A Depleted Argon TPC
for DUSEL
F. Calaprice, C. Galbiati
for the Depleted Argon Group
Lead, SD
April 24, 2008
The Depleted Argon
Group
F. CALAPRICE1, P. COLLON2, E. HUNGERFORD3,
C. GALBIATI1, S. JANSEN-VARNUM4, J. MARTOFF4,
P. MEYERS1, S. PORDES5, A. SONNENSCHEIN5
1Princeton
University
2Notre Dame University
3Houston University
4Temple University
5Fermi National Accelerator Laboratory
An Interdisciplinary
Effort
• Geo
• Martin Cassidy, Chris Ballentine, Stuart Gilfillan,
Martin Schoell, Sujoy Mukhopadhyay
• Engineers
• Bob Parsells, Ernst de Haas, Augusto Goretti,
Andrea Ianni
• Graduate Students
• Alvaro Chavarria, Ben Loer, Pablo Mosteiro,
Richard Saldanha, Daniel Robertson, Chris Shmitt
The Goal
• Develop S4/S5 bids for a Depleted Argon
TPC as part of the Initial Suite of
Experiments at DUSEL
• If selected, build a 10 (20) ton TPC wth
Depleted Argon to be deployed at DUSEL.
Science reach: 5×10-47cm2
Key developments on
argon dark matter
• WARP operation (LNGS, May 2004)
• Argon pulse shape discrimination (Hime and
Boulay, Nov 2004)
• Boulay and Hime, Astropart. Phys. 25, 179 (2006)
• WARP two-fold discrimination + cm-like spatial
resolution (LNGS, Apr 2005)
• Benetti et al., Astrop. Phys. 28, 495 (2008)
• •Underground argon depleted in
Acosta-Kane et al., NIMA 587, 46 (2008)
39Ar
(July 2007)
Argon Discrimination
Highest discrimination of minimum ionizing events, in favor of potential WIMP
recoils, with three simultaneous and independent criteria:
•
Pulse shape discrimination of primary scintillation (S1) based on the very large
difference in decay times between singlet (≈ 7 ns) and triplet (1.6 µs)
components of the emitted UV light
Minimum ionizing: triplet/singlet ~ 3/1
Nuclear recoils: triplet/singlet ~ 1/3
Hitachi et al., Phys. Rev. B 27, 5279 (1983)
Theoretical Identification Power exceeds 108 for > 60 photoelectrons
Boulay & Hime astro-ph/0411358
•
Both prompt scintillation (S1) and drift time-delayed ionization (S2) are
simultaneously detected with a pulse ratio strongly dependent from
recombination of ionizing tracks.
Rejection ~ 102-103
P. Benetti et al., NIM A 332, 395 (1993)
•
Precise determination of events location in 3D: 5 mm x-y, 1 mm z
Additional Rejection for multiple neutron recoils and γ background
Detector Size
•
•
•
Serious contamination from 39Ar in atmospheric
argon, 1 Bq/kg
•
1 kHz/ton count rate from 39Ar
Powerful three-fold discrimination - especially, pulse
shape - can offset contamination from 39Ar
Detector size limited at ~0.5 ton fiducial, due to pileup and limitations on electronics in handling GHz-rate
digitization for hundreds of microseconds at >1 kHz
count rates
Requirement for 10-Ton
Detector
• Required factor >=20 depletion for 10-Ton
deetctor
• Required capability of producing 10-Ton
target in 2 years
Why is underground
argon desirable?
•
39Ar-depleted
argon available via centrifugation or thermal
diffusion, but prohibitevely expensive at the ton scale!
•
•
•
•
$40,000/kg on small scale, production rate of a few kg per
month
$400M for a 10-Ton detector if depleted argon produced by
centrifugation
39Ar
production by cosmic rays strongly suppressed
underground
Borexino found that in petrochemicals 14C/C~10-18, six orders
of magnitude lower than in atmospheric carbon (14C/C~10-12)
39Ar: Early
Results
•
Underground gas samples collected on the field and process
by Princeton researchers, measurements in Bern Argon Lab
•
Two sources studied:
first source <5% atm. (reduced x20), 100 kg/day
second source <10% atm. (reduced x10), 3 kg/day
•
•
•
No detection of 39Ar! Upper limit due to technique
systematics, size of gas sample available
•
•
Production capacity of ~30 tons per year for the first source
Reduction by factor 20 makes 10-Ton detector possible!
Next Steps for Underground
Argon R&D
•
•
Continue Investigation of Other Sources of Interest
•
•
Factor of 20 depletion already sufficient
There is time to identify best source before large scale
production!
Upgrade Sensitivity for 39Ar to 1/1000 of atmospheric
value
•
•
AMS (see summary of progress by Frank)
Low-Background LAr Detectors: DUSEL R&D Proposals
from Princeton and FNAL
Challenge #1:
Collection of Underground Argon
•
Collection of multi-ton argon with reduction of 39Ar by
factor 20, already identified, is extremely compelling
•
Feasibility evaluated by engineering firm, supported by
DUSEL R&D NSF Grant. Conceptual design completed in
October 2007. Detailed engineering completed
•
Proven experience in Princeton group in construction and
operation of purification plants (Borexino)
•
DUSEL R&D Proposal submitted in Dec 2007. Requested
funds for construction of prototype separation and
purification plant at small scale (1 kg/day)
Challenge #2:
Large Scale Collection of
Underground Argon
• Plants: ~$1M
• Operations: ~$400k/ton
• Storage: ~$20k/ton
• Cost per units mass 1% of cost for
centrifugation
• Extremely competetive with argon from
centrifugation (1% of cost), competitive with
xenon (10% of cost)
Challenge #3:
Towards a 10-ton Detector
• Optimization of Light Collection
• Optimization of Ionization Collection
• Control of Impurities to Ultra-Trace Levels
• Material Radiopurity (neutrons from PMT and vessel
background still major concerns)
• Water Neutron Shield
• Phased Program with 100-kg and 1-Ton Detectors
at SUSEL
Challenge #4: S4/S5
• Anticipated Request of ~$400k/year for
engineering
• FNAL/Princeton/PPPL
• Anticipated Cost of $30M for 10-Ton
detector
R&D Funds
• Pending Proposals (DUSEL R&D) from
Princeton and FNAL
• 1-kg size liquid phase detectors with
sensitivity at 1/1000 of atmospheric 39Ar
activity
• Prototype plants for collection and
refinement of depleted argon
Two Phase Liquid Argon
Detectors for WIMP Search
Detector Design Requirements
• Two-phase LAr with scintillation/ionization
detection.
–Detect the scintillation light and the ionization
charge
• Use underground argon
– 39Ar level < 5%
• Background and detector size: ton-scale
–Neutrons/betas/gammas
–External shielding/internal shielding
Advantages of 2-phase
Ionization/Scintillation Detector
• Scintillation and ionization parameters provide
better rejection of beta and neutron
backgrounds than scintillation alone.
• Ionization provides position measurement
– Detection of neutron multi-hit events
– Fiducial volume cut to reject:
• single neutron hits near vessel surface
• surface events due to 210Po alpha decay.
Two Phase Ionization/Scintillation
• Scintillation detection:
– Wavelength in deep UV:
• 128 nm stopped by glass, quartz- LiF, CaF2 ok
• Detect with glass or quartz window PMTs with wavelength
shifter: 128 -> 400 nm (TPB is most common)
– Light yield (S1):
• 40 photons/keV for electrons (2-3 pe/keVe not difficult)
• 10 photons/keV for recoil Ar atoms (quenching ~ 25%)
• Ionization
– Yield similar to photon yield
– Detection
• Drift to surface with electric field of 0.25-1.0 kV/cm
– drift velocity ~ 1 mm/µsec
• Extract from liquid to gas with electric field of 3-4 kV/cm
• Accelerate in gas to produce S2 scintillation: S2 ~ 100 S1
WARP 140 kg-Detector
• Current status:
– Under construction at LNGS
• Design features
– Large external veto detector of
LAr (~25 tons-9 active)
• Detects neutrons that scattered in
the internal detector
– Inner detector not sealed
• Inner detector and outer veto
detector share same liquid argon.
• New Inner detector is needed
to use argon depleted in 39Ar.
Main Features of LAr Detector
39
Depleted in Ar
• Efficient use of argon
• High light collection efficiency
• Low threshold energy
• Active external shielding with water tank
• Internal shielding against PMT
radioactivity with high purity acrylic.
–U, Th: ~10 ppt
Conceptual Design for 1-ton LAr Detector
Detector Summary
– Conservative assumptions
• Depletion of 39Ar:
• Low Light yield:
• Existing PMT technology
– Glass envelop (low radioactivity)
– Low quantum efficiency
5%
2.4 pe/keV
200 mBq/PMT
17%
• Low wavelength shifter efficiency
40%
– Room for improvement
• Lower 39Ar depletion
• Higher Q.E. PMT
• Lower background PMT’s
PMT
< 0.1%
25-30%
<10 mBq/
Simulations
• Neutron background
– Rock + Cosmogenic
– PMTs
• Beta background (39Ar)
– Pulse shape and S2/S1 discrimination
– Statistical calculation of pulse shape discrimination
• Method confirmed with data from small WARP detector
• Background rates of ~1 event with ~70% WIMP
acceptance:
– 100 kg x 3 year (Fiducial = 80 kg)
– 1-ton x 3 year (Fiducial = 800 kg)
– 10-ton x 3 year (Fiducial = 7.8 ton) (20-ton?)
Simulation Results
Latest CDMS Results
Future Improvements to
WIMP Sensitivity
– Underground argon with lower 39Ar (<0.1%)
• Possibly already very low
• Need better measurements
• Continue searches
– Photo-detectors with low background
• Fused silica PMT’s, etc.
– Photo-detectors with high quantum efficiency.
A 10-ton detector with 30 keV threshold and zero
background can reach WIMP sensitivity < 10-47
cm2.
Search for Argon with low
39Ar
• Need Efficient Method to Measure 39Ar with small
samples of argon gas
• Direct Counting Limits
– Gas samples:
• Bern laboratory, 1 atm-liter: sensitivity ~5%
– Liquid samples:
• Kg-samples could achieve 0.1% sensitivity
• Kg-sample too large for survey of many wells
• Proposal under review for detector to measure small number of
samples
• Accelerator Mass Spectrometry
– Small samples possible ( few atm-cc’s)
– Current sensitivity at ANL ATLAS Facility:
5%
– Work in Progress to improve sensitivity to 0.1%
ATLAS at Argonne
National Labs
ECR Ion
Sources
ATLAS Linear
Accelerator
Spectrograph
39Ar AMS
Collaboration
• ANL
Pardo, Richard Vondrasek, Ernst Rehm, Hye
• Richard
Young Lee, Robert Scott
Dame
• Notre
• Philippe Collon, Daniel Robertson, Chris Schmitt
University
• Hebrew
• Michael Paul
• Princeton
Burgers, Frank Calaprice, Alvaro Chavarria, Ernst de
• Alex
Haas, Cristiano Galbiati, Ben Loer, Allan Nelson, Richard
Saldanha, Mike Souza
of Vienna
• University
• Walter Kutschera
Recent Development in AMS
• High purity aluminum and precision cleaning
procedures for ATLAS ECR ion source parts
yielded a lower K background.
• Improvements in the spectrograph ΔE detectors
yielded better separation between 39K and 39Ar.
• New run being planned in next few months.
• Sensitivity of 0.1% for 39Ar appears to be
feasible.
• Strong collaboration led by Philippe Collon
(Notre Dame) working on improvements.
Conclusions
• Two phase liquid argon detector with
underground argon depleted in 39Ar is a sensitive
detector for WIMPs.
• Conservative estimates of WIMP sensitivity with
existing materials and methods are excellent,
reaching into the 10-46 cm2 range.
• Improvements in sensitivity could occur in near
future, especially regarding 39Ar issues and
underground argon.
The End