering support, URAs, funds for new source development extra shifts, …

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

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

Background rates in DEAP-1 (120 -240 pe) Date Background Rate (in WIMP ROI) Configuration Improvements for this rate April 2006 20 m. Bq First run (Queen’s) Careful design with input from materials assays (Ge g couting) August 2007 7 m. Bq Water shield (Queen’s) Water shielding, some care in surface exposure (< a few days in lab air) January 2008 2 m. Bq Moved to SNOLAB 6000 m. w. e. shielding August 2008 0. 4 m. Bq Clean v 1 chamber at SNOLAB Glove box preparation of inner chamber (reduce Rn adsorption/implantation on surfaces) March 2009 0. 15 m. Bq Clean v 2 chamber at SNOLAB Sandpaper assay/selection, improved purging, PTFE instead of BC-620 reflector (from Rn emanation measurements), Rn diffusion mitigation, UP water in glove box, documented procedures; Rn Trap@SNOLAB for filling. March 2010 ? Clean v 3 chamber at SNOLAB Acrylic monomer purification for coating chamber. TPB purification. Table from Mark Boulay

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

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

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: 1. Correct for clipping (currently gives ~10% energy resolution) 2. Develop independent alpha energy scale (currently gives ~3% energy resolution)

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

Radon 220 Coincidences 220 Rn Fit T½ = 0. 15 ±. 02 s Real T½ = 0. 15 s 216 Po

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

Correcting for Nonlinearity

Correcting for Nonlinearity Corrected Prompt = Total Prompt _ 1 – 0. 05 Prompt. Z + 1. 33 Prompt. Z 2 Prompt. Z = Prompt 0 – Prompt 1 Prompt 0 + Prompt 1

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