A large water shield at 4850 Homestake

Report
A large water shield for dark
matter, double beta decay and low
background screening.
T. Shutt - Case
R. Gaitskell - Brown
Water shields for dark matter or bb decay.
• Conventional Pb + Poly shield for DM, bb decay
expensive, inflexible at large size.
– Ancient Pb (or Cu) to avoid 210Pb - $$.
– Thick polyethylene - $$.
– Higher intrinsic gamma background than water shield.
• Existing water shields
– SNO light water.
– Borexino’s CTF: surrounds 2m Ø liquid scintillator
– Boulby - UKDM
• Liquid noble detectors: At a 1st order phase transition.
– Hundred-kg LXe, LAr, bubble chamber modules not expensive.
– Rapid evolution and scale-up to ton scale could happen very
rapidly...
…. if shielding weren’t prohibitive.
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Multiple User Facility
• Tom Bowles proposal at first Lead meeting, 2001.
• Modular approach from 100 kg - ton scale for
modular dark matter experiments.
– Dual-phase detectors have some natural size limit (as
opposed to XMASS/CLEAN/DEAP).
• Modular approach will accommodate other
experiments
– Experiments may not have the same internal backgrounds.
Spacing, arrangement.
• Good platform for advanced screening
– Ge counters
– Beta cage, alpha screening.
– Moderate-sized liquid scintillator.
Shielding summary
L. DeViveiros, R. Gaitskell, Brown
Gammas:
• 2 m ~ 105 expected from 20 cm Pb shield.
• 4 m affords extraordinarily low background.
• Final rate will depend on water purity.
High energy neutrons from muons
• Muons in rock, outside
of veto
(Mei and Hime, astroph/0512125)
– Low rate, but important
• Cross section on
hydrogen dropping
• Conversion in Pb
multiplies them. N ~ 20.
High energy neutrons in water
• Elastic scattering primarily on O.
– But forward scattered
• Overcome by simple thickness
• 2m water better than
feasible Pb/Poly shield
10-46 cm2
WIMP
rates
• 4m water sufficient for
10-46 cm2 (~1 ton)
sensitivity at 4850 mwe
• Can we live at shallow
depth?
4850 mwe depth
L. DeViveiros, R. Gaitskell, Brown
Water purity
• Assumption: bulk contaminants will be very low with
moderate cost commercial purification
– 18 MΩ deionization
• Radon is main question.
– From initial water: let decay. (3.82 half-life).
– From Ra.
• Main concern of SNO
• Borexino’s CTF: ~ 1 mBq/m3 with commerical system.
– Make-up water. Membrane stripping/degassing.
• Stable water
– SNO, Kamland: should get stagnant water -> Rn decays.
– Chiller with recirculation to enforce gradient.
• Dark matter with discrimination may not drive high
requirement.
– Screening, other experiments may drive this.
From a proposal for Homestake
DUSEL
(R. Gaitskell, Brown /
XENON)
• 10 module system
1.75 m
• 4 m shielding
16 m
– Could be reduced to 3
• Cavern: 16m x 10m x
15 m.
10 m
• Davis cavern +3m
depth.
Mechanics
• Detector grid hangs from
ceiling, supports modules.
• Detector modules either
water-tight, or sealed in
plastic
14 m
• Feedthrough plate handles
sealing of each module.
• Muon veto: Based on
CTF3, ~ 20 PMTs should
give 99.9% or better
efficiency.
Sealing against Rn
• Cavern lined same as SNO cavern. 107
reduction.
• Deck structure sealed to walls with flexible
membrane.
• Each detector module contains all conduit
seals.
– Use same mechanism for sealing against water.
• N2 pure on blanket.
Where?
• Possible “Early Implementation” at DUSEL.
– Strong endorsement by both Homestake and Henderson
DUSEL sites.
• Implementation soon would provide very powerful
boost to promising next-generation, very large scale
detectors.
• Tremendous opportunity for collaborative effort for
liquid-noble gas DM detectors
• SNOLAB?
Noble Liquid Dark Matter Consortium
Adam Bernstein54
David Cline54
Rick Gaitskell54
Yongsheng Gao54
Andrew Hime10,18
Ed Kearns10,18
Dan McKinsey10,18,54
Tom Shutt54
Hanguo Wang54
James White10,54
Frank Wolfs54
So far:
US based effort from
CLEAN+DEAP, XENON,
ZEPLIN.
Open to further participation.
Consortium
• Follows informal discussions over last ~1.5 years.
– Previous DMSAG meeting catalyzed letter to committee.
• First step: letter to DMSAG (6/26/06):
– “We believe it would be beneficial to operate a US
consortium, which could exploit common infrastructure
and specific shared R&D projects. A prime example is a
large multi-module water shield that could be used by a
number of experiments (and also for ultra-sensitive low
background screening).”
– “In addition to benefiting the next phase of technical
development, this consortium will also help lay the
groundwork for what we anticipate to be a very largescale experiment based on the noble liquid technology
(or technologies) that prove most sensitive for detecting
WIMP dark matter.”
PMTs
• PMT radioactivity, g and n, is dominant issue in all
experiments.
• Idea: unified R&D effort with manufacturer(s)
• Development efforts to date:
– Hamamatsu: XMASS
– ETL: ZEPLIN, DAMA, WARP
• Larger effort may gain critical mass
• Overlap of goals:
– Radioactivity: common goal
– Size: large, apart from top dual-phase array
– Temperature: need extra metal coating for Ar/Ne.
Liquid-phase purification
• Liquid phase purification needed at large mass
– Heat load from gas phase: XENON10 rate -> 0.5 kW @ 100 kg
• Common purifier technology:
– “Spark-gap” or cold-getter
– Ne can also use charcoal.
• Key technical challenge: Clean fluid pumping
Internal neutron backgrounds
• From PMTs, will become and issue below WIMP
sensitivity of ~10-45 cm2 (nominal 100 kg active
mass).
• Common approach to mitigating makes sense,
especially in context of water shield.
– Outer liquid (or solid?) scintillator
– Gd doping in water?
– LAr/Ne shield?
• Measurements to calibration of Monte Carlos?
Other possible joint R&D activities
• Waveshifter.
– Essential for Ar + Ne, may be good for Xe.
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Cryogenics
DAQ
Rn screening
Monte Carlo
Nuclear recoil calibration techniques

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