Transcript DEAP-1 @SNOLAB

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
~4x10-8
Detector stability
(120-240 pe)
Measured at 511 keV
2.9
2.8
2.7
arXiv:0904.2930
DEAP-1 moved to SNOLAB in 2007
• Runs underground:
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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
Clean v1 chamber
Glove box preparation of inner chamber (reduce Rn
adsorption/implantation on surfaces)
222Rn
in DEAP-1 (Gen 1)
alpha
222Rn
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 TiO2 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
v2, ~2.5 pe/keV
v3, ~4.7 pe/keV
60 keV gammas from 241Am in AmBe neutron
calibration runs
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 ....
Manufacturer
Model No.
Eff. Relative to
PMT in testing facility at Queen’s
R1408
Hamamatsu
R1408 (SNO)
1.00
Hamamatsu
R5912
1.30
Hamamatsu
R5912 HQE
1.40
Photonis
XP1806
1.18
Electron Tubes
ET9354KB
1.20
5912 SPE
R5912 HQE will be used for DEAP-3600.
<75 ppb U/Th for
R5912
Background rates in DEAP-1 versus time
120-240 pe region
v3 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.
NaOH
Inlet
Water trap
(coils at -60C)
ChromaSorb
Trap at -110C
(ethanol slush)
Lucas cell
DEAP Rn tube
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.
PSD Underground
DAQ
Sampling
Data Rate
Ev/sec
500 MHz
Scope
<~150 /s
10 sec
V1720 & 250 MHz
~350 /s
MIDAS
16 sec
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 TeraBytes 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
(arXiv: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 22Na source
2
1
Place source in bicron
BC-490 plastic scint in
mold.
Double-tag:
1- positron in plastic
1 cm
PMT
2- back 511keV
Source in design stages. Early testing with BC-490
successful.
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
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 DEAP3600 > 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.
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
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Services (IT, logistics …)
LN2,
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…
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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 mBq
First run (Queen’s)
Careful design with input from materials
assays (Ge g couting)
August 2007
7 mBq
Water shield (Queen’s)
Water shielding,
some care in surface exposure
(< a few days in lab air)
January 2008
2 mBq
Moved to SNOLAB
6000 m.w.e. shielding
August 2008
0.4 mBq
Clean v1 chamber at
SNOLAB
Glove box preparation of inner chamber
(reduce Rn adsorption/implantation on
surfaces)
March 2009
0.15 mBq
Clean v2 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
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Clean v3 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
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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
238U
chain
232Th
• Timing coincidences for
alpha decays give calibration
points for the alpha energy
scale
Chain
Radon 220 Coincidences
220Rn
Fit T½ = 0.15 ± .02 s
Real T½ = 0.15 s
216Po
Polonium 214 Coincidences
214Bi
Fit T½ = 163 ± 27 us
Real T½ = 164 s
214Po
Correcting for Nonlinearity
Correcting for Nonlinearity
Corrected Prompt =
Total Prompt
_
1 – 0.05 PromptZ + 1.33 PromptZ2
PromptZ = Prompt0 – Prompt1
Prompt0 + Prompt1
Calibrated Alpha Spectrum
Daughter
222Rn
218Po
214Po
210Po
220Rn
216Po
212Po
Χ2/dof
Constrained Fit
267 ± 14
267 ± 14
41 ± 7
35 ± 18
68 ± 7
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%